Response mechanisms of Chlorella sorokiniana to microplastics and PFOA stress: Photosynthesis, oxidative stress, extracellular polymeric substances and antioxidant system

Co-pollution of microplastics and per-and polyfluoroalkyl substances (PFAS) is prevailing in the aquatic envi-ronment. However, the risks of coexisting microplastics and PFAS on organisms remain unknown. This study investigated the response mechanisms of Chlorella sorokiniana (C. sorokiniana) under polystyrene microplastics (PS-MPs) and perfluorooctanoic acid (PFOA) stress, including toxicity and defense mechanisms. C. sorokiniana was exposed to PS-MPs (10 mg/L) and PFOA (0.05, 0.5, and 5 mg/L) and their mixtures for 96 h, respectively. We found that the dominant toxicity mechanism of PFOA and PS-MPs to C. sorokiniana was dissimilar. PS-MPs mainly inhibited photosynthesis through shading effect, while PFOA mainly induced oxidative stress by reactive oxygen species. The co-exposure of PFOA and PS-MPs aggravated biotoxicity (maximum inhibition rate: 27.27 +/- 2.44%), such as photosynthesis inhibition, physical damage, and oxidative stress, compared with individuals. To alleviate toxicity, C. sorokiniana activated defense mechanisms. Extracellular polymeric substances were the first barrier to protect cells, the effect on its secretion was ordered PS-MPs+5PFOA > PS-MPs > 5PFOA, and IBRv2 values were 2.37, 1.35, 1.11, respectively. Antioxidant system was thought of second defense pathway, the in-fluence order of treatment groups was PS-MPs+5PFOA > 5PFOA > PS-MPs, and its IBRv2 values were 2.89, 1.69, 0.25, respectively. Our findings provide valuable information on the complex impacts of PFOA and PS-MPs, which facilitate the ecological risk assessment of multiple pollutants.