Plastisphere as a unique metabolic hotspot in river water: Impact of plastic substrate biodegradability

Microplastics in aquatic environments predominantly exist as carriers of plastispheres, yet the microbial community structures and co-occurrence patterns within submillimeter plastispheres (formed by approximately 100 μm plastic particles) remain poorly understood. The effects of plastic biodegradability on plastisphere microbiome assembly and function have also not been fully characterized. Hence, this study investigated the micro-ecological composition of submillimeter plastispheres in river water, focusing on plastic degradation and carbon-nitrogen biogeochemical cycling. Compared to surrounding water, these plastispheres create novel ecological niches that serve as microbial "metabolic hotspots," fostering complex, functionally interconnected networks with enhanced carbon and nitrogen metabolic potential through strong selective pressures. Plastispheres derived from biodegradable plastics create nutrient-rich environments that favor polymer-degrading taxa, promoting niche differentiation and more stable ecological networks. Higher biodegradability was associated with greater enrichment of genes encoding plastic-degrading enzymes and elevated potential for nitrogen fixation, denitrification, and methane production. In contrast, plastispheres from non-biodegradable plastics exhibited intensified interspecific competition and increased species diversity, while limited carbon availability and tight microbial interactions facilitated enrichment of methane oxidation genes. These findings highlight submillimeter plastispheres as potential hotspots for greenhouse gas emissions, providing new insights into the ecological risks of microplastics in aquatic environments.