Biofilm-forming traits enrich the plasmid diversity and functional potential in particle-attached bacteria in coastal ecosystems.

Planktonic microorganisms play a central role in aquatic biogeochemical processes and are commonly divided into particle-attached (PA) and free-living (FL) fractions. Although these two lifestyles differ in ecological strategy, the contribution of plasmids to their niche differentiation remains poorly resolved. Here, we conducted a plasmid-centric metagenomic analysis of two anthropogenically impacted coastal ecosystems in South China, the Pearl River Estuary (PRE), and Daya Bay (DYB), to determine the environmental and biological drivers of plasmid diversity, and their functional potenitial. We found that plasmid diversity was jointly shaped by different fractions and environmental stressors. The PA fraction contained significantly higher plasmid abundance and richness than the FL fraction, and was enriched in multifunctional and conjugative plasmids. These plasmids were associated with genes adapting to the PA lifestyle or microenvironments, suggesting linkage between particle attachment and plasmid maintenance. Structural equation modeling indicated that different fractions shaped plasmid diversity primarily through biofilm-forming genes. Along an anthropogenic gradient from DYB to PRE, increasing pollution levels were accompanied by higher plasmid diversity and greater abundances of antibiotic and metal resistance genes. Plasmid diversity was strongly correlated with resistance gene abundance. The enrichment of transferable plasmids in the PA fraction, where cell densities are high and intercellular distances are close, suggested that particle-associated habitats favor genetic exchange and the persistence of resistance traits. Together, these results demonstrate that particle-associated microbial communities represent key reservoirs of plasmid diversity and resistance potential in coastal ecosystems and highlight the combined influence of lifestyles and anthropogenic stress on plasmid-mediated microbial adaptation. IMPORTANCE: Plasmids play an important role in microbial adaptation by mediating horizontal gene transfer, yet the ecological contexts that favor their persistence and diversification in natural environments remain poorly understood. This study showed that particle-attached microbial communities in coastal waters harbored substantially higher plasmid diversity and resistance potential than free-living communities, and that this enrichment is strongly linked to biofilm-associated traits. By demonstrating how particulate habitats and pollution gradients jointly shape plasmid diversity and resistance gene abundance, our findings identify particle-associated microenvironments as critical reservoirs for plasmid-mediated functions in coastal ecosystems. These results advance understanding of how microbial lifestyle and human activities influence microbial evolution and the environmental dissemination of resistance traits.