Abstract: Microplastics (MPs) and nanoplastics (NPs) entering freshwater environments interact with freshwater biofilms, potentially impacting ecosystem processes. There are few reports on the effects of MPs and NPs on the microbial community of freshwater biofilms. In this study, water from Lake Taihu was collected. Based on the microcosm experiment and metagenome technology, polystyrene plastics with different concentrations (1 mg·L^-1, 10 mg·L^-1) and particle sizes (1 μm: MPs; 100 nm: NPs) were exposed to freshwater biofilms cultured in the laboratory to explore its effects on the structure and function of biofilms microbial community. The results showed that the dominant phyla of microbial community in freshwater biofilm were mainly Proteobacteria, Bacteroidota, Actinobacteria, Planctomycetota, and Verrucomicrobiota. Both MPs_1 and NPs_1 decreased the relative abundance of Proteobacteria, and the effect of MPs_1 was more significant. Both MPs_10 and NPs_10 decreased the relative abundance of Planctomycetota and Verrucomicrobiota. Both MPs and NPs significantly decreased the relative abundance of degrading functional bacteria such as Rhodoferax and Acidovorax, but promoted the abundance of tolerant bacteria such as Pseudomonas. NPs_1 increased the relative abundance of Bacteroidota and Actinobacteria, reflecting the differential effect of concentration and particle size. Both MPs and NPs increased community richness and decreased Shannon and Simpson diversity. Such impacts depended on both concentration and particle size, with larger particle sizes and higher concentrations leading to more significant changes. Moreover, they altered the biofilm microbial community structure, and the effect of MPs was stronger than that of NPs, reflecting the particle size effect. Function analysis revealed that MPs and NPs reduced the relative abundance of functional genes for amino acid and carbohydrate metabolism, which may interfere with microbial-mediated carbon and nitrogen cycling processes in freshwater ecosystems, thereby affecting ecological functions.

Key words: microplastics, nanoplastics, freshwater biofilm, metagenome, community structure, diversity, function analysis