How Microplastics Influence the Microbial Communities of Periphytic Biofilm Between the Paddy Soil and Water Interface: A Microcosm Study

Microplastics (MPs) are emerging pollutants that affect soil–microbe interactions in paddy ecosystems. Periphytic biofilms (PBs) are complex microbial consortia that ubiquitously distribute at the soil–water interface of paddy ecosystems, playing essential roles in nutrient cycling and pollutant migration. However, whether MPs affect the community composition of PBs remains largely unknown. This microcosm study investigated the effects of three types of MPs (polyacrylonitrile, PAN; polyethylene, PE; and polyethylene terephthalate, PET) on the community characteristics of PBs via high-throughput sequencing (16S/18S rRNA) technology. Results showed that the addition of all MPs significantly increased the biomass and chlorophyll-a content of PBs, with PAN inducing the maximum increase (by 331.9% and 128.6%). However, all MPs had no significant effect on the PB α-diversity of bacterial and eukaryotic communities (p > 0.05). As for PB composition, PAN and PET increased the relative abundance of Cyanobacteria, Proteobacteria and Holozoa, PE increased that of Cyanobacteria, Bacteroidota and Blastocladiomycota, and all MPs decreased the relative abundance of Chloroflexi, Actinobacteriota and Basidiomycota. Furthermore, PET decreased the predicted functional potential of natural polymer degradation (cellulolysis, ligninolysis, xylanolysis, ureolysis), nitrogen fixation and nitrate ammonification, while PE increased predicted potential for plastic degradation, nitrate reduction and denitrification. Co-occurrence network analysis suggested that the PE network showed higher connectivity and lower modularity, while the PAN network showed higher modularity. This study advances our understanding of soil MPs–microbe interactions under high-concentration conditions. It also suggests that PB community characteristics may serve as potential bioindicators for soil MP pollution.