Arsenic Speciation–Thallium Synergy and AsB-Mediated Drivers of Arsenic Resistance Genes in a Large Reservoir

Arsenic (As) poses risks to drinking water safety, while arsenic resistance genes (AsRGs) modulate As transformation and fate. Although the occurrence and enrichment of AsRGs under high As pressure is well studied, their patterns and drivers in low-As aquatic environments remain unclear. Here, we investigated AsRGs in the Danjiangkou Reservoir, a low-arsenic source water, using seasonal sampling and metagenomics. Arsenate [As(V)] was the dominant inorganic species, while arsenite [As(III)] showed significant seasonal variation, higher in spring and summer. Total AsRGs were dominated by arsC, followed by arsB/ACR3, and varied seasonally. AsRGs correlated with As speciation, environmental factors, and heavy metals, with arsM and multiple other AsRGs strongly correlated with Tl, Sb, and Mo. Redundancy analysis identified As(V), Mo, pH, dissolved oxygen (DO), dimethylarsinic acid (DMA), Pb, Cd, Tl, W, and Cr as core variables, with heavy metals having the highest explanatory power. Mediation analysis revealed that arsenobetaine (AsB) fully mediated the effect of Zn on arsB. These findings support a triple-driving framework—direct As stress, heavy metal co-selection, and AsB-mediated regulation—elucidating AsRGs proliferation in low-As waters and supporting reservoir risk management.