Temporal Feedback Loop Drives the Coevolutionary Fate of Microplastics and Surface Biofilms in River Sediments

Microplastics (MPs) are pervasive in river sediments, where surface biofilm formation critically regulates their environmental behavior. However, mechanisms governing the dynamic MP-biofilm interactions remain underexplored. A 60-day in situ sediment incubation, coupled with continuous monitoring of MP physicochemical properties and microbial community characteristics, was conducted to elucidate the interactions between biofilms and MPs with different polymer types (poly(ethylene terephthalate) (PET), polypropylene (PP), and poly(vinyl chloride) (PVC)) and preaging experiences. In the early stage, biofilm development was promoted by oxygen-containing functional groups (OFGs) of MPs, whereas the additive release from PVC suppressed microbial enrichment. Eventually, both the biofilm biomass and extracellular polymeric substances depended strongly on polymer types. Over time, biofilms progressively modified MP surface chemistry, increasing the O/C ratios of PVC, PET, and PP by 0.41, 0.26, and 0.11, respectively, by producing extracellular proteins, especially plastic-degrading enzymes. Therefore, a temporal feedback loop formed in which MP-derived OFGs acted as nutrient sources, reshaping the microbial community structure and selectively enriching plastic-degrading taxa and enzymes, which in turn accelerated MP degradation. This study provides novel insights into the dynamic reciprocal interactions between MP surface chemistry and biofilm communities and advances our understanding of the mechanisms controlling the ecotoxicity and fate of MPs in river sediments.