Response of Chlorella sorokiniana to Co-Exposure to Sulfamethoxazole and Polystyrene Microplastics: Toxicity Effect, Defense and Biodegradation Mechanism

Microalgal biotransformation plays an important role in determining the environmental fate of trace antibiotics. However, the role of polystyrene microplastics (PS-MPs) in regulating algal stress responses and antibiotic biotransformation remains poorly understood. In this study, PS-MPs and sulfamethoxazole (SMX) were selected as representative pollutants to investigate the toxic effects, defense responses, and SMX degradation mechanism of typical microalgae, i.e., Chlorella sorokiniana, under single- and combined exposure conditions (PS-MPs: 10 mg/L; SMX: 0.5 and 5 mg/L) over 8 days. The results showed that co-exposure significantly alleviated growth inhibition, photosynthesis inhibition, and reactive oxygen species (ROS) accumulation. Meanwhile, antioxidant enzyme activities decreased, whereas extracellular polymeric substance (EPS) secretion increased markedly, with the highest exopolysaccharide content reaching 83.22 ± 0.13 mg/L in the PS-MPs + 5 mg/L SMX group. Compared with the 5 mg/L SMX group, the biodegradation ratio and total removal ratio of SMX in the PS-MPs + 5 mg/L SMX group increased from 28.90% to 39.50% and from 40.90% to 50.50%, respectively. The relevant research proved that high EPS secretions facilitated SMX degradation and consequently reduced biotoxicity. Moreover, eleven degradation products and three potential transformation pathways were identified during the incubation. During the co-exposure of SMX and PS-MPs, most metabolites exhibited lower toxicity and bioaccumulation potential than SMX, indicating reduced ecological risk in the presence of PS-MPs. These findings provide new insight into the fate of antibiotics in microplastic-contaminated aquatic systems.