Enantioselective toxic effects and biodegradation of benalaxyl in Scenedesmus obliquus

Enantioselectivity in ecotoxicity and biodegradation of chiral pesticide benalaxyl to freshwater algae Scenedesmus obliquus was studied. The 96 h-EC50 values of rac-, R-(-)-, S-(+)-benalaxyl were 2.893, 3.867, and 8.441 mg L-1, respectively. Therefore, the acute toxicities of benalaxyl enantiomers were enantioselective. In addition, the pigments chlorophyll a and chlorophyll b, antioxidant enzyme activities catalase (CAT) and superoxide dismutase (SOD) as well as lipid peroxide malondialdehyde (MDA) were determined to evaluate the different toxic effects. Chlorophyll a was induced by S-(+)-benalaxyl but inhibited by R-(-)-benalaxyl at 1 mg L-1. Chlorophyll b were both induced at 1 mg L-1, but S-(+)-form was fourfold higher than R-(-)-form. S-(+)-benalaxyl inhibited more CAT activities at 3 mg L-1 and 5 mg L-1, induced less SOD activity and MDA content at 5 mg L-1 than R-(-)-benalaxyl. Based on these data, enantioselectivity occurred in anti-oxidative stress when S. obliquus response to benalaxyl. In the biodegradation experiment, the half-lives of S-(+)-benalaxyl and R-(-)-benalaxyl were 4.07 d and 5.04 d, respectively, resulting in relative enrichment of the R-(-)-form. These results showed that toxic effects and biodegradation of benalaxyl in S. obliquus were enantioselective, and such enantiomeric differences must be taken into consideration in pesticide risk. (C) 2011 Elsevier Ltd. All rights reserved.