Anthropogenic Gradients Drive Divergent Bacterial Communities and C/N/P/S Cycling Functional Gene Profiles in Urbanized Coastal Sediments

Coastal sediment bacteria mediate essential biogeochemical cycles, yet their functional gene potential to anthropogenic stress gradients remains insufficiently understood. This study presents a comparative analysis of sediment bacterial communities and carbon, nitrogen, phosphorus, and sulfur (C/N/P/S) cycling genes between the west and east coasts of Shenzhen, China, a region exemplifying sharp contrasts between intensive urbanization and relatively pristine marine environments. We integrated 16S rRNA gene high-throughput sequencing with QMEC (Quantitative Microbial Element Cycling), a high-throughput quantitative PCR chip to profile both bacterial taxonomic composition and quantitatively measured gene abundances across C/N/P/S biogeochemical pathways. Western sediments, under strong terrestrial input, exhibited higher ammonium and organic carbon, while eastern sediments had higher salinity. Bacterial alpha diversity was significantly higher in the west, and community structure differed markedly between regions (ANOSIM R = 0.334, p = 0.001). Although dominant phyla (Pseudomonadota, Bacteroidota, and Planctomycetota) were shared, genus-level taxa diverged: Sulfurovum (sulfur cycling) was enriched in the west, while organic-degrading Burkholderia-Caballeronia-Paraburkholderia and Phaeodactylibacter prevailed in the east. Notably, functional gene abundance for all four biogeochemical cycles was an order of magnitude higher in western sediments, with denitrification genes dominating there and nitrification genes in the east. Redundancy analysis (RDA) and the Mantel test identified nitrate as the primary environmental driver of bacterial variation (r = 0.574, p = 0.001), alongside salinity, temperature, ammonium, and total organic carbon. These findings reveal distinct bacterial adaptations to contrasting anthropogenic pressures and demonstrate that sediment bacterial functional gene profiles can serve as sensitive indicators of ecosystem health, providing a quantitative microbiological basis for targeted pollution management in urbanized coastal zones.