Toxic effects of carboxyl/amino polystyrene microplastics and macrolide antibiotics on Chlorella pyrenoidosa: Insights based on physiological level, microstructure, and toxicity prediction.

The combined ecological risks of microplastics and antibiotics in aquatic systems remain poorly understood, particularly regarding how functionalized polystyrene microplastics influence macrolide antibiotic toxicity. This study investigates the toxic effects and mechanisms of carboxyl-modified and amino-modified polystyrene microplastics (PS-COOH, PS-NH2) combined with macrolide antibiotics (Erythromycin, Roxithromycin, Azithromycin) to support ecological risk assessment of co-exposure. Results show that PS-NH2 was more toxic than plain PS or PS-COOH. At environmental concentrations (0.01 mg/L), binary mixtures showed negligible toxicity, whereas at high concentrations (10 mg/L), algal inhibition exceeded 60%. PS-COOH combinations generally exhibited higher toxicity (pEC50 = -0.574 to -2.038) than PS/PS-NH2 combinations (pEC50 = -0.322 to -2.476). PS-NH2 formed heterogeneous aggregates on algal surfaces and attenuated antibiotic effects. Under identical conditions, algal growth inhibition was highest with PS-NH2 (95.33%/85.28%) compared to PS (81.81%/67.02%) and PS-COOH (88.28%/80.91%). The independent influence model outperformed the concentration addition model in predicting mixture toxicity, indicating synergistic/additive or antagonistic effects. Mechanistically, the pollutants inhibited synthesis of photosynthetic pigments (Chla, Chlb, Car), disrupted TP metabolism, and induced oxidative stress-evidenced by increased MDA levels and a "low-promotion, high-inhibition" pattern in SOD and CAT activities. This study underscores the importance of microplastic surface functionality in modulating combined toxicity with antibiotics, offering a theoretical basis for risk control of co-pollution in aquatic environments.