Extracellular polymers substances towards the toxicity effect of Microcystis flos-aquae under subjected to nanoplastic stress

The widespread presence of nanoplastics in aquatic ecosystems and their harmful effects on algae have garnered significant attention. However, little is known about the mechanisms of extracellular polymeric substances (EPS) derived from algae in response to nanoplastic stress. This study investigated the impact of EPS on the toxicity of polyvinyl chloride (PVC, 537 nm) and polymethyl methacrylate (PMMA, 485 nm) nanoplastics on Microcystis flos-aquae (MFa)under nanoplastic stress. The results revealed that EPS removal reduced algal biomass. PVC nanoplastics (250 mg L) caused biomass inhibition of -16.87% before and -9.82% after EPS removal. PMMA nanoparticles exhibited a more significant inhibition of growth and chlorophyll synthesis compared to PVC. After EPS removal, algal cells gradually recovered their maximum quantum yield of photosystem II and exhibited increased superoxide dismutase (SOD) enzyme activity, suggesting a self-regulation mechanism. Nanoplastic stress elevated EPS protein and polysaccharide levels, with maxima of 12.38 mg L at 50 mg L PVC and 17.24 mg L at 100 mg L PMMA. At the same time, the polysaccharide content in nanoplastics was significantly higher than that of proteins, with the maximum value being 2.82 times that of proteins. Fourier-transform infrared spectroscopy (FTIR) and excitation-emission matrix (EEM) analyses showed that aldehyde functional groups on the surface of algal cells were oxidized into carboxylic acids by both types of nanoparticles. Exposure to different nanoplastics increased humic-like substances in tightly bound EPS (TB-EPS), indicating that EPS dynamically adjusts to reduce nanoplastic toxicity by enhancing viscosity and algal aggregation. These results demonstrate that EPS mitigates the direct contact between algal cells and nanoplastics by increasing viscosity and promoting algal self-aggregation, thereby reducing the toxicity of nanoplastics to algae. This phenomenon is consistent across various stress conditions, providing valuable insights into the self-protection mechanisms of microalgae against nanoplastic stress.