Toxicities Of Herbicide Paraquat To Microalgae

Herbicides could destroy ecological environment although they can improve crop production. Algae occupy an important position as primary producers of aquatic ecosystems. Toxicity of herbicide on algae will have an impact on the whole aquatic ecosystem. On the other hand, algae are sensitive to water pollution, so they are often used as target species to assess the toxicity of pollutants, Toxicity values of herbicides on algae have become the basis for effective evaluation of ecological effects. In this research, we choose some typical algal species selected from the less pollution water body in the downstream area of the Yangtze River, and test toxic effects of a wide used herbicide paraquat (1,1’-dimethyl-4,4’-bipyridinium dichlorine) on the algae in order to provide scientific evidence for setting standard of herbicide discharges and water quality management.Four cyanobacterial strains (Microcystis aeruginosa XW01, M. aeruginosa PCC7806, Merismopedia sp., Synechocystis PCC6803) and two green algal strains (Chlorella pyrenoidosa, Chlorococcum sp.) were used as target species. To assess the toxic effects of herbicide paraquat on the growths, the growth curve and the content of Ch1 a of 6 algal strains were measured in different concentrations of paraquat. The EC50 (Median Effect Concentration), LOEC (Lowest Observable Effective Concentration), NOEC (No Observable Effective Concentration) and ChV value (Chronic Value) of paraquat to 6 algal strains were calculated. Additionally, we measured the cell membrane permeability and the nitrate reductase activity of the 6 algal strains to explore the effects of paraquat on algal physiological metabolism.The experimental results showed that the cyanobacterial strains were more sensitive than green algae to paraquat, and the cell membrane permeability of cyanobacterial strains and algal strains were stronger under the higher concentrations of paraquat,96 h-EC50 value of C. pyrenoidosa was 7.4 times higher than the value of M. aeruginosa XW01. Stronger light intensity remarkably increased the toxicities of paraquat to the cyanobacteria and the algae. The 96 h-EC50 values of all strains under stronger light (45 μmol/m2/s) were significantly lower than under weak light (15 μmol/m/s). The 96 h-EC50 values of strain PCC7806 and C. pyrenoidosa under the weak light were 11.8 times and 2.28 times of their values under the stronger light respectively. The toxicities of paraquat to the algae depended on exposure time. The longer the algae exposed, the stronger the toxicity was. According to the experiment, the NOEC value of paraquat to cyanobacterial strains is 0.015 mg/L.The results obtained in this study could provide a basic data for the establishment of standards for paraquat pollution control in water body.