Ecotoxicity Of Chiral 1-Alkyl-3-Methylimidazole Tartrate Ionic Liquids To Scenedesmus Obliquus

As a kind of functional ionic liquid, chiral ionic liquids (CILs) have properties of chiral materials. CILs are used as chiral solvents, catalyst or chiral inducer in asymmetric synthesis, spectral analysis and the optical resolution of racemates. However, with the extensive use of CILs, the environmental load and ecological risks have been increasing. At present, the study on the toxicity of chiral ionic liquids is very short. In this paper, the enantioselective effect on growth inhibition rate, photosynthesis, cell structure and oxidative stress of Scenedesmus obliquus (S.obliquus) caused by 1-alkyl-3-methyl imidazolium tartrate (RMIMT) were studied, thus revealing the toxicity and toxicological mechanism of chiral ionic liquids.The results of acute toxicity on S.obliquus were showed:When the exposure time is constant, the growth inhibition rate of algae increased with the increasing of the treatment concentration. With the extension of exposure time, the growth inhibition rate of the low concentration treatment group was decreased, and the growth inhibition rate of the high concentration treatment group increased gradually, It was showed that S.obliquus has the ability to recovert the toxicity of RMIMT at low concentrations. The Logistic model fitting results can be drawn that the acute toxicity of L-(+)-RMIMT to the algae is higher than that of D-(-)-RMIMT and the increasing alkyl chain length increased the RMIMT toxicity to algae and decreased the enantioselective difference, indicating that cation properties have a larger effect on toxicity than anion properties. The toxicity order was that DeMIMT>OMIMT> HMIMT and L-(+)-RMIMT> D-(-)-RMIMT.After 96h, the chlorophyll content and chlorophyll fluorescence intensity were determined in this paper. The results showed that the concentration of chlorophyll a, chlorophyll b and total chlorophyll content decreased, and the photosynthetic efficiency decreased, which decreased the growth rate of the algae. The effect of RMIMT on the chlorophyll content increased with the concentration increasing. RMIMT has significant effect on the fluorescence parameters (F0, Fv/Fm, Fv/F0) and rapid light response curves (Y(II) and NPQ), and it was presenting an obvious does-effect relationship. The results revealed that the PS II reaction center of S.obliquus was inactivated or destroyed by RMIMT. There is an enantioselectivity of RMIMT on the effects of photosynthetic systems, and the impact of of L-(+)-RMIMT treatment is greater than that of D-(-)-RMIMT treatment. The toxicity of RMIMT on photosynthetic systems was increased with increasing of alkyl chain length of cation.Further studies showed that RMIMT affected cell membrane permeability and cellular structure to some extent. The cell membrane permeability of S.obliquus increased markedly when exposed to RMIMT, and the increase of RMIMT concentrations could raise the cell membrane permeability of S.obliquus, Meanwhile, there was a good linear relationship between the cell membrane permeability and growth inhibition ratio of S.obliquus compared with the experimentally measured values, which indicated that RMIMT damaged the normal physiological function of cells through changing the cell membrane permeability. Additionally, a longer alkyl-chain of RMIMT resulted in stronger inhibition of algal growth. It was also found that a series of changes occurred in cells by observing the ultrastructure of cells, such as plasmolysis, the decrease of the lamella structure of chloroplast, the damage of the pyrenoid and nucleoid. RMIMT exhibited enantioselective toxicity to the green algae S.obliquus, which may induced the chiral discrimination to the acute toxicity of S.obliquus.Oxidative stress is a direct response of plants to environmental changes, the results of this study revealed that:under the stress of RMIMT, the ROS content in the cells increased significantly, and there was a dose-effect relationship, both of membrane permeability and growth inhibition rate has a good linear relationship with ROS content, that is means oxidative stress response is activated. Meanwhile vivo antioxidant system is also activated to eliminate ROS, so that intracellular SOD and CAT activity increased. With the increase concentration, the SOD and CAT activity increased at first and then gradually decreases and reaches maximum at about EC50 concentration. Under the high concentration of RMIMT, the algal produced a large number of ROS and accumulated in the cell, which can destroy the biological macromolecules, cell structure, and so on, causing metabolic disorders, and eventually lead to cell death, there is an enantioselectivity toxicity of RMIMT on ROS content and antioxidant enzyme activities of S.obliquus, and the enantioselective was consistent with the growth inhibition rate. Similarly, the toxicity of RMIMT on algal cell ultrastructure increased with increasing alkyl chain length of cation.