Enantioselective toxicity, degradation and transformation of the chiral insecticide fipronil in two algae culture

The occurrence of pesticides and their metabolites in the environment can alter the ecological relationships between aquatic food chains. Fipronil is a broad-spectrum insecticide which release in the environment may harm the non-target organisms. However, the toxicity and biotransformation of its two enantiomers are far from fully understood. The present study aimed to investigate the aquatic toxicity and environmental behavior of fipronil at enantiomeric level using two freshwater algae, Scenedesmus quaclricauda (S. quaclricauda), and Chlorella vulgaris (C. vulgaris) through an integrative approach the transformation process of the individual enantiomer isolated and in racemic form. The 72 h-EC50 values of rac-, R-, S-fipronil varied from 3.27 to 7.24 mg L−1 with R-fipronil posing a more significant effect on algal growth inhibition. Chlorophyll a was more susceptible to fipronil exposure than chlorophyll b and carotenoids. Enantioselective alterations on physiological and biochemical parameters (chlorophyll a, chlorophyll b, carotenoids, and the activities of antioxidant enzyme catalase (CAT) and superoxide dismutase (SOD)) were also observed. The half-lives (T1/2) of R-fipronil and S-fipronil in algae culture were 3.4–3.5 d and 4.0–4.9 d, respectively. By the end of the 17-d exposure, the enantiomer fractions (EFs) increased to 0.59, indicating a preferential depuration of R-fipronil. The metabolites monitoring showed the fipronil sulfide was the main metabolite followed by fipronil sulfone. The results revealed that the enantiomers of fipronil pose enantiospecific behaviors induced by these two algae, with the R-enantiomer more toxic to algal growth and favorable in degradation. These analyses are beneficial for understanding the ecological effect of chiral pesticide in aquatic environment, and the enantiomeric differences of the toxicity, degradation and the formation of toxic metabolites could be helpful for the eco-environmental risk evaluation.