Convergent heavy metal profiles shape divergent bacterial community patterns in subtropical water-sediment system

Subtropical basins, globally valued for ecological and economic services, are increasingly impaired by severe heavy-metal contamination. Coupling seasonal field sampling, laboratory analysis and 16S rDNA sequencing, this study elucidated spatiotemporal distributions of cobalt (Co), copper (Cu), manganese (Mn), nickel (Ni) and zinc (Zn) and their consequent regulation of water and sediment bacterial communities across a representative subtropical catchment. Results demonstrated that heavy metal concentrations and risk indices were persistently higher during the dry season. Heavy metals exhibited spatial consistency in water bodies and sediments, with relatively high Co and Mn in upper reaches and elevated Ni, Cu and Zn in mid-lower segments. In surface waters, increased Cu and Zn concentrations reduced bacterial α-diversity, up-regulated detoxification and chemotaxis genes and shifted dominance from Actinobacteriota (67.5%) to Proteobacteria (49.9%). Sediment assemblages experienced mid-reach suppression of richness and keystone taxa under mixed-metal pressure; continued Cu and Zn accumulation downstream further enriched defense and ATP-transport genes and elevated Firmicutes abundance from 12.2% to 31.6%. These findings delineate basin-scale bacterial response patterns to heavy-metal stress, offering a mechanistic framework for risk assessment, targeted monitoring and bioremediation of metal-impacted subtropical basins characterized by elevated geochemical background.