Ecotoxicological assessment of industrial effluent using duckweed (Lemna minor L.) as a test organism

This study aimed at assessing the toxic effects of industrial effluents using duckweed (Lemna minor L.) plants as a test system. Growth inhibition test according to standardized protocol (ISO 20079) was performed. The suitability of the Comet assay (indicates DNA damage) and certain parameters such as peroxidase activity and lipid peroxidation level, as biomarkers for environmental monitoring was evaluated. The water samples were collected monthly over a 3-month period from the stream near the industrial estate of Savski Marof, Croatia. All samples caused inhibition of growth rates based on frond number and biomass as well as decrease of chlorophylls content. In contrast, peroxidase activity, malondialdehyde content and tail extent moment (measure of DNA strand breaks) markedly increased. Obtained data demonstrate the relevance of duckweed as sensitive indicators of water quality as well as the significance of selected biological parameters in the reliable assessment of phyto- and genotoxic potential of complex wastewaters.     

A novel response of wild-type duckweed (Lemna paucicostata Hegelm.) to heavy metals

The responses of wild-type Lemna paucicostata, used as the experimental stock, to seven heavy metals were investigated with a hydroponic culture in a culture facility. The results showed that six of the metals could induce L. paucicostata to release daughter fronds from the mother frond before maturity, resulting in breakup of the colony. This phenomenon is novel and has not been reported before. Six of the heavy metals investigated-copper, cadmium, nickel, zinc, mercury, and chromium-could induce this response, but sensitivity and response of the plant varied. When Cu(2+) at a concentration of 0.2 micromol/L was added, the response was significantly different from that of the control after a 2-h exposure. The response to cadmium (0.8 micromol/L) was significantly different after a 6-h exposure; to zinc (10 micromol/L) and nickel (5 micromol/L), after 5 and 7 h, respectively. But for lead, even at a concentration of 80 micromol/L for 96 h, no colony breakup was induced. The response to heavy metals occurred within 24 h; increasing the exposure by another 24 h had nearly no additional effect on the colony breakup. When the response was over, the number of colonies with fronds varied with the inducement capacity of the metal. Nutrients could affect the sensitivity of L. paucicostata to some heavy metals, such as mercury, chromium, zinc, and nickel, but not to copper and cadmium. Sensitivity was also changed when CuCl(2) was used as the copper source instead of CuSO(4). These results indicate that some heavy metals could serve as a toxicant for inducing L. paucicostata to release daughter fronds in advance. This change might involve a stress response such as ethylene production. Also discussed is the possibility of this response as a biomarker in a bioassay.     

Allelopathic effects of the toxic cyanobacterium Microcystis aeruginosa on duckweed, Lemna gibba L

Cyanotoxins are a group of compounds produced by cyanobacteria that can have severe physiological effects on other organisms, including humans. The potential allelopathic effects of Microcystis aeruginosa, a toxic cyanobacterium, on the duckweed plant, Lemna gibba L., were examined using three experimental methods: (1) a series of toxicity bioassays, (2) evaluation of toxin production by M. aeruginosa in the direct and indirect presence of L. gibba, and (3) inhibition of oxygen evolution in photosynthesis. The results showed that, first, there were no clear dose-dependent effects of the microcystin-LR standard or the toxic M. aeruginosa culture filtrate on any of the end points measured in the toxicity bioassays (plant and frond number, dry weight, growth rate, chlorophyll content; one-way ANOVA, p > 0.05). In those cases in which an EC(50) value could be obtained, chlorophyll a was the most sensitive end point, as it had the lowest EC(50) value (14.47 microg/L microcystin-LR) of all the end points. Second, the presence of L. gibba did not result in higher microcystin-LR production in the toxic M. aeruginosa culture. And, last, oxygen evolution was not affected in isolated chloroplasts exposed directly to microcystin-LR. Therefore, microcystins from the toxic cyanobacterium Microcystis aeruginosa do not appear to have an allelopathic effect on the common aquatic macrophyte Lemna gibba.     

Alumina nanoparticles enhance growth of Lemna minor

The industrial use of nanoparticles is rapidly increasing, and this has given rise to concerns about potential biological impacts of engineered particles released into the environment. So far, relatively little is known about uptake, accumulation and responses to engineered nanoparticles by plants. In this study, the effects of alumina nanoparticles on growth, morphology and photosynthesis of Lemna minor were quantified. It was found that alumina nanoparticles substantially increase biomass accumulation of L. minor. Such a stimulatory effect of alumina nanoparticles on growth has not been reported previously. Enhanced biomass accumulation was paralleled by morphological adjustments such as increased root length and number of fronds per colony, and by increased photosynthetic efficiency. Metal nanoparticles have previously been shown to enhance the energy transfer efficiency of isolated reaction centres; therefore it is proposed that the mechanism underlying the alumina mediated enhancement of biomass accumulation in L. minor is associated with increased efficiencies in the light reactions of photosynthesis.     

Comparing the sensitivity of geographically distinct Lemna minor populations to atrazine

The objectives of this study were to compare the sensitivities of field populations and a laboratory culture of a duckweed species (Lemna minor) to the herbicide atrazine using three different endpoints and to determine whether sensitivity to atrazine was affected by past exposure to the herbicide. L. minor cultures were purchased commercially or collected from field sites within an agricultural watershed and exposed to atrazine for 7 days under greenhouse conditions. Populations differed significantly in their sensitivity to atrazine. Biomass was more sensitive than frond number, while chlorophyll fluorescence was not a sensitive endpoint. Overall, the sensitivity of the various populations to atrazine was not strongly related to measures of past exposure to agriculture stressors. Positive correlations between biomass twenty-five percent inhibition concentrations (IC₂₅s), biomass estimated marginal means and in-stream atrazine concentrations were observed, providing evidence that atrazine exposure is linked to a decrease in sensitivity to atrazine. However, IC₂₅s generated for each population were similar, ranging from 19 to 40 and 57 to 92 μg/L atrazine for biomass and frond data respectively, and likely do not represent biologically significant differences in atrazine sensitivity. Given the small range in sensitivity observed between populations, commercial laboratory cultures appear to provide a good estimate of the sensitivity of field populations of L. minor to atrazine and should continue to be used in regulatory phytotoxicity testing.     

Effects of the antimalarial lumefantrine on Lemna minor,Raphidocelis subcapitata and Chlorella vulgaris

Lumefantrine is used to treat uncomplicated malaria caused by pure or mixed Plasmodium falciparum infections and as a prophylactic against recrudescence following artemether therapy. However, the pharmaceutical is released into the aquatic environment from industrial effluents, hospital discharges, and human excretion. This study assessed the effects of lumefantrine on the growth and physiological responses of the microalgae Chlorella vulgaris and Raphidocelis subcapitata (formerly known as Selenastrum capricornutum and Pseudokirchneriella subcapitata) and the aquatic macrophyte Lemna minor. The microalgae and macrophyte were exposed to 200−10000 μg l⁻¹ and 16−10000 μg l⁻¹ lumefantrine, respectively. Lumefantrine had a variable effect on the growth of the aquatic plants investigated. There was a decline in the growth of R. subcapitata and L. minor post-exposure to the drug. Contrarily, there was stimulation in the growth of Chlorella vulgaris. All experimental plants had a significant increase in lipid peroxidation, which was accompanied by an increase in malondialdehyde content. Peroxidase activity of L. minor increased only at low lumefantrine concentrations, while the opposite occurred at higher levels of the drug. Incubation in lumefantrine contaminated medium significantly up-regulated the activity of R. subcapitata cultures. Glutathione S-transferase of L. minor exposed to lumefantrine treatments had substantially higher activities than the controls. Our findings suggest lumefantrine could have adverse but variable effects on the growth and physiology of the studied aquatic plants.     

Sites of toxicity of specific photooxidation products of anthracene to higher plants: inhibition of photosynthetic activity and electron transport in Lemna gibba L. G-3 (duckweed)

Sites of toxicity of polycyclic aromatic hydrocarbons (PAHs) were examined to determine if inhibition of photosynthetic activity could be correlated to whole-organism toxicity. The inhibition of photosynthesis was observed by detecting the induction kinetics of endogenous chlorophyll a (Chl a) fluorescence. Anthracene (ANT) photooxidation products were applied to the aquatic higher plant Lemna gibba L. G-3 at concentrations ranging from 0.01 to 10 ppm. The impact on Chl a fluorescence was found to correlate with whole-organism toxicity for the 13 PAH compounds tested in this in vivo study. The mechanism of toxic action starts with inhibition of photosystem I (PSI) or the cytochrome-b6/f complex, followed by photooxidative damage to photosystem II (PSII). To study the effects of oxygenated ANTs on photosynthesis in vivo, the IC(50)s for F(V)/F(M) (PSII activity) and F(Q)/F(M) (activity downstream from PSII) were determined. The IC(50)s for a decrease of F(Q)/F(M) for all 13 chemicals were on average twofold lower than those for F(V)/F(M). F(V)/F(M) was found to be a measure of acute toxicity, whereas F(Q)/F(M) was found to be a measure of chronic toxicity. Thus, Chl a fluorescence by use of the whole organism was able to detect the impacts of photomodified ANT products and indicate a site of action for the chemicals.     

Reciprocal allelopathic responses between toxic cyanobacteria (Microcystis aeruginosa) and duckweed (Lemna japonica).

We examined cyanobacterial toxin production in response to direct exposure to an axenically cultured aquatic plant (Lemna japonica Landolt) using two toxic monoclonal strains of Microcystis aeruginosa Küzing (NIES strains 103 and 107). The patterns of nutrient concentrations were similar between the controls and treatments. The exposure to toxic M. aeruginosa inhibited growth in L. japonica, and exposure to the aquatic plant caused an increase in microcystin (MC) production and inhibited growth in M. aeruginosa. The respective biomasses of M. aeruginosa and L. japonica differed significantly between control and treatments (p<0.05). Both intracellular MC and extracellular MC productions were significantly higher in the treatment than in the control for both strains (p<0.01). Increased MC production in response to direct exposure to this aquatic plant could be an induced defense mediated by the release of allelochemicals from the plant. This study revealed reciprocal allelopathic responses between Microcystis and Lemna, which can be applied to the management of eutrophic waters as well as an important information concerning strategies for recovering eutrophic waters.     

Evaluation of physiological changes induced by the fluoroquinolone antibiotic ciprofloxacin in the freshwater macrophyte species Lemna minor and Lemna gibba

The worldwide increase in the consumption of antibiotics is becoming a concern for the scientific community, since the presence of their residues in the wild poses specific challenges, especially in ecotoxicological terms. Currently, antibiotics are used for a wide range of purposes, being used against bacterial diseases but also as growth promoters. As a result, their environmental presence can affect wild organisms, especially those from the aquatic environment. This scenario leads to the need of characterizing the toxicity of antibiotics, especially towards non-target organisms. In this study we selected two species of aquatic macrophytes, Lemna minor and Lemna gibba, which are standard plant species inscribed in ecotoxicological testing guidelines. In this work we characterized the toxic effects of the quinolone antibiotic ciprofloxacin (in levels of 0.005, 0.013, 0.031, 0.078, and 0.195 mg/L), focusing on its potential toxicity towards photosynthetic mechanisms, and pro-oxidant effects. These objectives were attained by measuring the concentrations of chlorophyll a and b, and carotenoids levels. The determination of the quantum yield allowed assessing the effects of ciprofloxacin on the photochemical efficiency of the Photosystem II (PSII). The pro-oxidant effects induced by ciprofloxacin were evaluated by measuring oxidative stress biomarkers, such as catalase activity, and also by determining lipoperoxidation levels. The obtained results showed no differences in terms of the content of both chlorophylls a and b, or any change in the photochemical efficiency of the PSII; however, the global carotenoids content of L. gibba were significantly decreased. The activity of the anti-oxidant enzyme catalase was also significantly increased in L. minor. L. gibba showed a decrease in lipid peroxidation levels, but only for the two lowest concentrations of ciprofloxacin. The global set of data shows the activation of the anti-oxidant defensive system of both plant species, a response that was likely activated by the pro-oxidant character of ciprofloxacin. Our data demonstrate the interference of this therapeutic compound at different levels of plant metabolism, at ecologically relevant concentrations. In fact, the obtained results are of ecological relevance since they illustrate deleterious effects that may compromise the physiology of aquatic non-target plant species.     

Phosphate regulates uranium(VI) toxicity to Lemna gibba L. G3

The influence of phosphate on the toxicity of uranium to Lemna gibba G3 was tested in semicontinuous culture with synthetic mine water developed as an analogue of surface water of two abandoned uranium mining and ore processing sites in Saxony, Germany. Six concentrations of uranium were investigated under five different supply regimes of PO(4) (3-) at constant pH (7.0 +/- 0.5) and alkalinity (7.0 +/- 1.6 mg L(-1) total CO(3) (2-)). The results showed significant inhibition of specific growth rates in cultures exposed to the highest uranium concentrations (3500 and 7000 microg U L(-1)) at lowest PO(4) (3-) supply of 0.01 mg L(-1). An increase of phosphate concentration from 0.01 to 8.0 mg L(-1) resulted in an increase of EC(50) from 0.9 +/- 0.2 to 7.4 +/- 1.9 mg L(-1) (significant with Student's t test, P > 0.05). The accumulation of uranium in L. gibba increased exponentially with the increase in uranium concentration in cultures with 0.01 and 0.14 mg PO(4) (3-) L(-1). Accumulation also increased significantly when PO(4) (3-) supply was increased from 0.14 to 1.36 mg PO(4) (3-) L(-1) for all uranium concentrations. However, as the supply of PO(4) (3-) gradually increased from 1.36 to 8.0 mg PO(4) (3-) L(-1), uranium bioaccumulation increased slightly but insignificantly before leveling off. Uranium speciation modeling with PhreeqC geochemical code predicted increases in the proportions of uranyl phosphate species when PO(4) (3-) concentrations increase in the media. Most of these uranyl phosphate species have a high probability of precipitation [saturation indices (SI) > 0.93]. Therefore, the alleviation of uranium toxicity to L. gibba with phosphates is due to interactions among components of the media, mainly uranyl and phosphate which results in precipitation. Consequently, bioavailable fractions of uranium to L. gibba are reduced. This might explain lack of consistent EC(50) values for uranium to most aquatic organisms.     

Induction of frond abscission by metals and other toxic compounds in Lemna minor

Fronds of the duckweed Lemna minor L. clone St form colonies of different sizes on the basis of stipes connecting mother and daughter fronds for some time after the development of daughter fronds. All the metals (AsO₄³⁻, AsO₂⁻, Cd²⁺, CrO₄²⁻, Co²⁺, Cu²⁺, Ni²⁺, Hg²⁺, Tl⁺ and Zn²⁺) and one non-metal (SeO₄²⁻, SeO₃²⁻) tested here induced frond abscission, thus decreasing the colony size on the basis of a novel mechanism of abscission described recently. Concentration-response curves were created based on percentages of frond abscission after 7 and 24 h of toxic compound application, and response concentrations were calculated accordingly. The following conclusions could be drawn: (1) in most cases the response demonstrates less sensitivity than the bio test based on the ISO protocol 20079. (2) Even applying 1 mM of the metals, AsO₄³⁻, CrO₄²⁻, Co²⁺ and Zn²⁺ did not reach the half-maximal effects. (3) The concentration-response curves are bell-shaped with AsO²⁻, Cd²⁺, Hg²⁺, SeO₃²⁻ and Tl⁺, which demonstrates that abscission is induced by lower but not by higher concentrations. (4) Frond abscission shows fast and sensitive effects (24 h) for Ag⁺, Cu²⁺, AsO²⁻, SeO₄²⁻, SeO₃²⁻ and Tl⁺. The mechanisms and responses described here quantitatively for the first time complement and explain observations within the frame of the ISO protocol. Therefore, frond abscission should be regularly reported in the standard test protocols as abscission always indicates massive physiological effects.     

Effects of erythromycin, tetracycline and ibuprofen on the growth of Synechocystis sp. and Lemna minor

Pharmaceutically active substances have recently been recognised as an emerging environmental problem. Human and veterinarian therapeutic agents can contaminate aquatic ecosystems via sewage discharges (human and animal excretion), improper disposal or industrial waste. Very little is known on the effects of pharmaceutical pollutants on aquatic photosynthetic organisms. In this study the effects of erythromycin, tetracycline and ibuprofen on the growth of the cyanobacterium Synechocystis sp. PCC6803 and the duckweed Lemna minor FBR006 were studied at concentrations of 1-1000 microg l(-1). At dosage of 1 mg l(-1), erythromycin affected the growth of both Synechocystis and Lemna with a maximum inhibition of 70 and 20%, respectively. Tetracycline had inhibitory effects (20-22% reduction in growth) on Synechocystis at intermediate dosages. The same aminoglycoside antibiotic promoted growth in Lemna by 26% at 10 microg l(-1), while frond development was reduced at 1 mg l(-1) (tetracycline). The anti-inflammatory ibuprofen strongly stimulated the growth of Synechocystis at all concentrations tested (72% increase at 10 microg l(-1)) although inhibited Lemna in a linear dose-dependent manner with a 25% reduction over control levels at a dosage of 1 mg l(-1). The 7 days effective concentration (EC(50)) calculated for Lemna were 5.6, 1 and 4 g l(-1), respectively, for erythromycin, tetracycline and ibuprofen. Moreover, exposure to the three pharmaceuticals resulted in the production of the stress hormone, abscisic acid (ABA), in Lemna. Erythromycin and tetracycline were more effective in promoting ABA synthesis compared to ibuprofen. The effects shown by the three therapeutic drugs on Synechocystis and Lemna growth may have potential implications in the assessments of residual environmental risks associated with the presence of pharmaceuticals in freshwater ecosystems. Promotion of ABA synthesis in Lemna by the two antibiotics and by copper suggests that the plant hormone could be a suitable (additional) indicator for future evaluation of phytotoxicity that results in plant senescence.     

Real‐time visualization of oxidative stress in a floating macrophyte Lemna minor L. exposed to cadmium,copper, menadione,and AAPH

An ultra‐sensitive digital imaging system was employed to visualize oxidative stress in intact L. minor plants exposed to Cd, Cu, menadione, AAPH, and ascorbate in real time. The increase of ROS production was assessed by measuring the rate of fluorescence intensity increases of the test medium supplemented with a fluorescing probe (dichlorofluorescein diacetate). The addition of 100 μM CdCl₂ or 100 μM CuSO₄ to the growth medium resulted in a significant increase of medium fluorescence. Additionally, CuSO₄ caused a significantly higher fluorescence intensity than CdCl₂ did. A strong positive correlation (R² = 0.99) between menadione concentration and fluorescence intensity was observed. The positive correlation between AAPH concentration and fluorescence intensity was not as strong as in the case of menadione (R² = 0.81). Menadione induced a stronger oxidative stress than similar concentration of AAPH. The addition of 100 μM ascorbate to L. minor treated with 50 μM menadione significantly reduced the fluorescence intensity increase. A linear trend of the fluorescence increase was observed in all treatments, indicating that chemical‐induced oxidative stress is a gradual process and that the applied concentrations of the chemicals caused a constant increased production of ROS with different intensities, depending on the treatment. This is the combined result of a gradual diminishing of antioxidant reserves and accumulating oxidative damage. The observed rates of ROS production were slower than those in the studies using cell cultures. © 2009 Wiley Periodicals, Inc. Environ Toxicol 25: 573–580, 2010.     

Combined effects of cadmium and linear alkyl benzene sulfonate on Lemna minor L.

The effects of 0.1 ppm cadmium and 0.005% linear alkyl benzene sulfonates (LAS) on the uptake and metabolic incorporation of ¹⁴C glycine by Lemna minor L., after 2, 24 and 48 h were studied for antagonistic/synergistic effects. Combined exposure was found to decrease the ¹⁴C incorporation into proteins, DNA, RNA and phospholipids, to a greater extent than individual exposure. The presence of LAS increased the uptake of ¹⁰⁹Cd in the plants.     

Evaluation of monochloroacetic acid (MCA) degradation and toxicity to Lemna gibba,Myriophyllum spicatum,and Myriophyllum sibiricum in aquatic microcosms

The fate of monochloroacetic acid (MCA), a common phytotoxic aquatic contaminant, and its toxicity to the aquatic macrophytes Lemna gibba (L. gibba), Myriophyllum spicatum (M. spicatum), and Myriophyllum sibiricum (M. sibiricum) under semi-natural field conditions was studied. Replicate 12,000 l enclosures were treated with 0, 3, 10, 30 and 100 mg/l of MCA. Each microcosm was stocked with eight individual apical shoots of M. spicatum and M. sibiricum 1 day prior to initiation of exposure. Plants were sampled after 4, 7, 14 and 28 days of exposure and their response assessed using numerous somatic and biochemical endpoints. L. gibba was introduced into the microcosms the day of MCA treatment and monitored regularly for 21 days. The half-life of MCA in the water column ranged between 86 and 523 h. The most sensitive plant species was M. spicatum, followed by M. sibiricum and L. gibba. All species demonstrated toxicity within a threefold range of each other. Endpoint sensitivity varied depending on the duration of exposure and the level of effect chosen. Most species endpoint EC(x) values were less than an order of magnitude different. Citrate levels in Myriophyllum spp. were not influenced by exposure to MCA. The toxicity of MCA to M. spicatum and M. sibiricum was very similar and thus highly predictive of toxicity observed for each other. The EC(10) was a more conservative estimate of toxicity than the statistically derived no observed effect concentration. Current concentrations of MCA are not likely to pose a risk to these aquatic plants in surface waters.     

Growth response of the duckweed <Emphasis Type="Italic">Lemna gibba</Emphasis> L. to copper and nickel phytoaccumulation

To assess the tolerance and phytoaccumulation ability of the duckweed Lemna gibba L. to copper (Cu) and nickel (Ni), the plants were exposed to different concentrations of Cu and Ni (0.1–2.0 mg/l) under laboratory conditions. The results showed that Cu and Ni were tolerated by L. gibba at concentrations ≤0.3 and ≤0.5 mg/l, respectively. However, plant growth decreased by 50% (I₅₀) when the medium contained 0.45 mg Cu/l or 0.75 mg Ni/l. The observed LCI (lowest concentration causing complete inhibition) were 0.5 and 1.0 mg/l respectively in the presence of Cu and Ni. Results from metal analysis in plant biomass revealed a high accumulation of Cu (1.5 mg g⁻¹ DW), a low accumulation of Ni (0.5 mg g⁻¹ DW) within the plants and a corresponding decrease of metals in the water. The removal percentage of Cu was about 60–80%. We conclude that the duckweed L. gibba L. showed a higher accumulation potential for Cu from polluted water than Ni after 4 days of exposure.     

Simultaneous effects of two fungicides (copper and dimethomorph) on their phytoremediation using Lemna minor

Effects of two fungicides, copper and dimethomorph ((E,Z)4-[3-(4-chlorophenyl)-3-(3-4dimethoxyphenyl) acryloyl] morpholine) on Lemna minor growth and phytoremediation were evaluated. The toxicity of copper and dimethomorph alone and in combination, was assessed by growth inhibition of L. minor cultures after 96 and 168 h. Copper had a severe impact on growth (max. inhibition: 90 % at 1,000 μg L^?1) while dimethomorph (as pure ingredient or formulated as Forum) did not (inhibition <45 % at 1,000 μg L^?1) after 168 h of treatment. When both chemicals were combined, synergism was observed after 96 h of exposure to copper and Forum. However, this interaction was a simple additivity after 168 h. Additivity was also observed when the pure active ingredient (dimethomorph) replaced Forum in the mixture of copper and dimethomorph at 96 and 168 h. L. minor showed an excellent performance in removing copper from the medium since after 96 h, 36, 60, and 76 % removal were reached for 10, 20, and 30 μg L^?1 of Cu respectively. Copper accumulated in the plants. The removal of copper increased with Forum concentration. After 96 h copper (10 μg L^?1 initial concentration) elimination increased from 36.39 ± 5.86–60.70 ± 6.06 % when Forum concentration increased from 0 to 500 μg L^?1. Accumulation of copper in plants was also increased by Forum but not by the active ingredient alone. Depuration of Forum by L. minor varied between 10 and 40 % after 96 h and it was generally more efficient than that of the pure ingredient. This depuration decreased in the presence of copper possibly due to the metal toxicity.     

Phytotoxic effects of cyanobacteria extract on Lemna minor and Myriophyllum spicatum phyto-tolerance and superoxide dismutase activity

The research on the effects of microcystins on aquatic plants has increased. Some aquatic plants have some tolerance to microcystins but the mechanism of the tolerance is still unknown. In this experiment, we used microcystins of different concentrations to study the toxic effect in Lemna minor and Myriophyllum spicatum. Experiments were carried out with a range of microcysitns levels (equivalent to 0, 0.1, 0.5, 1.0, and 4.3 mg/L). The growth of L. minor (as fresh weight) and chlorophyll a content were significantly reduced and superoxide dismutase (SOD) activity was significantly decreased at microcystins concentration up to 0.5 mg/L. The growth of M. spicatum was affected, only weakly, by microcystins and 0.5 mg/L and these treatments caused significant decrease in chlorophyll a content. Besides, the SOD activity of M. spicatum positively correlated to microcystins concentration (P < 0.01). The result indicated that M. spicatum was more tolerant to microcystins than L. minor and the induced SOD activity may contribute to the tolerance. The experiment also indicated that catalase (CAT) activity was not significantly influenced by microcystin for both the two tested aquatic plants.