Hetero-Aggregation of Nanoplastics with Freshwater Algae and the Toxicological Consequences: The Role of Extracellular Polymeric Substances

The presence of nanoplastics (NPs) in freshwater environments has received increasing attention in recent years. However, the hetero-aggregation behaviors of NPs with the co-existing algae and the influence on NP toxicity, especially the potential role of extracellular polymeric substances (EPS) during the entire process, are poorly understood. In this study, the hetero-aggregations of polystyrene (PS) and polylactic acid (PLA) NPs with <i>Chlorella vulgaris</i>, along with their toxicological consequences, were investigated in EPS-containing and EPS-free conditions. The results in the 12 h settling experiments showed that the ΔOD_reduced values ranged from 0.33 to 0.74, and the PS NPs exhibited higher aggregation efficiency with algae than the PLA NPs, which was inconsistent with previous microplastic studies and the Derjaguin-Landau-Verwey-Overbeek calculations. This can be attributed to the unique properties of NPs and the mediating effects of tightly bound and loosely bound EPS during the formation of stable heteropolymers. In the 96 h toxicological experiments, various endpoints for algal growth inhibition, pigment synthesis disturbance, cell membrane damage, and oxidative stress were measured. Both the ΔOD_reduced values and integrated biomarker responses were positively associated with membrane damage and superoxide dismutase activity, demonstrating a view that the hetero-aggregation behavior could affect the membrane integrity and oxidative stress of algal cells, and exacerbate the toxicity of NPs on algae. The present study underscores the material-specific uniqueness of NPs in interactions with freshwater algae. Further studies are needed to broaden our knowledge of the hetero-aggregation behaviors and toxicological effects of NPs.