Toxicity of polystyrene nanoplastic variants to cyanobacteria: Evidence from algal growth, physiology and...

Plastics are continuously discharged into the aquatic environment, and their toxicity to microalgae has attracted widespread attention. However, the toxic mechanisms of differently charged nanoplastics (NPs) remain to be elucidated. In this study, we used different types of polystyrene nanoplastics (PS-NPs) with surface charges (PS-NH₂, PS, and PS-COOH) to investigate their toxicity and toxic mechanisms in Microcystis aeruginosa under relatively low concentration conditions (10∼20 mg L^-1). We found that the PS-NH₂ system can inhibit the growth, photosynthetic parameters of photosystem II and I (PSII and PSI) and reduce the ROS antioxidant system-related enzyme activity levels at relatively high concentrations, especially during the early exposure stage. Electrostatic attraction causes extensive aggregation of cationic PS-NH₂ with algal cells, inducing lipid peroxidation and membrane damage. Detailed analysis showed that differently charged PS-NPs affected photosynthetic efficiency by damaging the electron transport and reaction center of PSII and PSI. Moreover, PS-NH₂ had a negative impact on algal cells compared to the unmodified PS and PS-COOH. Transcriptomic analysis of M. aeruginosa suggests that photosynthesis appears to be the most sensitive metabolic pathways in response to PS-NH₂. In particular, genes related to photosynthesis showed altered expression upon exposure to differently charged PS-NPs. This study reveals molecular toxic mechanisms of charge-variant PS-NPs on M. aeruginosa, verifying surface charge as a key determinant of nanoplastic ecotoxicity in freshwater cyanobacteria.