Eutrophication drives taxonomic and functional trajectories in plastic-associated biofilms

A significant amount of plastic debris enters our aquatic ecosystems annually, serving as vectors for pathogens and harmful chemical contaminants. Such plastic pollution is expected to worsen, but the interplay between plastic-associated microbiomes and other aquatic stressors remains underexplored. This study investigates the effect of eutrophication and algal blooms on the structure and function of plastic-associated biofilms in freshwater. Low-density polyethylene (LDPE) films were incubated in constructed freshwater microcosms to assess biofilm succession over a 42-day period. The microcosms were subjected to light and nutrient enrichment simulating eutrophic conditions, and then compared with light-restricted controls with lower nutrient levels. Our analysis revealed that eutrophication significantly alters the composition of the plastic-associated biofilm community. Specifically, biofilms formed under simulated eutrophic conditions were enriched with bacterial strains harboring genes encoding key LDPE-degrading enzymes, such as alkane hydroxylase, copper oxidase, and esterase. The enrichment of these putative plastic-degrading strains co-occurred with an increase in taxa harboring gene homologs encoding enzymes for prokaryotic Wzy-dependent, ABC transporter-dependent, and Synthase-dependent biosynthesis of EPS. Both functional groups of bacteria primarily belonged to the classes Alphaproteobacteria, Gammaproteobacteria, and Cyanobacteriia. We propose that the enhanced biofilm matrix formed under eutrophic conditions is consistent with facilitating colonization of putative plastic degraders. These findings demonstrate the synergistic effects of plastic pollution and eutrophication on freshwater ecosystems, and call for holistic water management strategies.