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.

► R-(−)-benalaxyl was more toxic than the S-(+)-isomer to Scenedesmus obliquus.
► Toxic effects of benalaxyl racemate and enantiomers to S. obliquus were different.
► The biodegradation of benalaxyl in S. obliquus was enantioselective.