Oligotrophic–Mesotrophic Divergence Shapes Plastisphere Bacterial Assemblages in Drinking-Water Source Reservoirs

Microplastics in freshwater environments provide persistent substrates for microbial colonization, forming the plastisphere. However, how trophic conditions shape plastisphere bacterial communities in drinking-water source reservoirs remains poorly understood. In this study, nine major drinking-water source reservoirs in Longyan City, Fujian Province, China, were investigated. Water quality measurements, trophic state assessment, and 16S rRNA gene amplicon sequencing were combined to characterize plastisphere bacterial communities across oligotrophic and mesotrophic reservoirs. The comprehensive trophic level index classified four reservoirs as mesotrophic and five as oligotrophic. Bacterial alpha diversity indices showed no significant trophic-dependent pattern, whereas PERMANOVA revealed significant compositional divergence between trophic groups (p < 0.01). Electrical conductivity, pH, and dissolved oxygen were the strongest correlates of community variation. Mesotrophic reservoirs were enriched in Bacillota and Bacteroidota, with biomarkers mainly affiliated with Comamonadaceae, while oligotrophic reservoirs harbored more diverse biomarkers dominated by Pseudomonadota and Cyanobacteriota. Functional prediction indicated that only aliphatic non-methane hydrocarbon degradation differed significantly between trophic groups, whereas nitrogen-cycling functions showed no significant divergence. These findings demonstrate that trophic status acts as a significant environmental filter shaping plastisphere community structure in drinking-water source reservoirs, even within a narrow oligotrophic-to-mesotrophic gradient, providing new insights for ecological risk assessment of microplastics in source-water ecosystems.