Micro- and nanoplastic stress intensifies Microcystis aeruginosa physiology and toxin risks under environmentally relevant water chemistry conditions

Microplastics (MPs) and nanoplastics (NPs) have emerged as contaminants of concern in freshwater ecosystems, yet their impacts on cyanobacteria and toxin dynamics under environmentally relevant conditions remain poorly understood. In this study, we investigated the effects of 10 μm MPs and 20 nm NPs at environmentally relevant (0.5 mg L) and projected future (4 mg L) concentrations on Microcystis aeruginosa under simulated, natural ionic conditions. Our results demonstrated that both MPs and NPs significantly enhanced algal biomass and microcystin (MCs) production. MPs primarily promoted intracellular toxin synthesis, while NPs significantly facilitated extracellular MCs release under co-exposure scenarios. Linear mixed-effects modeling and correlation analysis revealed that total MCs accumulation was driven by a combination of increased biomass, elevated per-cell toxin production, and enhanced release. Furthermore, confocal laser scanning microscopy revealed strong spatial colocalization between plastic particles and extracellular polymeric substances (EPS) in high-concentration groups, suggesting potential formation of EPS-plastic microenvironments that may stabilize algal aggregates and prolong toxin persistence in the water column. Our findings highlight that even at environmental concentrations, MPs and NPs can synergistically intensify cyanobacterial toxicity via multiple pathways. These results underscore the need to incorporate EPS-plastic interactions and realistic environmental conditions into future risk assessments of plastic pollution in aquatic systems.