Seasonal Restructuring of Microbial Communities and Resistomes in the Shitalakshya River, Bangladesh Revealed by Shotgun Metagenomics.

Urban rivers supplying drinking water face mounting pollution and AMR threats. We combined shotgun metagenomics with physicochemical analysis to investigate microbial community and resistome dynamics in Bangladesh's Shitalakshya River, a drinking water source under increasing pollution pressure, during early and peak dry seasons. Peak dry season water quality deteriorated markedly, characterized by hypoxia and elevated nutrient and organic carbon levels, which drove pronounced restructuring of the river microbiome. A distinct shift occurred from Myroides dominance toward a more diverse assemblage enriched in pollution-tolerant and opportunistic genera, notably Comamonas, Brevundimonas, Tissierella, and Aeromonas. Metagenomic profiling revealed a diverse resistome encompassing antibiotic, metal, and biocide resistance genes. Although overall antibiotic resistance gene abundance declined slightly, metal resistance genes increased more than twofold, with strong enrichment of mercury resistance determinants such as merA. Concurrent increases in multidrug efflux pump genes suggested potential co-selection driven by metal and chemical stressors. These findings indicate that dry-season pollutant concentration reshapes both microbial communities and resistance profiles through non-antibiotic selective pressures. Despite limited sampling, this study provides a baseline metagenomic snapshot of antimicrobial resistance dynamics in a climate-stressed urban river system, offering vital insights for pollution abatement and the safeguarding of drinking water safety.