Enantioselective toxic effects and biodegradation of benalaxyl in Scenedesmus obliquus

Abstract

Enantioselectivity in ecotoxicity and biodegradation of chiral pesticide benalaxyl to freshwater algae Scenedesmus obliquus was studied. The 96h-EC50 values of rac-, R-(−)-, S-(+)-benalaxyl were 2.893, 3.867, and 8.441mgL−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 1mgL−1. Chlorophyll b were both induced at 1mgL−1, but S-(+)-form was fourfold higher than R-(−)-form. S-(+)-benalaxyl inhibited more CAT activities at 3mgL−1 and 5mgL−1, induced less SOD activity and MDA content at 5mgL−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 4.07d and 5.04d, 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.