Study On Combined Pollution And Enantioselective Toxicity Between Heavy Metal And Metolachlor On Scenedesmus Obliquus

Combined pollution between organics and inorganics, especially pesticides and heavy metal has been paied attention to. In this paper, the ecotoxicological effects of combined pollution between heavy metals (Zn2+, Cd2+) and chiral pesticides (metolachlor) have been studied on Scenedesmus obliquus. The main research includes four parts as follows:(1) The acute toxicity and chlorophyll content of S. obliquus were investigated to study and analysis the mechanism of enantioselective toxicity by metolachlor alone or coexisting with Zn2+. The results showed that the trend of the enantioselective toxicity of Rac-and S-metolachlor coexisting with Zn2+was similar to that of Rac-and S-metolachlor alone. The growth inhibition rate of S. obliquus was decreased by the coexistence of Zn2+with high concentrations of metolachlor. The inhibition rates with0.30mg·1-1Rac-and S-metolachlor alone at24h were49.61%and59.73%, and in the coexistence of Zn2+the values were38.41%and42.52%, respectively. The enantioselective toxicity of Rac-and S-metolachlor was expanded and the toxicity of S-metolachlor increased greater than that of Rac-metolachlor. The coexistence of Zn2+showed partial increase in toxicity of metolachlor in low concentrations, while there was antagonistic effect in high content of metolachlor. The trend of chlorophyll content of S. obliquus at96h was in accordance with the growth inhibition.(2) The acute toxicity of combined pollution between Cd2+and S-metolachlor to S. obliquus was studied. A positive correlation between the acute toxicity and concentration/exposure time of Cd2+and S-metolachlor was observed. The growth inhibition rate of S. obliquus with high concentration of Cd2+and S-metolachlor was beyond that of Cd2+and S-metolachlor alone, and was determined as antagonism effect, which is in accordance with the result by the combined toxicity evaluation methods. The independent EC50of Cd2+and S-metolachlor was decreased with the increasing time, the EC50of Cd2+was0.27,0.17,0.16,0.16mg·L-1, respectively, and the EC50of S-metolachlor was0.24,0.16,0.14,0.13mg·L-1, respectively, suggesting the stronger toxicity of S-metolachlor than Cd2+.(3) The effects of combined pollution between Cd2+and S-metolachlor on superoxide dismutase (SOD) activity and cellular membrane permeability to S. obliquus was studied. After a96h-exposure, the total soluble proteins content of S. obliquus decreased with the increasing concentration of Cd2+and S-metolachlor. And the effect of Cd2+on the total soluble proteins content of S. obliquus was more significant than of S-metolachlor. After a96h-exposure, the stimulation of SOD activity was first observed,and then the inhibition with the increasing concentration of Cd2+and S-metolachlor. And because of the high growth inhibition to S. obliquus by combined pollution between Cd2+and S-metolachlor in high concentration, SOD activity was inhibited obviously, though the combined toxicity was antagonism effect. A positive correlation between the cellular membrane permeability of S. obliquus and concentration/exposure time of Cd2+and S-metolachlor was observed. After a72h-exposure, with the increasing concentration of Cd2+, the cellular membrane permeability of algaes were1.46,4.11and7.20times of the control for Cd2+alone, and the effect of S-metolachlor on the cellular membrane permeability of S. obliquus was more significant than Cd2+. The combined toxicity of Cd2+and S-metolachlor on the cellular membrane permeability decreased with the coexistence of S-metolachlor in high concentration.(4) The effects of combined pollution between Cd2+and S-metolachlor on chlorophyll content and chlorophyll fluorescence parameters to S. obliquus was studied. After a96h-exposure, the trend of chlorophyll content of S. obliquus exposed to Cd2+and S-metolachlor was in accordance with the growth inhibition. The effect of Cd2+on chlorophyll fluorescence parameters such as Fo, Fv/Fm, Fv/F0, Y(Ⅱ), qP, NPQ and rETR of S. obliquus was more significant than of S-metolachlor. The effects of Cd2+and S-metolachlor on Y(Ⅱ), qP, NPQ and rETR increased with the increasing concentration of Cd2+and S-metolachlor, which lead to the damage of the PS Ⅱ in algaes, the inhibition of the primary reaction of photosynthesis, and the process of protosynthetic electron transport stopped, the ability of heat dissipation in algaes lost.