Nanoplastic-mediated disruption of freshwater carbon cycling via modulating of plankton communities

Nanoplastics (NPs) have emerged as environmental pollutants of global concern, attracting widespread scientific attention. Although previous studies have revealed the toxic effects of NPs across multiple biological levels, their effects on community structure and ecosystem functions in aquatic ecosystems remain poorly understood. Thus, in this study, we investigated the effects of polystyrene nanoplastics (PS-NPs; 80, 200, and 500 nm) at an environmentally relevant concentration (1 mg/L) on plankton-mediated carbon cycling processes through their exposure in freshwater mesocosms. Results showed that PS-NPs significantly altered the community structures of zooplankton and bacteria but had relatively limited effects on phytoplankton and fungal communities. They enhanced dissolved organic carbon decomposition by modulating the life-history strategies, carbon utilization capacity, and metabolic pathways of bacterial communities. Meanwhile, they inhibited the decomposition of particulate organic carbon in the water, thereby significantly reducing the CO release rate at the water-air interface (by 95-114 %) and ultimately disrupting carbon decomposition and release processes in aquatic ecosystems. PS-NPs of different sizes exhibited distinct regulatory effects on carbon cycling mediated by different plankton communities. In particular, smaller-sized PS-NPs promoted carbon source degradation by enhancing microbial carbon utilization capabilities, whereas larger-sized PS-NPs facilitated carbon fixation by increasing the photosynthetic activity of phytoplankton communities. This study elucidated how environmentally relevant concentrations of NPs interfere with carbon cycling in freshwater ecosystems, offering novel insights into how plastic pollution disrupts biogeochemical processes in aquatic ecosystems.