The Toxic Mechanisms Of Metolachlor To Scenedesmus Obliquus

Rational use and environmental security of chiral pesticides have gained the interest of many researchers. In this paper, the enantioselective effect on acute toxicity and oxidative stress of Scenedesmus obliquus by metolachlor were determined. The chlorophyll content, cellular membrane permeability and cell ultrastructures of S. obliquus caused by metolachlor were investigated as well to study and analysis the mechanism of enantioselective toxicity by metolachlor. It is to offer the theoretical foundation for the production and rational use of chiral pesticides, as well as the ecological risks. In our study, a positive correlation between the acute toxicity and concentration/exposure time of metolachlor was observed; For the early treatment time, low growth inhibition and even stimulation on algae growth was showed at the low concentration treatment (<0.15mg/L). And with the increasing time, the more obvious stimulation was showed, the cell growth was stimulated by 11.7%and14.6%, respectively, by Rac-and S-metolachlor after48h, indicating that S. obliquus under low concentration treatment could recover growth. The value ET(enantioselective toxicity)>1, from24h to96h, suggesting the stronger toxicity of S-metolachlor. And the enantioselective toxicity of metolachlor was relative stable with the increasing time. Further study on the oxidative stress and damage by metolachlor, showed that a significant increase of ROS content in herbicide treatment compared to the control, especially at S-metolachlor treatment. It was0.74times (0.05mg/L),1.92times (0.1mg/L),8.31times (0.2mg/L) and1.08times (0.3mg/L) of S-metolachlor to Rac-metolachlor. SOD and CAT at the relative low concentration were stimulated at varying degrees. SOD activity of0.2mg/L Rac-and S-metolachlor was1.58and3.18times of the control, correspondingly, CAT activity of that was2.14and3.10times of the control, indicating the enantioselective oxidative stress by Rac-and S-metolachlor. With the increasing concentration of herbicide, a decrease trend of antioxidant enzyme activity was observed, while a continuously going up trend in ROS production was showed, suggesting the balance of ROS production in algae cells might be disturbed, resulting in overproduction of ROS. In addition, significant effects on chlorophyll content, cellular membrane permeability and cell ultrastructures of S. obliquus by herbicide, especially S-metolachlor treatment, were observed in our study. Chl a and Chl b content at0.3mg/L Rac-metolachlor were69.4%and79.5%of the control, respectively. For S-metolachlor, they were94.1%and94.5%of the control, indicating the stronger effect on the photosynthesis of algaes. Accordingly, the cellular membrane permeability of algaes exposed to0.3mg/L Rac-and S-metolachlor were6.19and42.5times of the control. And strong ROS content and chlorophyll content/cellular membrane permeability/growth inhibition were observed. Moreover, damages on the chloroplast and cell membrane of S. obliquus were observed by transmission electron microscopy. The cell wall of treated algaes separating from the cell membrane, obscure chloroplast grana lamellae, declined starch granules in the chloroplast and an increase in the number of vacuoles with certain black deposits of S. obliquus exposed to the herbicide were observed, especially in the cells of S-metolachlor treatment.Two types of cyclodextrins, a-cyclodextrin (a-CD) and β-cyclodextrin (β-CD), were selected to evaluate the effects of the enantioselective ecotoxicity of Rac-and S-metolachlor on S. obliquus. The results showed that the toxicity of S-metolachlor was higher than Rac-metolachlor and that CDs enhanced the toxicity of metolachlor enantioselectively by increasing the toxicity of Rac-metolachlor rather than that of S-metolachlor to S. obliquus.64.5%and40.9%increase of the96h-acute toxicity of Rac-and S-metolachlor were observed, respectively, by a-CD; And for β-CD, they were68.0%and41.8%. The equilibrium constant:Kd (Rac-metolachlor-β-CD)> Kd (Rac-metolachlor-a-CD)> Kd (S-metolachlor-β-CD)> Kd (S-metolachlor-a-CD), which was responsible for the greater toxicity shift effect of Rac-metolachlor. Thermodynamic studies of cyclodextrin complexes, with△H<0,△S<0and△G<0, showed that inclusion for all of the complexes was primarily a exothermic, spontaneous and enthalpy-driven process.