Microbial succession and early transformation signatures of low-density polyethylene and polylactic acid in an urban freshwater stream.

Freshwater plastispheres are dynamic microbial interfaces that form on plastic debris, one of the most pervasive anthropogenic pollutants in freshwater ecosystems. Despite their environmental relevance as vectors for microbial dispersal, pollutant transfer, and plastic degradation, their role in early-stage polymer transformation remains poorly understood. This study combined in situ freshwater incubation, laboratory enrichment, isolation, screening, and chemical profiling to investigate microbial succession and early transformation of low-density polyethylene (LDPE) and polylactic acid (PLA). Amplicon sequencing revealed a two-phase succession, in which early colonizers were replaced by later-stage plastic-associated taxa, with functional prediction indicating transformation-related metabolic potential. Subsequent enrichment and isolation yielded 135 culturable isolates, from which hydrolytically active strains were selected for controlled polymer incubation. Scanning electron microscopy, infrared spectroscopy, reactive oxygen species imaging, and non-target metabolite profiling showed polymer-specific transformation signatures, gradual oxidation-associated modification in LDPE and more limited hydrolysis-oriented alteration in PLA. The field and isolate-based results highlight distinct LDPE and PLA responses and the need to evaluate plastic persistence under realistic freshwater conditions, not by material classification alone.