Spatiotemporal dynamics and dual-transfer mechanisms of the riverine resistome under antibiotic and metal co-pollution in the Xiangjiang River Basin.

The co-pollution of antibiotics and heavy metals severely exacerbates the dissemination of antimicrobial resistance, yet the distinct mechanisms driving resistome assembly across complex environmental matrices remain poorly understood. This study characterizes the spatiotemporal dynamics of the resistome under multi-pollutant stress within the water-sediment system of the Xiangjiang River, a basin historically impacted by intensive heavy metal smelting and contemporary antibiotic discharges. We elucidate a phase-dependent "dual-track" mechanism governing resistance evolution. In the flowing aqueous phase, particularly during wet-season runoff events, sub-inhibitory antibiotics serve as a primary stimulant that promotes a high potential for rapid horizontal gene transfer, facilitating the structural consolidation of multidrug resistance genes with highly mobile genetic elements. Conversely, in benthic sediments, persistent heavy metal legacies exert deterministic selective pressure that restructures the microbial host community, strongly implicating host-dependent vertical gene transfer as the dominant pathway for resistome enrichment. Crucially, our in vitro transformation models demonstrate that antibiotics and heavy metals exert a potent combined promotion effect on genetic exchange during co-exposure, with both contaminants concurrently driving the significant elevation of horizontal mobility. This potent co-selection transforms the riverine ecosystem into a dynamic "genetic reactor", enabling environmental microbiomes to acquire broad-spectrum resistance traits through singular transfer events. Ultimately, our findings highlight the urgent need for integrated water-sediment management and the synchronized co-regulation of mixed contaminants to mitigate escalating ecological and public health risks.