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, <i>R</i>-(-)-PTC and <i>S</i>-(+)-PTC. To address the concern of its environmental safety, the enantioselective toxic effects of PTC on <i>Scendesmus obliquus</i> (<i>S. obliquus</i>) were investigated. PTC racemates (<i>Rac</i>-PTC) and enantiomers exhibited dose-dependent acute toxicity effects against <i>S. obliquus</i> at a concentration from 1 to 10 mg·L^-1. The 72 h-EC50 value of <i>Rac</i>-, <i>R</i>-(-)-, and <i>S</i>-(+)-PTC is 8.15, 16.53, and 7.85 mg·L^-1, respectively. The growth ratios and photosynthetic pigment contents of the <i>R</i>-(-)-PTC treatment groups were higher than the <i>Rac</i>- and <i>S</i>-(+)-PTC treatment groups. Both catalase (CAT) activities and esterase activities were inhibited in the <i>Rac</i>- and <i>S</i>-(+)-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 <i>R</i>-(-)-PTC treatment groups. PTC could disrupt the cell morphology of <i>S. obliquus</i> and induce cell membrane damage, following the order of <i>S</i>-(+)-PTC ≈ <i>Rac</i>-PTC > <i>R</i>-(-)-PTC. The enantioselective toxic effects of PTC on <i>S. obliquus</i> provide essential information for its ecological risk assessment.