Enantioselective Toxic Effects of Prothioconazole toward Scenedesmus obliquus

Prothioconazole (PTC) is a broad-spectrum triazole fungicide with one asymmetric center and consists of two enantiomers, R-(−)-PTC and S-(+)-PTC. To address the concern of its environmental safety, the enantioselective toxic effects of PTC on Scendesmus obliquus (S. obliquus) were investigated. PTC racemates (Rac-PTC) and enantiomers exhibited dose-dependent acute toxicity effects against S. obliquus at a concentration from 1 to 10 mg·L⁻¹. The 72 h-EC50 value of Rac-, R-(−)-, and S-(+)-PTC is 8.15, 16.53, and 7.85 mg·L⁻¹, respectively. The growth ratios and photosynthetic pigment contents of the R-(−)-PTC treatment groups were higher than the Rac- and S-(+)-PTC treatment groups. Both catalase (CAT) activities and esterase activities were inhibited in the Rac- and S-(+)-PTC treatment groups at high concentrations of 5 and 10 mg·L⁻¹, and the levels of malondialdehyde (MDA) were elevated, which exceeded the levels in algal cells for the R-(−)-PTC treatment groups. PTC could disrupt the cell morphology of S. obliquus and induce cell membrane damage, following the order of S-(+)-PTC ≈ Rac-PTC > R-(−)-PTC. The enantioselective toxic effects of PTC on S. obliquus provide essential information for its ecological risk assessment.