Influence of extracellular polymeric substances on cell-NPs heteroaggregation process and toxicity of cerium dioxide NPs to Microcystis aeruginosa

Highlights: • The presence of EPS alter the heteroaggregation and toxicity process. • Adding back BEPS mitigate heteroaggregation process between n-CeO₂ and cyanobacteria. • BEPS act more effectively in buffering against the toxicity of n-CeO₂ to cells than SEPS. • SEPS extend the retention time of n-CeO₂ in the solution. The presence of abundant extracellular polymeric substances (EPS) play a vital role in affecting heteroaggregation process and toxicity of nanoparticles (NPs) to Microcystis aeruginosa. Interactions between n-CeO₂ and cyanobacteria with/without EPS and the toxicity of n-CeO₂ to M. aeruginosa were investigated in this study. Aggregation kinetics of n-CeO₂ under both soluble EPS (SEPS) and bound EPS (BEPS) indicated the presence of EPS could induced the formation of EPS-NPs aggregates. Heteroaggregation between cells and n-CeO₂ was confirmed through co-settling experiment and SEM-EDS observation. SEPS contributed to the observable heteroaggregation using spectral measurement. Heteroaggregation between cells and n-CeO2 under no BEPS was hardly obtained through spectral measurement, but SEM-EDS observation convinced this process. And the DLVO theory explained this heteroaggregation process under various EPS conditions, where the energy barrier decreased with gradual EPS extraction. In addition, the order for 96 h half growth inhibition concentration (IC₅₀) was Raw M9 > M9-SEPS > M9+BEPS > M9-BEPS. These results revealed that not all heteroaggregation between cell-NPs can lead to the NPs toxicity to cells. BEPS act more important role in buffering against the toxicity of NPs from ambient adverse factors, but SEPS increase the stability of NPs which could aggravate the adverse effects of NPs in the environment.