Research articleEffects of copper and cadmium on heavy metal polluted waterbody restoration by duckweed (Lemna minor)
Introduction
Over the last two decades there has been an ever-increasing awareness of how heavy metals are as environmental pollutants. Their presence in the atmosphere, soil, and water, even in trace concentrations, can cause serious problems to organisms [23].
Copper and cadmium are reported to be widespread heavy metal pollutants in natural and wastewaters in China resulting from agriculture and industrial activities such as pigments, mining, smelting and electroplating, etc. [23], [37]. Cu is an essential micronutrient and a component of several enzymes mainly participating in electron flow and catalyzing the redox reactions [6]. But it becomes toxic at high concentrations, whereas Cd has no known biological function and is a highly toxic metal to aquatic organism [50].
A variety of technologies, including chemical, physical and biological methods, have been applied to prevent and treat water pollutions. However, these methods present different efficiencies for different metals and they can be very expensive especially if large volumes, low metal concentration and high standards of cleaning are required [32]. In recent years, interest has been focused on using aquatic plants, such as Lemna minor, Microspora, and Pistia stratiotes et al., as a promising approach to take up heavy metals from water body [8], [13], [19], [26], [27], [36]. A study by Maine et al. [27] showed that cadmium separation by Pistia stratiotes could reach 85% for the concentrations of 1–6 mg l−1. Duckweed (Lemna minor) also has been found to have good ability to accumulate heavy metals at certain concentration. In the experiment Lemna minor could remove 75–90% of lead from water after three weeks' exposure to concentration of 5 mg l−1 [13]. Moreover, duckweed is widely distributed and commonly found year-round in fresh and brackish waters in China and therefore the bioremediation of metal pollution can be inexpensive compared to the traditional means of heavy metal removal. However, the removal of highly polluted water body by Lemna minor may not be a success owing to physiological characteristic of duckweed.
In this paper we studied the responses of duckweed to the increasing concentrations of copper and cadmium, respectively with reference to: (1) changes in soluble protein content; (2) changes in contents of chlorophyll a, chlorophyll b and carotenoid; and (3) changes in the contents of antioxidant enzymes such as, peroxidase (POD), catalase (CAT) and dismutase (SOD), and malondialdehyde (MDA). So this can determine the concentration extent to which Lemna minor is suitable for remediation of water body polluted by heavy metals.
Section snippets
Plant materials and growth condition
Lemna minor used in this study was collected from the region of Taihu lake in China, located at 31°17.408′N, 119°55.101′E. The duckweed fronds were maintained in glass aquariums (0.5 × 0.8 × 0.6 m3), which were placed in a controlled room at 26 ± 2 °C, under illumination provided by metal halide lamps with a light intensity of 72 μmol m−2 s−1 and a LD cycle of 16:10 h for three months as a pre-treatment before experiments. The medium of the pre-treatment was 1/10 Hutner medium [18] with a pump to keep
Effects of copper and cadmium on soluble protein in Lemna minor
Under the condition of heavy metal menace, changes of soluble protein content in Lemna minor were shown Fig. 1, which indicated that exposure to 0.05 mg l−1 of Cu2+ slightly decreased soluble protein content after 4 day of treatment, while 0.5 mg l−1 of Cu2+ decreased protein content significantly. Beyond the concentration, Cu2+ inhibited soluble protein content steadily. While the presence of 0.05 and 0.5 mg l−1 of Cd2+ in growth medium could result in a fast and strong inhibition of protein content,
Effects of copper and cadmium on soluble protein and photosynthetic pigments
The present results clearly indicate that the presence of heavy metals, copper and cadmium ions brought about the toxicity to Lemna fronds. Heavy metals had a strong inhibition effects on soluble protein and photosynthetic pigments of Lemna minor.
Soluble protein content in organisms, an important indicator of reversible and irreversible changes in metabolism, is known to respond to a wide variety of stressors such as natural and xenobiotic [44]. It was reported that Cd or pesticides resulted in
Conclusion
The toxic effects of copper and cadmium on Lemna minor were discussed in this paper. Based on the physiological responses of duckweed under increasing concentrations of heavy metals, following conclusions can be achieved.
- a.
Of two heavy metals, cadmium was more toxic than copper. The antioxidant system had become disordered when Lemna fronds were exposed to 0.5 mg l−1 of cadmium in grown medium.
- b.
At the lowest concentration (0.05 mg l−1), copper did little harm to duckweed. When the concentration
Acknowledgements
Financial assistance provided by Chinese basal keystone research project (973 project) (2002CB412300) and International Science and Technology Cooperation Key research project (2003DFB00018) is fully acknowledged.
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