Microbial Community Characteristics of Sediment Environment and Plastisphere in a Typical Highly Urbanized River Basin

Microplastics (MPs) provide surfaces for microbial colonization, yet field-based comparisons between plastisphere and surrounding sediment communities in urban rivers remain limited. We investigated sediments from a typical highly urbanized river basin—the Qinhuai River Basin, China—during normal flow (March 2025) and high flow (June 2025). We characterized MP abundance and particle features and compared bacterial and fungal communities in sediment and plastisphere samples using 16S rRNA V4 and ITS1 amplicon sequencing. MPs were detected in all valid samples, yielding 316 particles. Mean abundances were 178.35 ± 97.73 and 158.47 ± 143.18 particles kg-1 dry weight during normal and high flow, respectively; fragments and fibers dominated, and PET was the most frequent polymer among μ-FTIR-analyzed particles. Sediment communities had higher richness and diversity than plastisphere communities for both bacteria and fungi. Community composition differed between habitats (PERMANOVA: bacterial R2 = 0.06888, p = 0.001; fungal R2 = 0.04495, p = 0.002), although the proportion of variation explained by habitat was relatively small. Pseudomonadota and Ascomycota were relatively enriched in the plastisphere. CCA indicated that bacterial variation was associated with TN, NH3-N, water content, and SOC, whereas fungal variation was associated with NH3-N, water content, and TN. Mantel tests linked sediment bacterial community dissimilarity to between-sample differences in PET and PE variables, and fungal community dissimilarity to differences in PVC. These findings indicate modest habitat-associated differences in community composition, alongside associations with sediment properties and microplastic characteristics.