Many pesticides are chiral compounds and stereochemistry is an important factor for any reaction of chiral structures in biological systems. In this study, experiment about bioaccumulation of the two metalaxyl enantiomers in Tubifex (Oligochaeta, Tubificida) was conducted in laboratory aquatic ecosystems. Terrestrial soil spiked with two dose levels of metalaxyl was employed as the artificial bottom substrate. A method of determination of metalaxyl enantiomers in tubifex tissue, soil and overlying water were developed by HPLC. During a 14-day exposure, concentrations of metalaxyl in tubifex increased with the of soil concentration, however, the enantioselective bioaccumulation was only detected at high-dose exposure group, with the preferential accumulation of (-)-(R)-metalaxyl. The bioturbation activity of tubifex decreased water clarity and released soil-associated metalaxyl to overlying water. In those experiments where tubifex was exposed to metalaxyl from soil, pore water and overlying water, each route contributed to the total body burden, and our results indicated the pore water and soil are the primary exposure routes for high-dose exposure concentration treatment. Chirality 26:33–38, 2013. © 2013 Wiley Periodicals, Inc.

Enantioselectivity in ecotoxicity of chiral pesticides in the aquatic environment has been a subject of growing interest. In this study, the toxicological impacts of hexaconazole enantiomers were investigated with freshwater algae Scenedesmus obliquus. After 96 h of exposure, the EC₅₀ values for rac-hexaconazole, (+)-hexaconazole, and (−)-hexaconazole were 0.178, 0.355, and 0.065 mg l⁻¹, respectively. Therefore, the acute toxicities of hexaconazole enantiomers were enantioselective. In addition, the different toxic effects were evaluated when S. obliquus were exposed to 0.2, 0.5, and 1.0 mg l⁻¹ of rac-hexaconazole, (+)-hexaconazole, and (−)-hexaconazole during 96 h, respectively. The chlorophyll a and chlorophyll b contents of S. obliquus treated by (−)-hexaconazole were lower than those exposed to (+)-hexaconazole, whereas the malondialdehyde contents of S. obliquus treated by (−)-form were higher than those exposed to (+)-form at higher concentrations. In general, catalase activities were significantly upregulated by exposure to (−)-enantiomer than (+)-enantiomer at all three concentrations. However, superoxide dismutase activities exposed to (−)-hexaconazole were lower than that exposed to (+)-hexaconazole at 0.2 mg l⁻¹ and 0.5 mg l⁻¹. On the basis of these data, the acute toxicity and toxic effects of hexaconazole against S. obliquus were enantioselective, and such enantiomeric differences must be taken into consideration in pesticide risk assessment. Chirality 24:610–614, 2012. © 2012 Wiley Periodicals, Inc.