Seasonal Divergence between Microbiomes on Microplastics and Natural Particles Increases with Rising Water Temperatures in Urban Rivers

The "plastisphere," which comprises microplastics (MPs)-associated microbial communities, is an emerging component of urban river ecosystems. However, its seasonal dynamics remain poorly understood, especially compared with microbiomes on natural particles (NPs). We therefore conducted a year-long metagenomic study at 15 sites across 10 major urban rivers in Hong Kong to compare MP- and NP-associated microbiomes across four seasons. Representative high-quality metagenome-assembled genomes revealed significant seasonal variations in both taxonomic and functional compositions across particle types, with water temperature identified as the primary environmental driver. As temperatures increased, both MP and NP microbiomes exhibited increased taxonomic and functional diversity but reduced functional redundancy and network stability. Compared to NPs, MP microbiomes exhibited higher taxonomic and functional turnover, more complex and connected cooccurrence networks, and distinct taxonomic and functional traits along the temperature gradient. In MP microbiomes, warmer conditions were associated with a higher abundance of pollutant-degrading and putatively virulent taxa (particularly from Firmicutes and Actinobacteria), along with enhanced biosynthetic functions and increased potential microbial sharing and horizontal gene transfer with surrounding aquatic microbiomes. These findings highlight the temperature-dependent ecological impacts of MP microbiomes and underscore the need to consider climatic factors when assessing the long-term ecological risks of MPs in urban riverine ecosystems.