Duckweed Uptake of Phosphorus and PharmaceUTICALS FROM
WASTEWATER IN WELLSVILLE CITY (UTAH)
March 4, 2011
Duckweed Websites
Here is a list of some of the duckweed websites I've found useful:
Charms of Duckweed
Wayne Armstrong's treatment of the Lemnaceae
Dr. Wastewater's Duckweed Application Page
Popularity of Lemna over time (as per research papers)
USDA Plants database
Duckweed: A tiny plant with enormous potential for agriculture and the environment
Charms of Duckweed
Wayne Armstrong's treatment of the Lemnaceae
Dr. Wastewater's Duckweed Application Page
Popularity of Lemna over time (as per research papers)
USDA Plants database
Duckweed: A tiny plant with enormous potential for agriculture and the environment
February 10, 2011
Duckweed species native to Cache Valley (northern Utah)
Lemnacea (common name duckweed) grows naturally in almost every region with a growing season of at least five months. Most studies involving duckweed take place in climates with 9-10 month growing seasons. More rare are the duckweed studies taking place in regions like Cache Valley with only 5-7 month growing seasons [Culley, 1981]. Duckweed is a C3 plant—which helps grow in colder climates with shorter growing seasons. Since water freezes in the winter and duckweed floats on water, it does best in warmer climates. Nonetheless, of the four principal duckweed genera three are still found in Cache Valley. There are three principal species from this genera in Cache Valley, all of which are reported to be cold tolerant. The duckweed plants growing in Cache Valley Utah include Lemna turionifera (or L. minor), Wolffia Borrealis, and Spirodela Polyrhizza. Duckweed’s native presence in Cache Valley, its resilience to temperate climates, and its fast growth rates make it promising for nutrient removal.
The location of the Cache Valley duckweed varies depending on the species. L. minor/turionifera and W. borealis grow in a mixed culture on the Wellsville Municipal Sewage lagoons (56 acres). L. minor is more predominant in the field than Wolffia. The Wellsville lagoons receive some sheltering from wind due to their location in a recessed area bordered by trees along the Little Bear River (Fig. 1, left). These two species are only found in the parts of the Logan Wastewater lagoons that are protected from wind (i.e. culverts, chlorination basin, and wetlands). The majority of the 460 acre Logan lagoons are not protected from wind and do not contain duckweed (Fig. 1, right). The third Cache Valley species, S. Polyrhizza, can be found up Logan Canyon near third dam on the north side of the highway. These duckweed species, especially L. minor have adapted to temperate climates like that of northern Utah [Culley; Landolt 1986 p.421]. During the three years this study took place, full-duckweed coverage on the Wellsville lagoons occurred at the first part of May and continued until about the third week in November. The wastewater lagoons freeze during the winter forcing the duckweed plants into dormancy; however, duckweed fronds appear in the water at the first sign of ice melting off the lagoons in the Spring and occasionally on top of the ice in small puddles of water during particularly warmer periods during winter (Fig. 2).
The duckweed species native to Cache Valley need to tolerate temperate climates with cold winters. Spirodela polyrhizza, Wolffia borealis, and Lemna turionifera all develop special fronds called "turions" when the weather starts to cool. Turions are overwintering buds rich in starch. In the case of L. turionifera they look like single dark green fronds. In the case of W. borealis, they look like tiny spherical balls. Turions have a higher specific gravity than water. During winter, turions sink down into the sediments and then emerge under warmer conditions thus enabling them to survive freezing weather (Landolt, 1986, pp.421). Unlike the other three species, Lemna minor produces "resting fronds." Resting fronds look like the turions from L. turionifera but they do not sink to the sediment. The resting fronds also have a specific gravity > 1 which enables them to sink below the water surface and avoid being frozen at the surface by ice.
Figure1 : Duckweed growing atop 56 acres of Wellsville Municipal Sewage Lagoons which sits in a basin, but not growing atop 460 acres of Logan City lagoons.
Figure 2: (Left) L. minor and Wolffia duckweed species appearing immediately after the ice melts ca. March and April. (Right) L. Turionifera turions or L. minor "resting fronds" floating on 3 cm. water puddle above the ice on Feb. 4, 2011, following unusual rainy winter weather.
Lemnaceae species increase in size from Wolffia, L. minor, and S. polyrhizza species, respectively. Several factors contribute to the size of the fronds. Daughter fronds in lab studies are often smaller than the mother fronds (Al-Nozaily) which needs to be considered when basing growth rate on frond count. As plant density increases frond size often decreases. Landolt listed other factors contributing to an increase in frond size, including: increased light intensity; increased light duration; addition of sugar; increased nitrogen, phosphorus, potassium, calcium, and magnesium concentrations (which can also decrease frond size if too high); and increased temperature (Landolt, 1986, pp.36-37). Typically, full-size fronds for Wolffia, L. minor, and S. Polyrhizza range from 0.5-1mm, 3-5mm, and 1-1.5cm, respectively. L. minor fronds ranged in sizes depending on the development stage of the frond (i.e. whether a bud to a frond or fully separated frond w/ or w/o buds). Figure 3 show how L. minor fronds were characterized depending on development. The Lemnaceae species used in these laboratory experiments had a density of 815ug/ml (s.g. 0.82).
Figure 3: Characterization and digital imaging of L. minor and Wolffia fronds
Table 1: Characterization of L. minor fronds by area, mass, and dimensions
Table 2: Characterization of Wellsville duckweed by elemental composition
Some aquatic species tolerate cold climates as well or better than duckweed but unlike duckweed they do not currently grow on Wellsville nor Logan wastewater lagoons in Cache Valley. Watercress (Nasturtium officinale), have the ability to grow during the harsh Cache Valley winters provided that they are near flowing water like springs [Michaelis, 1976; personal observation]. Watercress grows during the winter in a canal running adjacent and north of Canyon Road in Logan, Utah, near the Utah Water Research Laboratory. Duckweed favors calm water, unfortunately, this turns to ice and limits the duckweed growing season to approximately six months in Cache Valley.
Azolla and pennywort have also been recommended as frost tolerant aquatic plants; unfortunately, their growth rates are lower than duckweed’s and they are not yet established on Cache Valley lagoons [personal correspondence with Louis Landesman, 2/12/09]. Duckweed species already grow on wastewater in Cache Valley and the literature about them is prolific. It is doubtful any other floating aquatic plant could outperform the duckweed since ice formation, not frost resistance, is the limiting factor of winter growth on wastewater lagoons in Cache Valley.
More on duckweed inhibitors
A follow-up on factors inhibiting duckweed growth:
1. Fungi--probably pythium fungi, cause plant tissues to deteriorate consequently converting duckweed into a meal for micro-organisms (Suren, 1989). Pythium blight starts small and spreads until it eventually infects all the duckweed. (Rejmankova, 1986; website http://www.mobot.org/jwcross/duckweed/duckweed-pests.html)
Methods to prevent and/or eliminate fungi:
a) Fungicide. Apply fungicide like Ridomil Gold EC at a rate of 0.3uL/L-nutrient solution. To do this, you can make up "Subdue" solution (0.3ml-Ridomil GoldEC/L) and then apply "Subdue" solution at a rate of 1mL-"Subdue"/L-nutrient solution. I only had to apply this fungicide to my 100L reactor once to get rid of all the fungi. I had no problem with the fungi until I began to experiment with lower duckweed densities (below 30g(dry)/m^2) and low nutrient solutions (<1ppm P and N). After the fungi reappeared following the latter growth conditions, I accidentally applied Ridomil GoldEC at a rate of 1mL/L-soln. Oops! I came back the next day to find a room that smelled like paint thinner and a reactor void of duckweed (all of it sunk to the bottom of the reactor).
b) Silicon. Add silicon to nutrient solution which might have the effect of "toughening" cell tissue to resist fungi and disease. The USU Crop Physiology Laboratory has noticed that their hydroponic solutions containing silicon are more resistant to disease. They have a recipe to produce 2KOH + SiO2 --> K2SiO3 + H2O. The instructions to make this are as follows: First, dissolved 44.9g KOH in ~3.5L distilled water; Second, stir until clear (~15 min.); Third, add 24 g SiO2 (fumed silica--before to use a fume hood and/or filter mask); Fourth, stir until clear (~4-8 hrs.); and Fifth, bring volume to 4 L. Note: they have also noticed disease resistance by simply adding chunks of potassium silicate glass to the sediment to supply the silicon.
Figure 1: Duckweed fronds with early onset of fungi infestation.
Figure 2: Duckweed fronds 3-7 days after Fig. 1.
Figure 3: Duckweed fronds completely infected by fungi and being decomposed by micro-organisms.
c) Temperature. Regarding fungi with duckweed and temperature effect, Elias Landolt wrote:
Ways to reduce/eliminate the growth of algae with duckweed include:
a) rinse duckweed in 0.05% sodium hypochlorite soln. (aka. bleach)--this is comparable to chemotherapy for duckweed plants and it's a matter of "survival of the fittest." You only need to rinse for 5-30 seconds. I tried this procedure once and rinsed fronds for 30 seconds which destroyed virtually all of the fronds (and algae).
b) as part of "a" growing cultures in the lab should be as aseptic as possible (i.e. autoclave or filter nutrient solution, laminar flow hood, autoclaved glassware, etc.). This is difficult for me to do which is probably why I seem to always end up with algae showing up.
c) foil or dark material to cover all but the surface of the growing vessel reducing the light available for algae growth beneath the water suface.
d) maintain a crop density of at least 20-30g(dry)/m^2; anything lower than this allows too much light to pass through the duckweed cover.
e) filter using sand and/or diatomaceous earth (Naghavi, 1986)
f) periodic spraying ponds with algicide copper sulphate at a concentration of 2mg/L; spray at noon when temperatures and algae concentrations are high. This algicide procedure was used to remove filamentous alga Oedognium (Edwards, 1992).
Figure 4: Algae attaching to pythium fungi-infected duckweed.
Figure 5: Healthy duckweed fronds and roots without algae, for bioassay
Figure 6: Signs of algae infestation (progressively worse L to R).
Figure 7 (top-bottom pairs): Logan City wastewater w/o and w/ chlorine;
Wellsville City wastewater; Hydrosol nutrient solution.
Figure 8: Bioassays (APHA Std. Method 8211) by frond count. Logan w/ and w/o chlorine.
Figure 9: Bioassay method 8211 Hydrosol and Wellsville City ww solutions.
Figure 10: Bioassay method 8211; 3-5 days (typ. 96 hrs.); Wellville City and Logan City w/Cl ww.
Figure 11: Duckweed plants following bioassay;
removed one-by-one for frond/colony counting.
Figure 12: More duckweed plants following bioassay;
removed one-by-one for frond/colony counting.
Figure 13: Bioassay results comparing effect of nutrient solutions and chlorination on duckweed growth. It appears that nutrient solution has a larger effect on duckweed growth than whether or not it has been chlorinated (see below).
3. Chlorine. Chlorine was investigated as a potential growth inhibitor in duckweed . Duckweed was collected first collected from the chlorination basin and wastewater collected just following the chlorination basin. After noticing that this duckweed had lower growth rates than the Wellsville City duckweed, a series of bioassays were conducted to compare the effect of chlorination on duckweed growth. Results showed that nutrient solution had a greater effect on growth rate than chlorination. In some cases, duckweed on chlorinated Logan City wastewater (ww) experienced higher growth than on non-chlorinated ww; nonetheless, both Hydrosol nutrient solution and raw Wellsville City ww produced higher growth rates than Logan City ww. The lower growth rates on Logan City ww are likely due to the algae competition. Fig 7 above shows the relative amount of algae in Logan City ww compared to the other solutions. One report showed that macrophyte growth can be inhibited by total residual chlorine concentrations greater than 0.05ppm (TRC) (Watkins, 1984). Tap water is not recommended for producing nutrient solution probably due to TRC concentrations. TRC in Logan City tap water at the UWRL is below 0.05mg-TRC/L. Residual chlorine is believed to have negligible effects on the duckweed growth in this study.
Figure 14: Boxplots showing effect of nutrient solution and chlorination on duckweed growth. Results show that duckweed growth slows over time (i.e. harvest more frequently) and chlorination effect is negligible.
4. pH--Hydroponic solutions typically recommend low pH (4.5-6) because more nutrients are available in solution. As pH rises, so does the likelyhood that nutrients like phosphorus, iron, and calcium will precipitate from solution. Competition between algae and duckweed was discussed earlier. One of the advantages algae has over duckweed is the ability to tolerate higher pH. Algae removes carbonate alkalinity, respires oxygen, and add hydroxyl alkalinity to the water, thus rising pH values. It is not uncommon for algae rich waters to have pH value ca. 9.3 or higher. Prior to installing pH control (via solenoid, controller, and CO2 gas similar to the system described by Bugbee) the pH in the 100L laboratory reactors reached as high 11 (8.3-9.6 typical). On 11 May 2010, the pH at the Wellsville City Municipal Sewage Lagoons (WMSL) ranged from 7.37 near the first pond (influent) to 8.09 in the fourth/final pond (effluent). As pH increases, an interesting phenomenon between duckweed species occurred in the laboratory. At the WMSL the dominating duckweed species is L. minor (possibly L. turionifera or both) and the less abundant species is W. borealis. However, once brought into the lab and grown indoors on increasingly high pH water then the balance between the two species shifted so that W. borealis became almost 100% dominant. In our case, pH most likely produced the species shift, but harvesting methods also lead to dominating Wolffia cultures since they are easily suspended in the water column and small enough evade harvesting--leading to its abundance and the removal of other larger species (Culley, 1981).
References:
Suren, A.M., "Histological changes in macrophyte tissue during decomposition," Aquatic Botany, 33 (1989) 27-40.
Rejmankova, E., "Dynamics of fungal infection in duckweeds (Lemnacea)", Veroff.Geobot.Inst.ETH, Zurich, 87 (1986), pp 178-189.
Landolt, E. (1986): The family of lemnaceae-a monographic study. Vol. 1 of the monograph: Morphology; karyology; ecology;geographic distribution; systematic position; nomenclature; descriptions. Published in the "Veroffentilichungen des Geobotanischen Institutes ETC, Stiftung Rubel, Zurich." This is also listed as vol. 2 (No. 71) of publications on "Biosystematic investigations in the family of duckweeds Lemnaceae."
Szabo, S., et al., "Influences of nine algal species isolated from duckweed-covered sewage miniponds on Lemna gibba L.," Aquatic Botany 60 (1998) pp189-195.
Szabo, S., et al., "A simple method for analysing the effects of algae on the growth of Lemna and preventing algal growth in duckweed bioassays," Arch. Hydrobiol. 157 (4) pp.567-575 (2003).
Szabo, S., et al., "The strength of limiting factors for duckweed during algal competition," Arch. Hydrobiol. 164 (1) pp. 127-140 (2005).
Roijackers, R., et al., "Experimental analysis of the competition between algae and duckweed," Arch. Hydrobiol. 160 (3) pp. 401-412 (2004).
Smart, R.M., et al., "Laboratory culture of submersed freshwater macrophytes on natural sediments," Aquatic Botany, 21 (1985) pp251-263.
Naghavi, B., et al., "Algae removal by fine sand/silt filtration," Wat. Res. (1986) Vol 20, No. 3, pp. 377-383.
Edwards, P., et al., "Cultivation of Duckweeds in septage-loaded earthen ponds," Bioresource Technology, 40, pp.109-117 (1992).
Watkins, C., "The toxicity of chlorine to a common vascular aquatic plant," Water Res. 8 (1984) pp. 1037-1043.
Culley, D.D., et al., "Production, chemical quality, and use of duckweed (Lemnaceae) in aquaculture, waste management, and animal feeds," Journal of the World Mariculture Society, Vol. 12 (2), pp.27-49 (1981).
1. Fungi--probably pythium fungi, cause plant tissues to deteriorate consequently converting duckweed into a meal for micro-organisms (Suren, 1989). Pythium blight starts small and spreads until it eventually infects all the duckweed. (Rejmankova, 1986; website http://www.mobot.org/jwcross/duckweed/duckweed-pests.html)
Methods to prevent and/or eliminate fungi:
a) Fungicide. Apply fungicide like Ridomil Gold EC at a rate of 0.3uL/L-nutrient solution. To do this, you can make up "Subdue" solution (0.3ml-Ridomil GoldEC/L) and then apply "Subdue" solution at a rate of 1mL-"Subdue"/L-nutrient solution. I only had to apply this fungicide to my 100L reactor once to get rid of all the fungi. I had no problem with the fungi until I began to experiment with lower duckweed densities (below 30g(dry)/m^2) and low nutrient solutions (<1ppm P and N). After the fungi reappeared following the latter growth conditions, I accidentally applied Ridomil GoldEC at a rate of 1mL/L-soln. Oops! I came back the next day to find a room that smelled like paint thinner and a reactor void of duckweed (all of it sunk to the bottom of the reactor).
b) Silicon. Add silicon to nutrient solution which might have the effect of "toughening" cell tissue to resist fungi and disease. The USU Crop Physiology Laboratory has noticed that their hydroponic solutions containing silicon are more resistant to disease. They have a recipe to produce 2KOH + SiO2 --> K2SiO3 + H2O. The instructions to make this are as follows: First, dissolved 44.9g KOH in ~3.5L distilled water; Second, stir until clear (~15 min.); Third, add 24 g SiO2 (fumed silica--before to use a fume hood and/or filter mask); Fourth, stir until clear (~4-8 hrs.); and Fifth, bring volume to 4 L. Note: they have also noticed disease resistance by simply adding chunks of potassium silicate glass to the sediment to supply the silicon.
Figure 1: Duckweed fronds with early onset of fungi infestation.
Figure 2: Duckweed fronds 3-7 days after Fig. 1.
Figure 3: Duckweed fronds completely infected by fungi and being decomposed by micro-organisms.c) Temperature. Regarding fungi with duckweed and temperature effect, Elias Landolt wrote:
8. Fungi. The hypochytridiomycetes Reessia amoeboides and Reessia lemnae live endobiotically in dying Lemnacea, accoding to Wagner (1969) and Kandeler (1979). Colbaugh (1981) reports of a lethal foliar blight of Lemnaceae in water cultures, which is caused by the oomycete Pythium aphanidermatus. The reduction occurs due to foliar blight and dying of the fronds. Greatest foliar blighting activity occurs at temperatures of 24'C and 27'C (better than at 18'C, 21'C, and 30'C). Rejmankova et al. (1986) isolated Pythium myriophyllum from L. gibba growing in a dairy farm of Louisiana. The authors were able to show that this gungus is the cause of duckweed kills. Under natural conditions and temperatures above 22'C the amount of duckweeds killed by the gungus grows exponentially and the whole stand dies within several days. Six species of lemnaceae have been tested in the laboratory: L. gibba, L. minor, and S. polyrrhiza proved to be most susceptible to the fungla infection. L. valdiviana showed more resistance whereas L. aequinoctialis and S. punctata never exhibited symptoms of desease. Optimum temperature for infection was about 32'C. It is interesting to note that the susceptibility to a fungal disease might be a factor limiting the distribution of certain Lemnaceae species. Rhizoctonia solani is able to infect L. minor, but the plants only get small irregular lesions (Joyner and Freeman 1973). A smut, tracya lemnae, is known from Spirodela (Fisher 1953, Zogg 1985).2. Algae--when duckweed densities are low, then algae grow by the light that otherwise would be absorbed by floating duckweed. Algae grows in the water column and the surface. It can attach to duckweed tissue. I've personally observed that air bubble form underneath duckweed in solution with algae--cutting off its interface directly with the water column. Elias Landolt said:
Citer from (Landolt, 1986, pp.194-195)
Algae are most competitive with Lemnaceae in nutrient-rich waters. Filiform algae, which form dense mats on the surface of the water (e.g. Spirogyra) especially can prevent Lemnaceae from spreading successfully. Very often, the algae cover is raised by development of gas, thus breaking the contact of the Lemnaceae with the water and causing the drying of fronds.Further reading: (Szabo, 1998/2003/2005; Roijackers, 2004; Smart, 1985)
Cited from (Landolt, 1986, p.203)
Ways to reduce/eliminate the growth of algae with duckweed include:
a) rinse duckweed in 0.05% sodium hypochlorite soln. (aka. bleach)--this is comparable to chemotherapy for duckweed plants and it's a matter of "survival of the fittest." You only need to rinse for 5-30 seconds. I tried this procedure once and rinsed fronds for 30 seconds which destroyed virtually all of the fronds (and algae).
b) as part of "a" growing cultures in the lab should be as aseptic as possible (i.e. autoclave or filter nutrient solution, laminar flow hood, autoclaved glassware, etc.). This is difficult for me to do which is probably why I seem to always end up with algae showing up.
c) foil or dark material to cover all but the surface of the growing vessel reducing the light available for algae growth beneath the water suface.
d) maintain a crop density of at least 20-30g(dry)/m^2; anything lower than this allows too much light to pass through the duckweed cover.
e) filter using sand and/or diatomaceous earth (Naghavi, 1986)
f) periodic spraying ponds with algicide copper sulphate at a concentration of 2mg/L; spray at noon when temperatures and algae concentrations are high. This algicide procedure was used to remove filamentous alga Oedognium (Edwards, 1992).
Figure 4: Algae attaching to pythium fungi-infected duckweed.
Figure 5: Healthy duckweed fronds and roots without algae, for bioassay
Figure 6: Signs of algae infestation (progressively worse L to R).
Figure 7 (top-bottom pairs): Logan City wastewater w/o and w/ chlorine;Wellsville City wastewater; Hydrosol nutrient solution.
Figure 8: Bioassays (APHA Std. Method 8211) by frond count. Logan w/ and w/o chlorine.
Figure 9: Bioassay method 8211 Hydrosol and Wellsville City ww solutions.
Figure 10: Bioassay method 8211; 3-5 days (typ. 96 hrs.); Wellville City and Logan City w/Cl ww.
Figure 11: Duckweed plants following bioassay;removed one-by-one for frond/colony counting.
Figure 12: More duckweed plants following bioassay;removed one-by-one for frond/colony counting.
Figure 13: Bioassay results comparing effect of nutrient solutions and chlorination on duckweed growth. It appears that nutrient solution has a larger effect on duckweed growth than whether or not it has been chlorinated (see below).
Figure 14: Boxplots showing effect of nutrient solution and chlorination on duckweed growth. Results show that duckweed growth slows over time (i.e. harvest more frequently) and chlorination effect is negligible.
References:
Suren, A.M., "Histological changes in macrophyte tissue during decomposition," Aquatic Botany, 33 (1989) 27-40.
Rejmankova, E., "Dynamics of fungal infection in duckweeds (Lemnacea)", Veroff.Geobot.Inst.ETH, Zurich, 87 (1986), pp 178-189.
Landolt, E. (1986): The family of lemnaceae-a monographic study. Vol. 1 of the monograph: Morphology; karyology; ecology;geographic distribution; systematic position; nomenclature; descriptions. Published in the "Veroffentilichungen des Geobotanischen Institutes ETC, Stiftung Rubel, Zurich." This is also listed as vol. 2 (No. 71) of publications on "Biosystematic investigations in the family of duckweeds Lemnaceae."
Szabo, S., et al., "Influences of nine algal species isolated from duckweed-covered sewage miniponds on Lemna gibba L.," Aquatic Botany 60 (1998) pp189-195.
Szabo, S., et al., "A simple method for analysing the effects of algae on the growth of Lemna and preventing algal growth in duckweed bioassays," Arch. Hydrobiol. 157 (4) pp.567-575 (2003).
Szabo, S., et al., "The strength of limiting factors for duckweed during algal competition," Arch. Hydrobiol. 164 (1) pp. 127-140 (2005).
Roijackers, R., et al., "Experimental analysis of the competition between algae and duckweed," Arch. Hydrobiol. 160 (3) pp. 401-412 (2004).
Smart, R.M., et al., "Laboratory culture of submersed freshwater macrophytes on natural sediments," Aquatic Botany, 21 (1985) pp251-263.
Naghavi, B., et al., "Algae removal by fine sand/silt filtration," Wat. Res. (1986) Vol 20, No. 3, pp. 377-383.
Edwards, P., et al., "Cultivation of Duckweeds in septage-loaded earthen ponds," Bioresource Technology, 40, pp.109-117 (1992).
Watkins, C., "The toxicity of chlorine to a common vascular aquatic plant," Water Res. 8 (1984) pp. 1037-1043.
Culley, D.D., et al., "Production, chemical quality, and use of duckweed (Lemnaceae) in aquaculture, waste management, and animal feeds," Journal of the World Mariculture Society, Vol. 12 (2), pp.27-49 (1981).
January 14, 2011
Duckweed Performance in Cold Climates
Duckweed grows approximately 6+ months in Cache Valley Utah. This presents an issue when using duckweed for removing nutrients from the wastewater. I've observed duckweed as early as March and as late as the end of November; however, I've only noticed full duckweed coverage from about the first of May to November 19th (World Toilet Day). Fortunately, Wellsville currently has the capacity to store water (i.e. save for treatment in the warm season) during the winter, which they do. Also, their discharge permit allows for 432 kg-P/yr (72kg during the warm season and 360kg cold season). Wellsville discharges into the Little Bear River. This excellent report by JUB Engineering provides a summary of the treatment plant where we propose using duckweed nutrient removal. I've met a few engineers from JUB and would highly recommend them.
Video: Water Cress growing green in Cold Cache Valley Climate (25sec)
Figure: Temperature Trend two weeks preceding video on 15 Jan 2011--Cache Valley Winter
Figure: Temperature Trend before video on 15 Jan 2011 Cache Valley Winter
Most research with duckweed is conducted in climates with a 9+month growing season. Laboratory tests are usually performed at 25'C. A difficult to find article, but frequently referenced, by Culley, D.D. Jr., et al., has this to say about winter and duckweed:
"Several species rarely flower and form no turions. During the winter season the fronds are greatly reduced but remain at the surface. Occasionaly, turion-like fronds will form, but the plants continue to slowly reproduce vegatatively. These plants are probably the best plants to utilize in a culture system as restocking is virtually assured. Lemna gibba, L. valdiviana, L. minor, L. trisulca and L. minuscula are five such plants that frequently show some growth in the cool season. In some cases, L. gibba (Culley 1978) also shows rapid growth under summer conditions, making it a candidate for continuous culture. Lemna minor, S. intermedia and S. biperforata also do not form turions and rarely flower. L. minor may be a candidate for continuous culture, but the latter two are more suited for culture under warm condition. L. trisulca is a delicate, submerged form and thus will be more difficult to culture and harvest.
"At present, mixed cultures appear preferable to monoculture to insure the best yield on a yearly basis. Maintaing mixed cultures can present management problems, due to competition for space, variation in growth rates, and harvesting techniques that favor removal of certain species. For example, Wolffia species are easily suspended in the water column when harvesting in a manner that disturbs the water. Over time, the larger and more buoyant species are removed, leaving an increasing biomass of Wolffia for expansion. L. gibba, a very buoyant plant, will rise above the more flattened fronds of, for example, Spirodela polyrrhiza. Unless the plants are carefully harvested to prevent crowding the culture will gradually be dominated by L. gibba."
-Culley
Full-scale operations:
Mr. Dudley D. Culley has researched full-scale duckweed systems; however, the majority of research and duckweed work is still limited to lab and small pilot-scale studies. I am aware of only a few full-scale operations with duckweed plants; and of those, I believe only one is still operable. Duckweed grows on many wastewater lagoons, but it's rare to find one periodically harvesting the duckweed for nutrient (or BOD, TSS, etc.) treatment and even rarer to find a system with a cold climate like Cache Valley, UT. Here is a small list of the full-scale operations I'm aware of:
Most Successful: Agriquatics Mirzapur System (Bangladesh): article, website, video
Closest to Utah: Boulder City, NV wastewater treatment Lemna Corporation project supervised by Don Donahue--11 acre facultative lagoon with duckweed for BOD reduction; abandoned after 10 years. Produced high amounts of duckweed biomass but due to inadequate solids handling became too burdensome too continue. Don mentioned in personal correspondence that the duckweed stopped growing around 10'C.
Problems: Biloxi, Miss. (terminated) and Paragould, Ark. (never successful due to algae/fungus issues) as discussed here.
Figure: Duckweed (L. minor and Wolffia) at Wellsville Sewage Lagoons 22 March 2010--duckweed appeared immediately along shore as ice melted.
Video: Water Cress growing green in Cold Cache Valley Climate (2.5min)
Video: Water Cress growing green in Cold Cache Valley Climate (25sec)
The figures below show temperature trends for Cache Valley, including the cold temperatures in January when that the water cress survives. These charts were retrieved online from a database/network ran by Jeff Horsburgh of the Utah Water Research Laboratory (UWRL). Jeff continuously monitors environmental data along the Little Bear River and makes it available to the public online. This is a fantastic research tool made possible by data logging equipment from Campbell Scientific, Inc. (a local Cache Valley company).
Figure: Temperature Trend two weeks preceding video on 15 Jan 2011--Cache Valley Winter
Figure: Temperature Trend before video on 15 Jan 2011 Cache Valley WinterMost research with duckweed is conducted in climates with a 9+month growing season. Laboratory tests are usually performed at 25'C. A difficult to find article, but frequently referenced, by Culley, D.D. Jr., et al., has this to say about winter and duckweed:
"Several species rarely flower and form no turions. During the winter season the fronds are greatly reduced but remain at the surface. Occasionaly, turion-like fronds will form, but the plants continue to slowly reproduce vegatatively. These plants are probably the best plants to utilize in a culture system as restocking is virtually assured. Lemna gibba, L. valdiviana, L. minor, L. trisulca and L. minuscula are five such plants that frequently show some growth in the cool season. In some cases, L. gibba (Culley 1978) also shows rapid growth under summer conditions, making it a candidate for continuous culture. Lemna minor, S. intermedia and S. biperforata also do not form turions and rarely flower. L. minor may be a candidate for continuous culture, but the latter two are more suited for culture under warm condition. L. trisulca is a delicate, submerged form and thus will be more difficult to culture and harvest.
"At present, mixed cultures appear preferable to monoculture to insure the best yield on a yearly basis. Maintaing mixed cultures can present management problems, due to competition for space, variation in growth rates, and harvesting techniques that favor removal of certain species. For example, Wolffia species are easily suspended in the water column when harvesting in a manner that disturbs the water. Over time, the larger and more buoyant species are removed, leaving an increasing biomass of Wolffia for expansion. L. gibba, a very buoyant plant, will rise above the more flattened fronds of, for example, Spirodela polyrrhiza. Unless the plants are carefully harvested to prevent crowding the culture will gradually be dominated by L. gibba."
-Culley
Full-scale operations:
Mr. Dudley D. Culley has researched full-scale duckweed systems; however, the majority of research and duckweed work is still limited to lab and small pilot-scale studies. I am aware of only a few full-scale operations with duckweed plants; and of those, I believe only one is still operable. Duckweed grows on many wastewater lagoons, but it's rare to find one periodically harvesting the duckweed for nutrient (or BOD, TSS, etc.) treatment and even rarer to find a system with a cold climate like Cache Valley, UT. Here is a small list of the full-scale operations I'm aware of:
Most Successful: Agriquatics Mirzapur System (Bangladesh): article, website, video
Closest to Utah: Boulder City, NV wastewater treatment Lemna Corporation project supervised by Don Donahue--11 acre facultative lagoon with duckweed for BOD reduction; abandoned after 10 years. Produced high amounts of duckweed biomass but due to inadequate solids handling became too burdensome too continue. Don mentioned in personal correspondence that the duckweed stopped growing around 10'C.
Problems: Biloxi, Miss. (terminated) and Paragould, Ark. (never successful due to algae/fungus issues) as discussed here.
January 13, 2011
Investigating Duckweed for Phytoremediation and as a Toxicity Indicator of Chemicals
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Title: Investigating Duckweed for Phytoremediation and as a
Toxicity Indicator of Chemicals
Created: May 2010
Author: Jon Farrell
Project: USU Environmental Toxicology 6270
Abstract:
Duckweed is an aquatic plant that can hyperaccumulate various nutrients and toxic metals, and can metabolize several organic chemicals. Duckweed is uniquely suited for phytoremediation of polluted surface water and for toxicity testing. Polyaromatic hydrocarbons (PAHs) are particularly toxic to duckweed and other plants due to metabolites, formed by UV photo-degradation of the parent compound, which ultimately lead to lower photosynthetic rates. On the other hand, plants like duckweed are useful from an environmental standpoint because they contribute to 45% of PAH degradation. This report reviews several studies that investigated the ability of plants like duckweed to uptake specific chemicals of concern. It also investigated the toxic effect certain chemicals like PAHs, As, Se, Cd, Cu, and chlorophenols had on duckweed—which in some instances like Cd, showed how toxic chemicals bioaccumulate through the food chain to humans. These toxic chemicals caused phytotoxicity via CYP-450 oxidation, free radicals, DNA damage, and cell membrane damage.
Background:
Duckweed has been researched extensively for more than 40 years (Clatworthy et al., 1960; Culley et al., 1981) as an ecological way of remediating polluted water and as a toxicity indicator. Researchers have investigated its ability to remove metals, inorganic nutrients, and more recently, organic chemicals including pharmaceuticals. Regarding phytoremediation, it has been noted that the best plants, including duckweed, hyperaccumulate the chemical(s) of concern, grow quickly, and harvest easily. Duckweed hyperaccumulates several toxic metals including Cd, Cu, and Se (Lone et al., 2008); has a high relative growth rate (RGR) of 0.06 up to 0.31 (Chaipraprat et al., 2005); and is a native species throughout the United States and the world (see Fig. 1), and simplifies harvesting due to the fact that it floats on water. These same characteristics make it ideally suited for toxicity testing because it uptakes many chemicals, grows on water including wastewater, and is adaptable to many environments. Both the American Public Health Association (APHA) and Environment Canada have official testing methods for duckweed toxicity testing.
Retrieved from USDA Plants Database
Figure 1: Distribution of Lemna minor (a.k.a. duckweed)
The majority of research concentrates on using duckweed for phytoremediation. A handful of studies have concentrated on using duckweed merely as toxicity indicators of aquatic environments (Tront et al., 2007; Saygideger et al., 2004) and a few of these studies have used duckweed to identify sentinel species. Sentinel species identify hazards to human health, similar to a canary in a coal mine. In a study done in Louisiana, Cd-enriched duckweed was fed to crayfish and the effect of bioaccumulation was seen as acetylcholinesterase activity diminished (Devi et al., 1996). The crayfish were labeled as sentinel species since they are only one step in the food chain away from humans. Duckweed not only shows promise for remediating polluted water, it also has proved itself in identifying harmful chemicals—some of which may end up on a dinner plate. In a very recent report from the President’s Cancer Panel, it was pointed out that 41% of Americans will be diagnosed with cancer and 21% will die from it. The United States was criticized of having a “reactionary rather than precautionary” regulatory approach. And the report pointed out that “only a few hundred of the more than 80,000 chemicals in use in the United States have been tested for safety” (USDHHS). While duckweed does not always uptake the same chemicals as humans nor experience the same toxic effects, at times it does behave similarly and also provides a relatively quick indication of hazards from certain chemicals like pharmaceuticals (Brain, 2004) and Polyaromatic Hydrocarbons (Kummerova et al., 2007).
While duckweed has been employed in full-scale remediation projects (Alaerts et al., 1996; Donahue, 2009), the majority of research has been conducted in laboratory settings and shows the effect of specific chemicals on the plant rather than entire ecological system (Bonairdi, Linton et al., 1998). Some compelling full-scale projects might include studies in North Carolina investigating duckweed as a means of cleaning up swine wastewater, specifically to remove nitrogen and phosphorus that would otherwise cause eutrophication and river impairment downstream from where lagoons discharge into rivers (Chaipraprat et al., 2005). In additional to removing inorganic nutrients, duckweed has also been shown to remove uranium and arsenic from mine drainage in Saxon, Germany (Makandawire 1 & 2) and hard to remove selenium (Ornes et al., 1991). There are several physiological characteristics that make duckweed suitable to remediation and toxicity testing.
Plant Physiology:
Duckweed is the common name for the Lemnaceae family of plants, with species like Lemna minor, Lemna Gibba, and Spirodela Polyrhizza. Duckweed is one of the smallest macrophytes, a monocot, angiosperm, and C-3 plant—which allows it to grow in colder climates with only 5-7 month growing seasons. Since water freezes in the winter and duckweed grows on water, it does best in warmer climates. Duckweed can produce seeds called turions, but typically reproduces asexually by growing more daughter fronds which eventually separate into their own colonies of 2-4 fronds.
Phytoremediation began as an interest to understand how nutrient metals were physiologically taken up by plants. These early studies looked into what factors made nutrients available to the plant from the soil. Phytoremediation as it’s known today, is concerned with using plants to remove harmful chemicals from the environment. Phytoremediation typically involves large fast growing plants rooted in soil that hyperaccumulate chemicals of concern. These plants include willows and poplars (Yifru et al., 2006); however, duckweed and other floating plants are uniquely able to remediate large water bodies without soil. Abiotic factors like pH and redox potential are the main drivers of metal availability in plants, but uptake mechanism are not solely limited to pH and ORP. Plant uptake of solutes and water depend on mechanisms such as osmotic potential, vapor potential, enzymes, microbes, and the physical nature of the chemical and the plant.
Osmotic potential is important to understanding plant-chemical relationships. Osmotic potential is the pressure gradient between the soil and plant, and is governed by the equation:
Where T=Total osmotic potential (approx. -2 to -1 Mpa); S=Solute potential (directly proportional to concentration and ionization of salts); P=Turdor potential (positive pressure as plant cells fill with water); M=Matrix potential (affinity of soils for water); and Z=pressure due to gravity (negligible)
Soil and healthy plants have a negative pressure. Depending on which pressure is greater drives where the water will go. Soil pressure is driven primarily by S + M while plant pressure is driven by S + P. If the osmotic pressure is greater in the substrate (i.e. soil or water) due to concentrated salts then the plant will not be successful at taking up chemicals.
Vapor pressure is essential to plant physiology and phytoremediation because it decreases P. as water transpires from the plant, thus drawing more water into the plant through the roots. Vapor pressure also governs whether certain chemicals, like trichloroethylene will volatilize through plant leaves (Orchard et al., 2000).
pH and redox potential effect the protonation of chemical compounds which in turn determines whether it is in a bioavailable form (Tront et al., 2007). pH and redox can be influenced by microbes near plant surfaces that hydrolyze chemicals (Makandawire). Enzymes enable certain chemicals (i.e. PO4-P) to be actively transported into and utilized by the plant functional groups.
The physical characteristics of chemicals and plants determine the type of chemicals taken up by plants and kinetic rate at which they’re taken up. Physical characteristics of the chemical include the octanol/water coefficient (Kow), molecular charge, and size. Typically, plants uptake organic chemicals with moderate to low hydrophobic Kow’s ranging from 0.5-3.5 into their tissue (i.e. lipid compounds); while on the other hand, aqueous chemicals (i.e. Kow<0.0) are easily transferred through plants via the xylem (Kim et al., 2004) or in some cases not taken up at all if the pKa is too low (Boutonnet). The uptake of hydrophobic to hydrophilic compounds by the plant via different mechanisms is shown via the root’s anatomy.
The plant’s roots can be visualized as circular layers of cells, beginning from the outermost layer: epidermis, cortex, endodermis, pericycle, and xylem. Different transport mechanisms exist to transfer chemicals through the different layers. According to Taiz, “Mineral nutrients absorbed by the root are carried to the shoot by the transpiration stream moving through the xylem” (Taiz et al., 1998) via the apoplast. Hydrophobic compounds don’t move through the apoplast; rather, they move through a network of interconnected lipid-cells known as the symplast (Taiz et al., 1998). According to Kim, non-aqueous chemicals (i.e. Kow>0) require symplastic movement through inner cells, aqueous chemicals require apoplastic movement through cell walls, and inorganic nutrients require “specific carrier- and channel-proteins” (Kim et al, 2004). Once a chemical is taken up by a plant it can be stored, metabolized (i.e. assimiliated), mineralized, or volatilized.
Physiological characteristics of plants (i.e. floating vs. rooted) make specific species more adept at removing specific chemicals. Individual laboratory tests frequently concentrate on one specie’s ability to uptake one type of chemical. In reality, ecological systems can contain multiple chemicals of concern. Some studies have looked into using multiple types of plants, each one with a particular ability to remove a specific chemical (Ornes et al. 1991). Other studies have observed competition between species that promote or inhibit multiple species (Clatworthy et al., 1960; Edwards et al., 1992, Wang et al., 2002).
Materials and Methods/Indicators of Phytotoxicity:
The majority of these studies investigating the use of duckweed and other plants for chemical uptake and toxicity were performed in the laboratory. One advantage of lab studies is that they “generate information about the fate of organic chemicals prior to field-scale tests because laboratory tests are less expensive, easier to control, and better enable investigators to elucidate fate mechanisms” (Kim et al., 2004). Each study regarding duckweed has a slightly different focus, ranging from finding uptake rates to discovering mechanisms of action. Due to a variety of focuses, each study also performed different tests (i.e. substrate solutions) and analyzed the results differently (i.e. chemical concentration vs. EC50). To begin with, a decision must be made as to what solution should be used to grow the plants, and whether or not the plants will be grown in large-several liter reactors, small shake flasks, or petri dishes. Standard methods exist for carrying out toxicity tests, but even these vary depending on what chemicals are being tested. For example, one test might require chelating agents in the nutrient solutions that in turn might make certain chemicals unavailable to the plant (Saygidegar). Nutrient solutions range from Hoagland, to Huntner, to industrial flue gas water (Sundberg et al., 2006), to wastewater.
The physical environment is often measured for light intensity, temperature, pH, ORP, and DO (Nzengung et al., 2004). These variables were used by Nzengung to demonstrate that uptake of perchlorate increased in aerobic conditions and enriched nitrate conditions; however, rhizodegradation decreased-and rhizodegradation is often preferred over plant uptake since it destroys perchlorate. In another study, pH measurements and comparison between pKa and percent protonated species revealed that chemical uptake utilized both abiotic (i.e. pH) and biotic (enzymatic transformation) mechanisms (Tront et al., 2007).
Enzymatic activity within plants were measured frequently to indicate toxicity. Activation of antioxidative enzymes were associated with free radical formation and subsequent physiological damage (Wang et al., 2004). Whereas, deactivation of acetylcholinesterase enzymes were associated with phytotoxicity due to Cd hyperaccumulation (Devi et al., 1996).
Mass balances showed the fate and motility of chemicals. The majority of studies reported the Bio Concentration Factor (BCF) which is the ratio of the amount of chemical in the plant tissue compared to the amount in the substrate solution. High BCF (typ. >1000) represent hyperaccumulation (Odjegba et al., 2004). A number of studies used C14 labeled chemicals to show the ratio of chemical in roots, to shoots, to the unaccounted chemical volatilized or degraded (Botcher).
Various extraction solutions provided similar data as C14 labeled compounds and were used to identify the chemicals connected to the roots and those in the shoots (Vadas et al., 2007), identified by the Root Concentration Factor (RCF). Similarly, extraction can be used to determine the amount of chemical adsorbed vs. absorbed/assimilated in plants. Comparable to BCF, the Transpiration Stream Concentration Factor (TSCF) shows the ratio of chemical in the transpiration stream to that in the external solute (Gomez-Hermosillo et al., 2006).
A study by Utah State University (USU) recognized the importance of carefully measuring and controlling the air to determine the fraction of chemical that was volatilized by plants. Trichloroethylene can be volatilized by fruit trees and poplar trees. USU created growth chambers specifically designed to keep track of CO2in/out, O2in/out, and also utilized CO2 traps to keep track of the fraction of mineralized chemicals (Orchard et al., 2000).
Phytotoxic effects were determined based on mass and size decrease and compared to chemical concentrations to determine toxic doses. Mass and size phytotoxic effects of chemicals were measured by relative growth rates (RGR) and Leaf Area Ratio (LAR). The toxic doses were usually reported as EC50 and LD50.
Phytotoxic effects to plants were measured indirectly in relation to photosynthesis. A decline in a plants ability to photosynthesize correlated with chlorophyll pigment reduction, which in turn was an indicator of more phytotoxic damage downstream. Several studies used non-destructive chlorophyll fluorometers to measure photosynthesis rates as they related to PAH toxicity (Slaski et al., 2002; Kummerova et al., 2007; Kapustka et al., 2004).
Chemical concentrations in solution and plant tissue required a variety of instruments ranging from Nuclear Magnetic Resonance (NMR), Inductively Couple Plasma (ICP), Atomic Absorption Spectrophotometry (AAS), Gas Chromatography (GC), and High Pressure Liquid Chromatography (HPLC).
Mechanisms of Action:
The materials and methods were often used to predict the mechanism of action (MOA) driving chemical uptake or phytotoxicity. The effect of PAHs on plants was frequently studied and the MOA driving toxicity was explained on various levels. For example, some studies simply stated that PAHs affected the chlorophyll, others stated that the damage was done in the thylakoid membrane where electron transport reactions occur (Marwood et al., 2003), and others reported MOA on the protein level (i.e. CYP-450 oxidation) (Kummerova et al., 2007). Reports show that PAH occurs naturally and anthropogenically as a parent compound (Marwood et al., 2003), but becomes more toxic after the metabolites are formed (El-Alawi et al., 2002) which can be an order of magnitude more toxic (Kummerova et al., 2007). PAH could be absorbed in solution or from the air, the principal toxic by-product is “UV-mediated photo-modification and subsequent disruption of photosynthesis [on aquatic plants]” (Kapustka et al., 2004). “Plants [uptake], translocate, transform, and accumulate PAHs,” and are responsible for eliminated up to 45% of the PAHs from the environment (Kummerova et al., 2007).
The MOA induced by Cu and Cd toxicity created free radicals that led to peroxidation of membrane lipids, which then led to loss of membrane integrity. “Plasma membrane permeability [could] result in leakage of potassium ions and other solutes and, finally, cause cell death” (Wang et al., 2004).
Uranium can enter inside of cells via mechanisms like: i) biomethylisation, which uptakes Ur via Methylobacterium spp. that symbiotically feed off of toxic plant by-products like CH4 (Taiz et al.); ii) assimilation, and iii) compartamentalization with specific enzymes (Makandawire et al., 2004, 2005).
Certain enzymes are only activated due to phytotoxicity and not physical cell damage. These enzymes include peroxidases (i.e. duckweed SpEx), superoxide dismutase, guaiacol peroxidase, and ascorbate peroxidase (Jansen et al., 2004; Wang et al., 2004). These enzymes were activated—toxicity indicator—after exposure with 2,4,6-trichlorophenol (TCP) and Copper. Chlorophenols were also enzymatically turned into glucoconjugates and incorporated into vacuoles and cell walls (Day et al., 2004).
Several MOAs were identified in a study observing the effect of duckweed exposure to pharmaceuticals. The studies quantified toxicity with plant necrosis, chlorophyll pigment, and carotenoid. The studies showed that fluoroquinolone-, sulfonamide-, and tetracycline-type compounds had the most toxic effect on duckweed. Fluoroquinolone toxicity caused bleaching of new fronds due to “inhibiting the activity of DNA gyrases.” The MOA of sulfamethoxazole is that they’re “folate antoagonists [which block] the conversion of p-aminobenzoid acid to the coenzyme dihydrofolic acid.” And the chlortetracycline inhibited protein synthesis (Brain et al., 2004, 2006).
Chemicals Studied:
Duckweed is suitable for phytoremediation of polluted surface water. It has demonstrated an ability to hyperaccumulate N, P, Cu, Cd, and Zn. Additionally, it accumulates more toxic metals such as As, Ag, Al, Cr, Fe, Hg, Ni, Pb, Ur (Objegba, Wang et al., 2004; Mkandawire et al., 2005, Olguin et al., 2005).
Organic chemicals taken up by duckweed and other plants include: N-nitrosodimethylamine (NDMA), perchlorate, trichloroethylene (TCE), di-chlorophenol (4-Cl-2-FP), triphenyltin, naphthalene, creosote, 2,4,6-trichlorophenol (TCP), chlorobenzene, and phenanthrene, plus various pharmaceuticals which have been found in the environment.
Engineering Significance & Applications:
The plants in the studies for this review were used to measure chemical uptake and toxicity in the following applications: Treatment of wastewater, mine drainage water, flue gas water, and aquatic pollution in wetlands, rivers, and lakes. Referring back to the canary in the coal mine analogy, plants like duckweed have the potential to point out the hazards associated with certain chemicals (some of which might be harmful to human health), while also removing these chemicals from impaired waters.
References:
Alaerts, G. J., Mahbubar, M. R., Kelderman, P. (1996). Peformance analysis of a full-scale duckweed-covered sewage lagoon. Water Research. 30(4), 843-852.
Boniardi, N., R. Rota, et al. (1999). Effect of dissolved metals on the organic load removal efficiency of Lemna gibba. Water Research 33, 530-538.
Bottcher, T. and R. Schroll (2007). The fate of isoproturon in a freshwater microcosm with Lemna minor as a model organism. Chemosphere. 66(4), 684-689.
Boutonnet, J. C., P. Bingham, et al. (1999). Environmental risk assessment of trifluoroacetic acid. Human and Ecological Risk Assessment. 5(1), 59-124.
Brain, A. R., Johnson, D. J., Richards, S. M., Hanson, M. L., Sanderson, H., Lam, M. W., Young, C., Mabury, S. A., Sibley, P. K., Solomon, K. R. (2004). Microcosm evaluation of the effects of an eight pharmaceutical mixture to the aquatic macrophytes Lemna gibba and Myriophyllum sibiricum. Aquatic Toxicology. 70, 23-40.
Brain, R. A., Sanderson, H., Sibley, P. K., Solomon, K. R. (2006). Probabilistic ecological hazard assessment, Evaluating pharmaceutical effects on aquatic higher plants as an example.
Ecotoxicology and Environmental Safety. 64, 128-135.
Chaiprapat S., Cheng, J. J., Classen, J. J., Liehr, S. K. (2005). Role of internal nutrient storage in duckweed growth for swine wastewater treatment. Trnasaction of the ASAE. 48(6), 2247-2258.
Clatworthy, J. N., Harper, J. L. (1960). The comparative biology of closely related species living in the same area. Comparative Biology. 307-324.
Culley Jr., D. D., Rejmankova, E., Kvet, J., Frye, J. B. (1981) Production, chemical, quality, and use of duckweed (Lemnaceae) in aquaculture, waste management, and animal feeds. Journal of World Mariculture Society. 12(2), 27-49.
Day, J. A. and F. M. Saunders (2004). Glycosidation of chlorophenols by Lemna minor. Environmental Toxicology and Chemistry. 23(3), 613-620.
Devi, M., D. A. Thomas, et al. (1996). Accumulation and physiological and biochemical effects of cadmium in a simple aquatic food chain. Ecotoxicology and Environmental Safety. 33(1), 38-43.
Donahue, D. (2009) Personal Correspondence.
Edwards, P., Hassan, M. S., Chao, C. H., Pacharaprakiti, C. (1992). Cultivation of duckweeds in setage-loaded earthen ponds. Bioresource Technology. 40, 109-117.
El-Alawi, Y. S., X. D. Huang, et al. (2002). Quantitative structure-activity relationship for the photoinduced toxicity of polycyclic aromatic hydrocarbons to the luminescent bacteria Vibrio fischeri. Environmental Toxicology and Chemistry. 2110), 2225-2232.
Gomez-Hermosillo, C., Pardue J. H., et al. (2006). Wetland plant uptake of desorption-resistant organic compounds from sediments. Environmental Science & Technology 40, 3229-3236.
Jansen, M. A. K., L. M. Hill, et al. (2004). A novel stress-acclimation response in Spirodela punctata (Lemnaceae), 2,4,6-trichlorophenol triggers an increase in the level of an extracellular peroxidase, capable of the oxidative dechlorination of this xenobiotic pollutant. Plant Cell and Environment. 27(5), 603-613.
Kapustka, L. A. (2004). Establishing Eco-SSLs for PAHs, Lessons revealed from a review of literature on exposure and effects to terrestrial receptors. Human and Ecological Risk Assessment. 10(2), 185-205.
Kim, J., M. C. Drew, et al. (2004). Uptake and phytotoxicity of TNT in onion plant. Journal of Environmental Science and Health Part a-Toxic/Hazardous Substances & Environmental Engineering. 39(3), 803-819.
Kummerova, M., S. Zezulka, et al. (2007). Photoinduced toxicity of fluoranthene on primary processes of photosynthesis in lichens. Lichenologist. 39, 91-100.
Linton, S., Goulder, R. (1998). The duckweed Lemna minor compared with the alga Selenastrum capricornutum for bioassay of pond-water richness. Aquatic Botany. 60, 7-36.
Lone, M. I., He Z., Stoffella, P. J., Yang, X. (2008). Phytoremediation of heavy metal polluted soils and water: progresses and perspectives. University of Science B. 9, 210-220.
Marwood, C. A., K. T. J. Bestari, et al. (2003). Creosote toxicity to photosynthesis and plant growth in aquatic microcosms. Environmental Toxicology and Chemistry. 22(5), 1075-1085.
Mkandawire, M. and E. G. Dudel (2005). Accumulation of arsenic in Lemna gibba L. (duckweed) in tailing waters of two abandoned uranium mining sites in Saxony, Germany. Science of the Total Environment. 336(1-3), 81-89.
Mkandawire, M., B. Tauert, et al. (2004). Capacity of Lemna gibba L. (Duckweed) for uranium and arsenic phytoremediation in mine tailing waters. International Journal of Phytoremediation. 6(4), 347-362.
Nzengung, V. A., H. Penning, et al. (2004). Mechanistic changes during phytoremediation of perchlorate under different root-zone conditions. International Journal of Phytoremediation. 6(1), 63-83.
Odjegba, V. J. and I. O. Fasid (2004). Accumulation of trace elements by Pistia stratiotes, implications for phytoremediation. Ecotoxicology. 13(7), 637-646.
Olguin, E. J., G. Sanchez-Galvan, et al. (2005). Surface adsorption, intracellular accumulation and compartmentalization of Pb(II) in batch-operated lagoons with Salvinia minima as affected by environmental conditions, EDTA and nutrients. Journal of Industrial Microbiology & Biotechnology. 32(11-12), 577-586.
Orchard, B. J., W. J. Doucette, et al. (2000). A novel laboratory system for determining fate of volatile organic compounds in planted systems. Environmental Toxicology and Chemistry. 19(4), 888-894.
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Reinhold, D. M. and E. M. Saunders (2006). Phytoremediation of fluorinated agrochemicals by duckweed. Transactions of the Asabe. 49(6), 2077-2083.
Saygideger, S. and M. Dogan (2004). Lead and cadmium accumulation and toxicity in the presence of EDTA in Lemna minor L. and Ceratophyllum demersum L. Bulletin of Environmental Contamination and Toxicology. 73(1), 182-189.
Slaski, J. J., D. J. Archambault, et al. (2002). Physiological tests to measure impacts of gaseous polycyclic aromatic hydrocarbons (PAHs) on cultivated plants. Communications in Soil Science and Plant Analysis. 33(15-18), 3227-3239.
Sundberg, S. E., S. M. Hassan, et al. (2006). Enrichment of elements in detritus from a constructed wetland and consequent toxicity to Hyalella azteca. Ecotoxicology and Environmental Safety. 64(3), 264-272.
Taiz, E., Zeiger, E. (1998). Plant Physiology. 2nd ed. 147-152; 173-193.
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Title: Investigating Duckweed for Phytoremediation and as a
Toxicity Indicator of Chemicals
Created: May 2010
Author: Jon Farrell
Project: USU Environmental Toxicology 6270
Abstract:
Duckweed is an aquatic plant that can hyperaccumulate various nutrients and toxic metals, and can metabolize several organic chemicals. Duckweed is uniquely suited for phytoremediation of polluted surface water and for toxicity testing. Polyaromatic hydrocarbons (PAHs) are particularly toxic to duckweed and other plants due to metabolites, formed by UV photo-degradation of the parent compound, which ultimately lead to lower photosynthetic rates. On the other hand, plants like duckweed are useful from an environmental standpoint because they contribute to 45% of PAH degradation. This report reviews several studies that investigated the ability of plants like duckweed to uptake specific chemicals of concern. It also investigated the toxic effect certain chemicals like PAHs, As, Se, Cd, Cu, and chlorophenols had on duckweed—which in some instances like Cd, showed how toxic chemicals bioaccumulate through the food chain to humans. These toxic chemicals caused phytotoxicity via CYP-450 oxidation, free radicals, DNA damage, and cell membrane damage.
Background:
Duckweed has been researched extensively for more than 40 years (Clatworthy et al., 1960; Culley et al., 1981) as an ecological way of remediating polluted water and as a toxicity indicator. Researchers have investigated its ability to remove metals, inorganic nutrients, and more recently, organic chemicals including pharmaceuticals. Regarding phytoremediation, it has been noted that the best plants, including duckweed, hyperaccumulate the chemical(s) of concern, grow quickly, and harvest easily. Duckweed hyperaccumulates several toxic metals including Cd, Cu, and Se (Lone et al., 2008); has a high relative growth rate (RGR) of 0.06 up to 0.31 (Chaipraprat et al., 2005); and is a native species throughout the United States and the world (see Fig. 1), and simplifies harvesting due to the fact that it floats on water. These same characteristics make it ideally suited for toxicity testing because it uptakes many chemicals, grows on water including wastewater, and is adaptable to many environments. Both the American Public Health Association (APHA) and Environment Canada have official testing methods for duckweed toxicity testing.
Retrieved from USDA Plants Database
Figure 1: Distribution of Lemna minor (a.k.a. duckweed)
The majority of research concentrates on using duckweed for phytoremediation. A handful of studies have concentrated on using duckweed merely as toxicity indicators of aquatic environments (Tront et al., 2007; Saygideger et al., 2004) and a few of these studies have used duckweed to identify sentinel species. Sentinel species identify hazards to human health, similar to a canary in a coal mine. In a study done in Louisiana, Cd-enriched duckweed was fed to crayfish and the effect of bioaccumulation was seen as acetylcholinesterase activity diminished (Devi et al., 1996). The crayfish were labeled as sentinel species since they are only one step in the food chain away from humans. Duckweed not only shows promise for remediating polluted water, it also has proved itself in identifying harmful chemicals—some of which may end up on a dinner plate. In a very recent report from the President’s Cancer Panel, it was pointed out that 41% of Americans will be diagnosed with cancer and 21% will die from it. The United States was criticized of having a “reactionary rather than precautionary” regulatory approach. And the report pointed out that “only a few hundred of the more than 80,000 chemicals in use in the United States have been tested for safety” (USDHHS). While duckweed does not always uptake the same chemicals as humans nor experience the same toxic effects, at times it does behave similarly and also provides a relatively quick indication of hazards from certain chemicals like pharmaceuticals (Brain, 2004) and Polyaromatic Hydrocarbons (Kummerova et al., 2007).
While duckweed has been employed in full-scale remediation projects (Alaerts et al., 1996; Donahue, 2009), the majority of research has been conducted in laboratory settings and shows the effect of specific chemicals on the plant rather than entire ecological system (Bonairdi, Linton et al., 1998). Some compelling full-scale projects might include studies in North Carolina investigating duckweed as a means of cleaning up swine wastewater, specifically to remove nitrogen and phosphorus that would otherwise cause eutrophication and river impairment downstream from where lagoons discharge into rivers (Chaipraprat et al., 2005). In additional to removing inorganic nutrients, duckweed has also been shown to remove uranium and arsenic from mine drainage in Saxon, Germany (Makandawire 1 & 2) and hard to remove selenium (Ornes et al., 1991). There are several physiological characteristics that make duckweed suitable to remediation and toxicity testing.
Plant Physiology:
Duckweed is the common name for the Lemnaceae family of plants, with species like Lemna minor, Lemna Gibba, and Spirodela Polyrhizza. Duckweed is one of the smallest macrophytes, a monocot, angiosperm, and C-3 plant—which allows it to grow in colder climates with only 5-7 month growing seasons. Since water freezes in the winter and duckweed grows on water, it does best in warmer climates. Duckweed can produce seeds called turions, but typically reproduces asexually by growing more daughter fronds which eventually separate into their own colonies of 2-4 fronds.
Phytoremediation began as an interest to understand how nutrient metals were physiologically taken up by plants. These early studies looked into what factors made nutrients available to the plant from the soil. Phytoremediation as it’s known today, is concerned with using plants to remove harmful chemicals from the environment. Phytoremediation typically involves large fast growing plants rooted in soil that hyperaccumulate chemicals of concern. These plants include willows and poplars (Yifru et al., 2006); however, duckweed and other floating plants are uniquely able to remediate large water bodies without soil. Abiotic factors like pH and redox potential are the main drivers of metal availability in plants, but uptake mechanism are not solely limited to pH and ORP. Plant uptake of solutes and water depend on mechanisms such as osmotic potential, vapor potential, enzymes, microbes, and the physical nature of the chemical and the plant.
Osmotic potential is important to understanding plant-chemical relationships. Osmotic potential is the pressure gradient between the soil and plant, and is governed by the equation:
Where T=Total osmotic potential (approx. -2 to -1 Mpa); S=Solute potential (directly proportional to concentration and ionization of salts); P=Turdor potential (positive pressure as plant cells fill with water); M=Matrix potential (affinity of soils for water); and Z=pressure due to gravity (negligible)
Soil and healthy plants have a negative pressure. Depending on which pressure is greater drives where the water will go. Soil pressure is driven primarily by S + M while plant pressure is driven by S + P. If the osmotic pressure is greater in the substrate (i.e. soil or water) due to concentrated salts then the plant will not be successful at taking up chemicals.
Vapor pressure is essential to plant physiology and phytoremediation because it decreases P. as water transpires from the plant, thus drawing more water into the plant through the roots. Vapor pressure also governs whether certain chemicals, like trichloroethylene will volatilize through plant leaves (Orchard et al., 2000).
pH and redox potential effect the protonation of chemical compounds which in turn determines whether it is in a bioavailable form (Tront et al., 2007). pH and redox can be influenced by microbes near plant surfaces that hydrolyze chemicals (Makandawire). Enzymes enable certain chemicals (i.e. PO4-P) to be actively transported into and utilized by the plant functional groups.
The physical characteristics of chemicals and plants determine the type of chemicals taken up by plants and kinetic rate at which they’re taken up. Physical characteristics of the chemical include the octanol/water coefficient (Kow), molecular charge, and size. Typically, plants uptake organic chemicals with moderate to low hydrophobic Kow’s ranging from 0.5-3.5 into their tissue (i.e. lipid compounds); while on the other hand, aqueous chemicals (i.e. Kow<0.0) are easily transferred through plants via the xylem (Kim et al., 2004) or in some cases not taken up at all if the pKa is too low (Boutonnet). The uptake of hydrophobic to hydrophilic compounds by the plant via different mechanisms is shown via the root’s anatomy.
The plant’s roots can be visualized as circular layers of cells, beginning from the outermost layer: epidermis, cortex, endodermis, pericycle, and xylem. Different transport mechanisms exist to transfer chemicals through the different layers. According to Taiz, “Mineral nutrients absorbed by the root are carried to the shoot by the transpiration stream moving through the xylem” (Taiz et al., 1998) via the apoplast. Hydrophobic compounds don’t move through the apoplast; rather, they move through a network of interconnected lipid-cells known as the symplast (Taiz et al., 1998). According to Kim, non-aqueous chemicals (i.e. Kow>0) require symplastic movement through inner cells, aqueous chemicals require apoplastic movement through cell walls, and inorganic nutrients require “specific carrier- and channel-proteins” (Kim et al, 2004). Once a chemical is taken up by a plant it can be stored, metabolized (i.e. assimiliated), mineralized, or volatilized.
Physiological characteristics of plants (i.e. floating vs. rooted) make specific species more adept at removing specific chemicals. Individual laboratory tests frequently concentrate on one specie’s ability to uptake one type of chemical. In reality, ecological systems can contain multiple chemicals of concern. Some studies have looked into using multiple types of plants, each one with a particular ability to remove a specific chemical (Ornes et al. 1991). Other studies have observed competition between species that promote or inhibit multiple species (Clatworthy et al., 1960; Edwards et al., 1992, Wang et al., 2002).
Materials and Methods/Indicators of Phytotoxicity:
The majority of these studies investigating the use of duckweed and other plants for chemical uptake and toxicity were performed in the laboratory. One advantage of lab studies is that they “generate information about the fate of organic chemicals prior to field-scale tests because laboratory tests are less expensive, easier to control, and better enable investigators to elucidate fate mechanisms” (Kim et al., 2004). Each study regarding duckweed has a slightly different focus, ranging from finding uptake rates to discovering mechanisms of action. Due to a variety of focuses, each study also performed different tests (i.e. substrate solutions) and analyzed the results differently (i.e. chemical concentration vs. EC50). To begin with, a decision must be made as to what solution should be used to grow the plants, and whether or not the plants will be grown in large-several liter reactors, small shake flasks, or petri dishes. Standard methods exist for carrying out toxicity tests, but even these vary depending on what chemicals are being tested. For example, one test might require chelating agents in the nutrient solutions that in turn might make certain chemicals unavailable to the plant (Saygidegar). Nutrient solutions range from Hoagland, to Huntner, to industrial flue gas water (Sundberg et al., 2006), to wastewater.
The physical environment is often measured for light intensity, temperature, pH, ORP, and DO (Nzengung et al., 2004). These variables were used by Nzengung to demonstrate that uptake of perchlorate increased in aerobic conditions and enriched nitrate conditions; however, rhizodegradation decreased-and rhizodegradation is often preferred over plant uptake since it destroys perchlorate. In another study, pH measurements and comparison between pKa and percent protonated species revealed that chemical uptake utilized both abiotic (i.e. pH) and biotic (enzymatic transformation) mechanisms (Tront et al., 2007).
Enzymatic activity within plants were measured frequently to indicate toxicity. Activation of antioxidative enzymes were associated with free radical formation and subsequent physiological damage (Wang et al., 2004). Whereas, deactivation of acetylcholinesterase enzymes were associated with phytotoxicity due to Cd hyperaccumulation (Devi et al., 1996).
Mass balances showed the fate and motility of chemicals. The majority of studies reported the Bio Concentration Factor (BCF) which is the ratio of the amount of chemical in the plant tissue compared to the amount in the substrate solution. High BCF (typ. >1000) represent hyperaccumulation (Odjegba et al., 2004). A number of studies used C14 labeled chemicals to show the ratio of chemical in roots, to shoots, to the unaccounted chemical volatilized or degraded (Botcher).
Various extraction solutions provided similar data as C14 labeled compounds and were used to identify the chemicals connected to the roots and those in the shoots (Vadas et al., 2007), identified by the Root Concentration Factor (RCF). Similarly, extraction can be used to determine the amount of chemical adsorbed vs. absorbed/assimilated in plants. Comparable to BCF, the Transpiration Stream Concentration Factor (TSCF) shows the ratio of chemical in the transpiration stream to that in the external solute (Gomez-Hermosillo et al., 2006).
A study by Utah State University (USU) recognized the importance of carefully measuring and controlling the air to determine the fraction of chemical that was volatilized by plants. Trichloroethylene can be volatilized by fruit trees and poplar trees. USU created growth chambers specifically designed to keep track of CO2in/out, O2in/out, and also utilized CO2 traps to keep track of the fraction of mineralized chemicals (Orchard et al., 2000).
Phytotoxic effects were determined based on mass and size decrease and compared to chemical concentrations to determine toxic doses. Mass and size phytotoxic effects of chemicals were measured by relative growth rates (RGR) and Leaf Area Ratio (LAR). The toxic doses were usually reported as EC50 and LD50.
Phytotoxic effects to plants were measured indirectly in relation to photosynthesis. A decline in a plants ability to photosynthesize correlated with chlorophyll pigment reduction, which in turn was an indicator of more phytotoxic damage downstream. Several studies used non-destructive chlorophyll fluorometers to measure photosynthesis rates as they related to PAH toxicity (Slaski et al., 2002; Kummerova et al., 2007; Kapustka et al., 2004).
Chemical concentrations in solution and plant tissue required a variety of instruments ranging from Nuclear Magnetic Resonance (NMR), Inductively Couple Plasma (ICP), Atomic Absorption Spectrophotometry (AAS), Gas Chromatography (GC), and High Pressure Liquid Chromatography (HPLC).
Mechanisms of Action:
The materials and methods were often used to predict the mechanism of action (MOA) driving chemical uptake or phytotoxicity. The effect of PAHs on plants was frequently studied and the MOA driving toxicity was explained on various levels. For example, some studies simply stated that PAHs affected the chlorophyll, others stated that the damage was done in the thylakoid membrane where electron transport reactions occur (Marwood et al., 2003), and others reported MOA on the protein level (i.e. CYP-450 oxidation) (Kummerova et al., 2007). Reports show that PAH occurs naturally and anthropogenically as a parent compound (Marwood et al., 2003), but becomes more toxic after the metabolites are formed (El-Alawi et al., 2002) which can be an order of magnitude more toxic (Kummerova et al., 2007). PAH could be absorbed in solution or from the air, the principal toxic by-product is “UV-mediated photo-modification and subsequent disruption of photosynthesis [on aquatic plants]” (Kapustka et al., 2004). “Plants [uptake], translocate, transform, and accumulate PAHs,” and are responsible for eliminated up to 45% of the PAHs from the environment (Kummerova et al., 2007).
The MOA induced by Cu and Cd toxicity created free radicals that led to peroxidation of membrane lipids, which then led to loss of membrane integrity. “Plasma membrane permeability [could] result in leakage of potassium ions and other solutes and, finally, cause cell death” (Wang et al., 2004).
Uranium can enter inside of cells via mechanisms like: i) biomethylisation, which uptakes Ur via Methylobacterium spp. that symbiotically feed off of toxic plant by-products like CH4 (Taiz et al.); ii) assimilation, and iii) compartamentalization with specific enzymes (Makandawire et al., 2004, 2005).
Certain enzymes are only activated due to phytotoxicity and not physical cell damage. These enzymes include peroxidases (i.e. duckweed SpEx), superoxide dismutase, guaiacol peroxidase, and ascorbate peroxidase (Jansen et al., 2004; Wang et al., 2004). These enzymes were activated—toxicity indicator—after exposure with 2,4,6-trichlorophenol (TCP) and Copper. Chlorophenols were also enzymatically turned into glucoconjugates and incorporated into vacuoles and cell walls (Day et al., 2004).
Several MOAs were identified in a study observing the effect of duckweed exposure to pharmaceuticals. The studies quantified toxicity with plant necrosis, chlorophyll pigment, and carotenoid. The studies showed that fluoroquinolone-, sulfonamide-, and tetracycline-type compounds had the most toxic effect on duckweed. Fluoroquinolone toxicity caused bleaching of new fronds due to “inhibiting the activity of DNA gyrases.” The MOA of sulfamethoxazole is that they’re “folate antoagonists [which block] the conversion of p-aminobenzoid acid to the coenzyme dihydrofolic acid.” And the chlortetracycline inhibited protein synthesis (Brain et al., 2004, 2006).
Chemicals Studied:
Duckweed is suitable for phytoremediation of polluted surface water. It has demonstrated an ability to hyperaccumulate N, P, Cu, Cd, and Zn. Additionally, it accumulates more toxic metals such as As, Ag, Al, Cr, Fe, Hg, Ni, Pb, Ur (Objegba, Wang et al., 2004; Mkandawire et al., 2005, Olguin et al., 2005).
Organic chemicals taken up by duckweed and other plants include: N-nitrosodimethylamine (NDMA), perchlorate, trichloroethylene (TCE), di-chlorophenol (4-Cl-2-FP), triphenyltin, naphthalene, creosote, 2,4,6-trichlorophenol (TCP), chlorobenzene, and phenanthrene, plus various pharmaceuticals which have been found in the environment.
Engineering Significance & Applications:
The plants in the studies for this review were used to measure chemical uptake and toxicity in the following applications: Treatment of wastewater, mine drainage water, flue gas water, and aquatic pollution in wetlands, rivers, and lakes. Referring back to the canary in the coal mine analogy, plants like duckweed have the potential to point out the hazards associated with certain chemicals (some of which might be harmful to human health), while also removing these chemicals from impaired waters.
References:
Alaerts, G. J., Mahbubar, M. R., Kelderman, P. (1996). Peformance analysis of a full-scale duckweed-covered sewage lagoon. Water Research. 30(4), 843-852.
Boniardi, N., R. Rota, et al. (1999). Effect of dissolved metals on the organic load removal efficiency of Lemna gibba. Water Research 33, 530-538.
Bottcher, T. and R. Schroll (2007). The fate of isoproturon in a freshwater microcosm with Lemna minor as a model organism. Chemosphere. 66(4), 684-689.
Boutonnet, J. C., P. Bingham, et al. (1999). Environmental risk assessment of trifluoroacetic acid. Human and Ecological Risk Assessment. 5(1), 59-124.
Brain, A. R., Johnson, D. J., Richards, S. M., Hanson, M. L., Sanderson, H., Lam, M. W., Young, C., Mabury, S. A., Sibley, P. K., Solomon, K. R. (2004). Microcosm evaluation of the effects of an eight pharmaceutical mixture to the aquatic macrophytes Lemna gibba and Myriophyllum sibiricum. Aquatic Toxicology. 70, 23-40.
Brain, R. A., Sanderson, H., Sibley, P. K., Solomon, K. R. (2006). Probabilistic ecological hazard assessment, Evaluating pharmaceutical effects on aquatic higher plants as an example.
Ecotoxicology and Environmental Safety. 64, 128-135.
Chaiprapat S., Cheng, J. J., Classen, J. J., Liehr, S. K. (2005). Role of internal nutrient storage in duckweed growth for swine wastewater treatment. Trnasaction of the ASAE. 48(6), 2247-2258.
Clatworthy, J. N., Harper, J. L. (1960). The comparative biology of closely related species living in the same area. Comparative Biology. 307-324.
Culley Jr., D. D., Rejmankova, E., Kvet, J., Frye, J. B. (1981) Production, chemical, quality, and use of duckweed (Lemnaceae) in aquaculture, waste management, and animal feeds. Journal of World Mariculture Society. 12(2), 27-49.
Day, J. A. and F. M. Saunders (2004). Glycosidation of chlorophenols by Lemna minor. Environmental Toxicology and Chemistry. 23(3), 613-620.
Devi, M., D. A. Thomas, et al. (1996). Accumulation and physiological and biochemical effects of cadmium in a simple aquatic food chain. Ecotoxicology and Environmental Safety. 33(1), 38-43.
Donahue, D. (2009) Personal Correspondence.
Edwards, P., Hassan, M. S., Chao, C. H., Pacharaprakiti, C. (1992). Cultivation of duckweeds in setage-loaded earthen ponds. Bioresource Technology. 40, 109-117.
El-Alawi, Y. S., X. D. Huang, et al. (2002). Quantitative structure-activity relationship for the photoinduced toxicity of polycyclic aromatic hydrocarbons to the luminescent bacteria Vibrio fischeri. Environmental Toxicology and Chemistry. 2110), 2225-2232.
Gomez-Hermosillo, C., Pardue J. H., et al. (2006). Wetland plant uptake of desorption-resistant organic compounds from sediments. Environmental Science & Technology 40, 3229-3236.
Jansen, M. A. K., L. M. Hill, et al. (2004). A novel stress-acclimation response in Spirodela punctata (Lemnaceae), 2,4,6-trichlorophenol triggers an increase in the level of an extracellular peroxidase, capable of the oxidative dechlorination of this xenobiotic pollutant. Plant Cell and Environment. 27(5), 603-613.
Kapustka, L. A. (2004). Establishing Eco-SSLs for PAHs, Lessons revealed from a review of literature on exposure and effects to terrestrial receptors. Human and Ecological Risk Assessment. 10(2), 185-205.
Kim, J., M. C. Drew, et al. (2004). Uptake and phytotoxicity of TNT in onion plant. Journal of Environmental Science and Health Part a-Toxic/Hazardous Substances & Environmental Engineering. 39(3), 803-819.
Kummerova, M., S. Zezulka, et al. (2007). Photoinduced toxicity of fluoranthene on primary processes of photosynthesis in lichens. Lichenologist. 39, 91-100.
Linton, S., Goulder, R. (1998). The duckweed Lemna minor compared with the alga Selenastrum capricornutum for bioassay of pond-water richness. Aquatic Botany. 60, 7-36.
Lone, M. I., He Z., Stoffella, P. J., Yang, X. (2008). Phytoremediation of heavy metal polluted soils and water: progresses and perspectives. University of Science B. 9, 210-220.
Marwood, C. A., K. T. J. Bestari, et al. (2003). Creosote toxicity to photosynthesis and plant growth in aquatic microcosms. Environmental Toxicology and Chemistry. 22(5), 1075-1085.
Mkandawire, M. and E. G. Dudel (2005). Accumulation of arsenic in Lemna gibba L. (duckweed) in tailing waters of two abandoned uranium mining sites in Saxony, Germany. Science of the Total Environment. 336(1-3), 81-89.
Mkandawire, M., B. Tauert, et al. (2004). Capacity of Lemna gibba L. (Duckweed) for uranium and arsenic phytoremediation in mine tailing waters. International Journal of Phytoremediation. 6(4), 347-362.
Nzengung, V. A., H. Penning, et al. (2004). Mechanistic changes during phytoremediation of perchlorate under different root-zone conditions. International Journal of Phytoremediation. 6(1), 63-83.
Odjegba, V. J. and I. O. Fasid (2004). Accumulation of trace elements by Pistia stratiotes, implications for phytoremediation. Ecotoxicology. 13(7), 637-646.
Olguin, E. J., G. Sanchez-Galvan, et al. (2005). Surface adsorption, intracellular accumulation and compartmentalization of Pb(II) in batch-operated lagoons with Salvinia minima as affected by environmental conditions, EDTA and nutrients. Journal of Industrial Microbiology & Biotechnology. 32(11-12), 577-586.
Orchard, B. J., W. J. Doucette, et al. (2000). A novel laboratory system for determining fate of volatile organic compounds in planted systems. Environmental Toxicology and Chemistry. 19(4), 888-894.
Ornes, W. H., K. S. Sajwan, et al. (1991). Bioaccumulation of Selenium by Floating Aquatic Plants. Water Air and Soil Pollution. 57-8, 53-57.
Prasad, M. N. V. and H. M. D. Freitas (2003). Metal hyperaccumulation in plants - Biodiversity prospecting for phytoremediation technology. Electronic Journal of Biotechnology. 6(3), 285-321.
Reinhold, D. M. and E. M. Saunders (2006). Phytoremediation of fluorinated agrochemicals by duckweed. Transactions of the Asabe. 49(6), 2077-2083.
Saygideger, S. and M. Dogan (2004). Lead and cadmium accumulation and toxicity in the presence of EDTA in Lemna minor L. and Ceratophyllum demersum L. Bulletin of Environmental Contamination and Toxicology. 73(1), 182-189.
Slaski, J. J., D. J. Archambault, et al. (2002). Physiological tests to measure impacts of gaseous polycyclic aromatic hydrocarbons (PAHs) on cultivated plants. Communications in Soil Science and Plant Analysis. 33(15-18), 3227-3239.
Sundberg, S. E., S. M. Hassan, et al. (2006). Enrichment of elements in detritus from a constructed wetland and consequent toxicity to Hyalella azteca. Ecotoxicology and Environmental Safety. 64(3), 264-272.
Taiz, E., Zeiger, E. (1998). Plant Physiology. 2nd ed. 147-152; 173-193.
Tront, J. M. and F. M. Saunders (2006). Role of plant activity and contaminant speciation in aquatic plant assimilation of 2,4,5-trichlorophenol. Chemosphere. 64(3), 400-407.
Tront, J. M. and F. M. Saunders (2007). Sequestration of a fluorinated analog of 2,4-dichlorophenol and metabolic products by L-minor as evidenced by F-19 NMR. Environmental Pollution. 145(3), 708-714.
Vadas, T. M., X. Zhang, et al. (2007). Fate of DTPA, EDTA, and EDDS in hydroponic media and effects on plant mineral nutrition. Journal of Plant Nutrition. 30(7-9), 1229-1246.
Wang, H., X. Q. Shan, et al. (2004). Responses of antioxidative enzymes to accumulation of copper in a copper hyperaccumulator of Commoelina communis. Archives of Environmental Contamination and Toxicology. 47(2), 185-192.
Wang, Q., Y. Cui, et al. (2002). Phytoremediation of polluted waters potentials and prospects of wetland plants. Acta Biotechnologica. 22(1-2), 199-208.
Yifru, D. D. and V. A. Nzengung (2006). Uptake of N-nitrosodimethylamine (NDMA) from water by phreatophytes in the absence and presence of perchlorate as a co-contaminant. Environmental Science & Technology. 40(23), 7374-7380.
Literature Review re: duckweed for phytoremediation and toxicity indicator
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COMMON THEMES in PHYTOREMEDIATION re: MECHANISMS OF ACTION
Wang, H., X. Q. Shan, et al. (2004). "Responses of antioxidative enzymes to accumulation of copper in a copper hyperaccumulator of Commoelina communis." Archives of Environmental Contamination and Toxicology 47(2): 185-192.
McFarlane, C., C. Nolt, et al. (1987). "The Uptake, Distribution and Metabolism of Four Organic Chemicals by Soybean Plants and Barley Roots." Environmental Toxicology and Chemistry 6: 847-856.
Topp, E., I. Scheunert, et al. (1986). "Factors Affecting the Uptake of C-14-Labeled Organic-Chemicals by Plants from Soil." Ecotoxicology and Environmental Safety 11(2): 219-228.
Sundberg, S. E., S. M. Hassan, et al. (2006). "Enrichment of elements in detritus from a constructed wetland and consequent toxicity to Hyalella azteca." Ecotoxicology and Environmental Safety 64(3): 264-272.
Odjegba, V. J. and I. O. Fasid (2004). "Accumulation of trace elements by Pistia stratiotes: implications for phytoremediation." Ecotoxicology 13(7): 637-646.
Yifru, D. D. and V. A. Nzengung (2006). "Uptake of N-nitrosodimethylamine (NDMA) from water by phreatophytes in the absence and presence of perchlorate as a co-contaminant." Environmental Science & Technology 40(23): 7374-7380.
Vadas, T. M., X. Zhang, et al. (2007). "Fate of DTPA, EDTA, and EDDS in hydroponic media and effects on plant mineral nutrition." Journal of Plant Nutrition 30(7-9): 1229-1246.
Flocco, C. G., A. Lobalbo, et al. (2002). "Some physiological, microbial, and toxicological aspects of the removal of phenanthrene by hydroponic cultures of alfalfa (Medicago sativa L.)." International Journal of Phytoremediation 4(3): 169-186.
Kim, J., M. C. Drew, et al. (2004). "Uptake and phytotoxicity of TNT in onion plant." Journal of Environmental Science and Health Part a-Toxic/Hazardous Substances & Environmental Engineering 39(3): 803-819.
Nzengung, V. A., H. Penning, et al. (2004). "Mechanistic changes during phytoremediation of perchlorate under different root-zone conditions." International Journal of Phytoremediation 6(1): 63-83.
Orchard, B. J., W. J. Doucette, et al. (2000). "A novel laboratory system for determining fate of volatile organic compounds in planted systems." Environmental Toxicology and Chemistry 19(4): 888-894.
LEMNA
Blackman, G. E., G. Sen, et al. (1959). "The Uptake of Growth Substances I. Factors Controlling the Uptake of Phenoxyacetic acids by Lemna Minor." Journal of Experimental Botany 10(28): 35-54.
Wang, Q., Y. Cui, et al. (2002). "Phytoremediation of polluted waters potentials and prospects of wetland plants." Acta Biotechnologica 22(1-2): 199-208.
Tront, J. M. and F. M. Saunders (2007). "Sequestration of a fluorinated analog of 2,4-dichlorophenol and metabolic products by L-minor as evidenced by F-19 NMR." Environmental Pollution 145(3): 708-714.
Tront, J. M. and F. M. Saunders (2006). "Role of plant activity and contaminant speciation in aquatic plant assimilation of 2,4,5-trichlorophenol." Chemosphere 64(3): 400-407.
Song, Z. H. and G. L. Huang (2005). "Toxic effect of triphenyltin on Lemna polyrhiza." Applied Organometallic Chemistry 19(7): 807-810.
Slaski, J. J., D. J. Archambault, et al. (2002). "Physiological tests to measure impacts of gaseous polycyclic aromatic hydrocarbons (PAHs) on cultivated plants." Communications in Soil Science and Plant Analysis 33(15-18): 3227-3239.
Saygideger, S. and M. Dogan (2004). "Lead and cadmium accumulation and toxicity in the presence of EDTA in Lemna minor L. and Ceratophyllum demersum L." Bulletin of Environmental Contamination and Toxicology 73(1): 182-189.
Reinhold, D. M. and E. M. Saunders (2006). "Phytoremediation of fluorinated agrochemicals by duckweed." Transactions of the Asabe 49(6): 2077-2083.
Prasad, M. N. V. and H. M. D. Freitas (2003). "Metal hyperaccumulation in plants - Biodiversity prospecting for phytoremediation technology." Electronic Journal of Biotechnology 6(3): 285-321.
Ornes, W. H., K. S. Sajwan, et al. (1991). "Bioaccumulation of Selenium by Floating Aquatic Plants." Water Air and Soil Pollution 57-8: 53-57.
Olguin, E. J., G. Sanchez-Galvan, et al. (2005). "Surface adsorption, intracellular accumulation and compartmentalization of Pb(II) in batch-operated lagoons with Salvinia minima as affected by environmental conditions, EDTA and nutrients." Journal of Industrial Microbiology & Biotechnology 32(11-12): 577-586.
Mkandawire, M., B. Tauert, et al. (2004). "Capacity of Lemna gibba L. (Duckweed) for uranium and arsenic phytoremediation in mine tailing waters." International Journal of Phytoremediation 6(4): 347-362.
Mkandawire, M. and E. G. Dudel (2005). "Accumulation of arsenic in Lemna gibba L. (duckweed) in tailing waters of two abandoned uranium mining sites in Saxony, Germany." Science of the Total Environment 336(1-3): 81-89.
Marwood, C. A., K. T. J. Bestari, et al. (2003). "Creosote toxicity to photosynthesis and plant growth in aquatic microcosms." Environmental Toxicology and Chemistry 22(5): 1075-1085.
Kummerova, M., S. Zezulka, et al. (2007). "Photoinduced toxicity of fluoranthene on primary processes of photosynthesis in lichens." Lichenologist 39: 91-100.
Kara, Y. (2004). "Bioaccumulation of copper from contaminated wastewater by using Lemna minor." Bulletin of Environmental Contamination and Toxicology 72(3): 467-471.
Kapustka, L. A. (2004). "Establishing Eco-SSLs for PAHs: Lessons revealed from a review of literature on exposure and effects to terrestrial receptors." Human and Ecological Risk Assessment 10(2): 185-205.
Jansen, M. A. K., L. M. Hill, et al. (2004). "A novel stress-acclimation response in Spirodela punctata (Lemnaceae): 2,4,6-trichlorophenol triggers an increase in the level of an extracellular peroxidase, capable of the oxidative dechlorination of this xenobiotic pollutant." Plant Cell and Environment 27(5): 603-613.
Gallardo-Williams, M. T., V. A. Whalen, et al. (2002). "Accumulation and retention of lead by cattail (Typha domingensis), hydrilla (Hydrilla verticillata), and duckweed (Lemna obscura)." Journal of Environmental Science and Health Part a-Toxic/Hazardous Substances & Environmental Engineering 37(8): 1399-1408.
Fayiga, A. O., L. Q. Ma, et al. (2004). "Effects of heavy metals on growth and arsenic accumulation in the arsenic hyperaccumulator Pteris vittata L." Environmental Pollution 132(2): 289-296.
El-Alawi, Y. S., X. D. Huang, et al. (2002). "Quantitative structure-activity relationship for the photoinduced toxicity of polycyclic aromatic hydrocarbons to the luminescent bacteria Vibrio fischeri." Environmental Toxicology and Chemistry 2110): 2225-2232.
Duxbury, C. L., D. G. Dixon, et al. (1997). "Effects of simulated solar radiation on the bioaccumulation of polycyclic aromatic hydrocarbons by the duckweed Lemna gibba." Environmental Toxicology and Chemistry 16(8): 1739-1748.
Drost, W., M. Matzke, et al. (2007). "Heavy metal toxicity to Lemna minor: studies on the time dependence of growth inhibition and the recovery after exposure." Chemosphere 67(1): 36-43.
Devi, M., D. A. Thomas, et al. (1996). "Accumulation and physiological and biochemical effects of cadmium in a simple aquatic food chain." Ecotoxicology and Environmental Safety 33(1): 38-43.
Debusk, T. A., R. B. Laughlin, et al. (1996). "Retention and compartmentalization of lead and cadmium in wetland microcosms." Water Research 30(11): 2707-2716.
Day, J. A. and F. M. Saunders (2004). "Glycosidation of chlorophenols by Lemna minor." Environmental Toxicology and Chemistry 23(3): 613-620.
Cecal, A., K. Popa, et al. (2002). "Bioaccumulation in hydrophytae plants of some microelements from alkaline sludge resulting in uranium ores processing." Revista De Chimie 53(4): 290-293.
Boutonnet, J. C., P. Bingham, et al. (1999). "Environmental risk assessment of trifluoroacetic acid." Human and Ecological Risk Assessment 5(1): 59-124.
Bottcher, T. and R. Schroll (2007). "The fate of isoproturon in a freshwater microcosm with Lemna minor as a model organism." Chemosphere 66(4): 684-689.
Boniardi, N., R. Rota, et al. (1999). "Effect of dissolved metals on the organic load removal efficiency of Lemna gibba." Water Research 33(2): 530-538.
Barber, J. L., G. O. Thomas, et al. (2004). "Current issues and uncertainties in the measurement and modelling of air-vegetation exchange and within-plant processing of POPs." Environmental Pollution 128(1-2): 99-138.
TYPES OF CHEMICALS TAKEN UP BY DUCKWEED
"Microcosm evaluation of the effects of an eight pharmaceutical mixture to the aquatic macrophytes Lemna gibba and Myriophyllum sibiricum"
Aquatic Toxicology, Volume 70, Issue 1, 18 October 2004, Pages 23-40
Richard A. Brain, David J. Johnson, Sean M. Richards, Mark L. Hanson, Hans Sanderson, Monica W. Lam, Cora Young, Scott A. Mabury, Paul K. Sibley, Keith R Solomon
"Probabilistic ecological hazard assessment: Evaluating pharmaceutical effects on aquatic higher plants as an example"
Ecotoxicology and Environmental Safety, Volume 64, Issue 2, June 2006, Pages 128-135
Richard A. Brain, Hans Sanderson, Paul K. Sibley, Keith R. Solomon
"Aquatic microcosm assessment of the effects of tylosin on Lemna gibba and Myriophyllum spicatum"
Environmental Pollution, Volume 133, Issue 3, February 2005, Pages 389-401
Richard A. Brain, Ketut (Jim) Bestari, Hans Sanderson, Mark L. Hanson, Christian J. Wilson, David J. Johnson, Paul K. Sibley, Keith R. Solomon
"Toxicity classification and evaluation of four pharmaceuticals classes: antibiotics, antineoplastics, cardiovascular, and sex hormones"
Toxicology, Volume 203, Issues 1-3, 15 October 2004, Pages 27-40
Hans Sanderson, Richard A. Brain, David J. Johnson, Christian J. Wilson, Keith R. Solomon
"Effects of a mixture of tetracyclines to Lemna gibba and Myriophyllum sibiricum evaluated in aquatic microcosms"
Environmental Pollution, Volume 138, Issue 3, December 2005, Pages 425-442
Richard A. Brain, Christian J. Wilson, David J. Johnson, Hans Sanderson, Ketut (Jim) Bestari, Mark L. Hanson, Paul K. Sibley, Keith R. Solomon
HOW DUCKWEED UPTAKES CHEMICALS (i.e. physio-chemico Phosphate system uptake of Aresenic (AS))
"Arsenic accumulation in duckweed (Spirodela polyrhiza L.): A good option for phytoremediation"
Chemosphere, Volume 69, Issue 3, September 2007, Pages 493-499
M. Azizur Rahman, Hiroshi Hasegawa, Kazumasa Ueda, Teruya Maki, Chikako Okumura, M. Mahfuzur Rahman
"The influence of Lemna gibba L. on the degradation of organic material in duckweed-covered domestic wastewater"
Water Research, Volume 32, Issue 10, October 1998, Pages 3092-3098
S. Körner, G. B. Lyatuu, J. E. Vermaat
"Toxicity of hexazinone and diquat to green algae, diatoms, cyanobacteria and duckweed"
Aquatic Toxicology, Volume 39, Issue 2, September 1997, Pages 111-134
Hans G. Peterson, Céline Boutin, Kathryn E. Freemark, Pamela A. Martin
"Ecophysiological tolerance of duckweeds exposed to copper"
Aquatic Toxicology, Volume 91, Issue 1, 18 January 2009, Pages 1-9
Myriam Kanoun-Boulé, Joaquim A.F. Vicente, Cristina Nabais, M.N.V. Prasad, Helena Freitas
COMMON THEMES in PHYTOREMEDIATION re: MECHANISMS OF ACTION
Wang, H., X. Q. Shan, et al. (2004). "Responses of antioxidative enzymes to accumulation of copper in a copper hyperaccumulator of Commoelina communis." Archives of Environmental Contamination and Toxicology 47(2): 185-192.
McFarlane, C., C. Nolt, et al. (1987). "The Uptake, Distribution and Metabolism of Four Organic Chemicals by Soybean Plants and Barley Roots." Environmental Toxicology and Chemistry 6: 847-856.
Topp, E., I. Scheunert, et al. (1986). "Factors Affecting the Uptake of C-14-Labeled Organic-Chemicals by Plants from Soil." Ecotoxicology and Environmental Safety 11(2): 219-228.
Sundberg, S. E., S. M. Hassan, et al. (2006). "Enrichment of elements in detritus from a constructed wetland and consequent toxicity to Hyalella azteca." Ecotoxicology and Environmental Safety 64(3): 264-272.
Odjegba, V. J. and I. O. Fasid (2004). "Accumulation of trace elements by Pistia stratiotes: implications for phytoremediation." Ecotoxicology 13(7): 637-646.
Yifru, D. D. and V. A. Nzengung (2006). "Uptake of N-nitrosodimethylamine (NDMA) from water by phreatophytes in the absence and presence of perchlorate as a co-contaminant." Environmental Science & Technology 40(23): 7374-7380.
Vadas, T. M., X. Zhang, et al. (2007). "Fate of DTPA, EDTA, and EDDS in hydroponic media and effects on plant mineral nutrition." Journal of Plant Nutrition 30(7-9): 1229-1246.
Flocco, C. G., A. Lobalbo, et al. (2002). "Some physiological, microbial, and toxicological aspects of the removal of phenanthrene by hydroponic cultures of alfalfa (Medicago sativa L.)." International Journal of Phytoremediation 4(3): 169-186.
Kim, J., M. C. Drew, et al. (2004). "Uptake and phytotoxicity of TNT in onion plant." Journal of Environmental Science and Health Part a-Toxic/Hazardous Substances & Environmental Engineering 39(3): 803-819.
Nzengung, V. A., H. Penning, et al. (2004). "Mechanistic changes during phytoremediation of perchlorate under different root-zone conditions." International Journal of Phytoremediation 6(1): 63-83.
Orchard, B. J., W. J. Doucette, et al. (2000). "A novel laboratory system for determining fate of volatile organic compounds in planted systems." Environmental Toxicology and Chemistry 19(4): 888-894.
LEMNA
Blackman, G. E., G. Sen, et al. (1959). "The Uptake of Growth Substances I. Factors Controlling the Uptake of Phenoxyacetic acids by Lemna Minor." Journal of Experimental Botany 10(28): 35-54.
Wang, Q., Y. Cui, et al. (2002). "Phytoremediation of polluted waters potentials and prospects of wetland plants." Acta Biotechnologica 22(1-2): 199-208.
Tront, J. M. and F. M. Saunders (2007). "Sequestration of a fluorinated analog of 2,4-dichlorophenol and metabolic products by L-minor as evidenced by F-19 NMR." Environmental Pollution 145(3): 708-714.
Tront, J. M. and F. M. Saunders (2006). "Role of plant activity and contaminant speciation in aquatic plant assimilation of 2,4,5-trichlorophenol." Chemosphere 64(3): 400-407.
Song, Z. H. and G. L. Huang (2005). "Toxic effect of triphenyltin on Lemna polyrhiza." Applied Organometallic Chemistry 19(7): 807-810.
Slaski, J. J., D. J. Archambault, et al. (2002). "Physiological tests to measure impacts of gaseous polycyclic aromatic hydrocarbons (PAHs) on cultivated plants." Communications in Soil Science and Plant Analysis 33(15-18): 3227-3239.
Saygideger, S. and M. Dogan (2004). "Lead and cadmium accumulation and toxicity in the presence of EDTA in Lemna minor L. and Ceratophyllum demersum L." Bulletin of Environmental Contamination and Toxicology 73(1): 182-189.
Reinhold, D. M. and E. M. Saunders (2006). "Phytoremediation of fluorinated agrochemicals by duckweed." Transactions of the Asabe 49(6): 2077-2083.
Prasad, M. N. V. and H. M. D. Freitas (2003). "Metal hyperaccumulation in plants - Biodiversity prospecting for phytoremediation technology." Electronic Journal of Biotechnology 6(3): 285-321.
Ornes, W. H., K. S. Sajwan, et al. (1991). "Bioaccumulation of Selenium by Floating Aquatic Plants." Water Air and Soil Pollution 57-8: 53-57.
Olguin, E. J., G. Sanchez-Galvan, et al. (2005). "Surface adsorption, intracellular accumulation and compartmentalization of Pb(II) in batch-operated lagoons with Salvinia minima as affected by environmental conditions, EDTA and nutrients." Journal of Industrial Microbiology & Biotechnology 32(11-12): 577-586.
Mkandawire, M., B. Tauert, et al. (2004). "Capacity of Lemna gibba L. (Duckweed) for uranium and arsenic phytoremediation in mine tailing waters." International Journal of Phytoremediation 6(4): 347-362.
Mkandawire, M. and E. G. Dudel (2005). "Accumulation of arsenic in Lemna gibba L. (duckweed) in tailing waters of two abandoned uranium mining sites in Saxony, Germany." Science of the Total Environment 336(1-3): 81-89.
Marwood, C. A., K. T. J. Bestari, et al. (2003). "Creosote toxicity to photosynthesis and plant growth in aquatic microcosms." Environmental Toxicology and Chemistry 22(5): 1075-1085.
Kummerova, M., S. Zezulka, et al. (2007). "Photoinduced toxicity of fluoranthene on primary processes of photosynthesis in lichens." Lichenologist 39: 91-100.
Kara, Y. (2004). "Bioaccumulation of copper from contaminated wastewater by using Lemna minor." Bulletin of Environmental Contamination and Toxicology 72(3): 467-471.
Kapustka, L. A. (2004). "Establishing Eco-SSLs for PAHs: Lessons revealed from a review of literature on exposure and effects to terrestrial receptors." Human and Ecological Risk Assessment 10(2): 185-205.
Jansen, M. A. K., L. M. Hill, et al. (2004). "A novel stress-acclimation response in Spirodela punctata (Lemnaceae): 2,4,6-trichlorophenol triggers an increase in the level of an extracellular peroxidase, capable of the oxidative dechlorination of this xenobiotic pollutant." Plant Cell and Environment 27(5): 603-613.
Gallardo-Williams, M. T., V. A. Whalen, et al. (2002). "Accumulation and retention of lead by cattail (Typha domingensis), hydrilla (Hydrilla verticillata), and duckweed (Lemna obscura)." Journal of Environmental Science and Health Part a-Toxic/Hazardous Substances & Environmental Engineering 37(8): 1399-1408.
Fayiga, A. O., L. Q. Ma, et al. (2004). "Effects of heavy metals on growth and arsenic accumulation in the arsenic hyperaccumulator Pteris vittata L." Environmental Pollution 132(2): 289-296.
El-Alawi, Y. S., X. D. Huang, et al. (2002). "Quantitative structure-activity relationship for the photoinduced toxicity of polycyclic aromatic hydrocarbons to the luminescent bacteria Vibrio fischeri." Environmental Toxicology and Chemistry 2110): 2225-2232.
Duxbury, C. L., D. G. Dixon, et al. (1997). "Effects of simulated solar radiation on the bioaccumulation of polycyclic aromatic hydrocarbons by the duckweed Lemna gibba." Environmental Toxicology and Chemistry 16(8): 1739-1748.
Drost, W., M. Matzke, et al. (2007). "Heavy metal toxicity to Lemna minor: studies on the time dependence of growth inhibition and the recovery after exposure." Chemosphere 67(1): 36-43.
Devi, M., D. A. Thomas, et al. (1996). "Accumulation and physiological and biochemical effects of cadmium in a simple aquatic food chain." Ecotoxicology and Environmental Safety 33(1): 38-43.
Debusk, T. A., R. B. Laughlin, et al. (1996). "Retention and compartmentalization of lead and cadmium in wetland microcosms." Water Research 30(11): 2707-2716.
Day, J. A. and F. M. Saunders (2004). "Glycosidation of chlorophenols by Lemna minor." Environmental Toxicology and Chemistry 23(3): 613-620.
Cecal, A., K. Popa, et al. (2002). "Bioaccumulation in hydrophytae plants of some microelements from alkaline sludge resulting in uranium ores processing." Revista De Chimie 53(4): 290-293.
Boutonnet, J. C., P. Bingham, et al. (1999). "Environmental risk assessment of trifluoroacetic acid." Human and Ecological Risk Assessment 5(1): 59-124.
Bottcher, T. and R. Schroll (2007). "The fate of isoproturon in a freshwater microcosm with Lemna minor as a model organism." Chemosphere 66(4): 684-689.
Boniardi, N., R. Rota, et al. (1999). "Effect of dissolved metals on the organic load removal efficiency of Lemna gibba." Water Research 33(2): 530-538.
Barber, J. L., G. O. Thomas, et al. (2004). "Current issues and uncertainties in the measurement and modelling of air-vegetation exchange and within-plant processing of POPs." Environmental Pollution 128(1-2): 99-138.
TYPES OF CHEMICALS TAKEN UP BY DUCKWEED
"Microcosm evaluation of the effects of an eight pharmaceutical mixture to the aquatic macrophytes Lemna gibba and Myriophyllum sibiricum"
Aquatic Toxicology, Volume 70, Issue 1, 18 October 2004, Pages 23-40
Richard A. Brain, David J. Johnson, Sean M. Richards, Mark L. Hanson, Hans Sanderson, Monica W. Lam, Cora Young, Scott A. Mabury, Paul K. Sibley, Keith R Solomon
"Probabilistic ecological hazard assessment: Evaluating pharmaceutical effects on aquatic higher plants as an example"
Ecotoxicology and Environmental Safety, Volume 64, Issue 2, June 2006, Pages 128-135
Richard A. Brain, Hans Sanderson, Paul K. Sibley, Keith R. Solomon
"Aquatic microcosm assessment of the effects of tylosin on Lemna gibba and Myriophyllum spicatum"
Environmental Pollution, Volume 133, Issue 3, February 2005, Pages 389-401
Richard A. Brain, Ketut (Jim) Bestari, Hans Sanderson, Mark L. Hanson, Christian J. Wilson, David J. Johnson, Paul K. Sibley, Keith R. Solomon
"Toxicity classification and evaluation of four pharmaceuticals classes: antibiotics, antineoplastics, cardiovascular, and sex hormones"
Toxicology, Volume 203, Issues 1-3, 15 October 2004, Pages 27-40
Hans Sanderson, Richard A. Brain, David J. Johnson, Christian J. Wilson, Keith R. Solomon
"Effects of a mixture of tetracyclines to Lemna gibba and Myriophyllum sibiricum evaluated in aquatic microcosms"
Environmental Pollution, Volume 138, Issue 3, December 2005, Pages 425-442
Richard A. Brain, Christian J. Wilson, David J. Johnson, Hans Sanderson, Ketut (Jim) Bestari, Mark L. Hanson, Paul K. Sibley, Keith R. Solomon
HOW DUCKWEED UPTAKES CHEMICALS (i.e. physio-chemico Phosphate system uptake of Aresenic (AS))
"Arsenic accumulation in duckweed (Spirodela polyrhiza L.): A good option for phytoremediation"
Chemosphere, Volume 69, Issue 3, September 2007, Pages 493-499
M. Azizur Rahman, Hiroshi Hasegawa, Kazumasa Ueda, Teruya Maki, Chikako Okumura, M. Mahfuzur Rahman
"The influence of Lemna gibba L. on the degradation of organic material in duckweed-covered domestic wastewater"
Water Research, Volume 32, Issue 10, October 1998, Pages 3092-3098
S. Körner, G. B. Lyatuu, J. E. Vermaat
"Toxicity of hexazinone and diquat to green algae, diatoms, cyanobacteria and duckweed"
Aquatic Toxicology, Volume 39, Issue 2, September 1997, Pages 111-134
Hans G. Peterson, Céline Boutin, Kathryn E. Freemark, Pamela A. Martin
"Ecophysiological tolerance of duckweeds exposed to copper"
Aquatic Toxicology, Volume 91, Issue 1, 18 January 2009, Pages 1-9
Myriam Kanoun-Boulé, Joaquim A.F. Vicente, Cristina Nabais, M.N.V. Prasad, Helena Freitas
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