Unveiling the role of sulfate-reducing bacteria in arsenic methylation in alluvial-lacustrine aquifers.

Arsenic (As) methylation is a crucial process within the geochemical cycle of arsenic in groundwater systems. The transformation of inorganic arsenic into less toxic monomethylarsenate (MMA) and demethylarsenate (DMA) holds the potential to partially mitigate the environmental risk of arsenic, thereby offering a promising strategy for regulating arsenic contamination in groundwater. Sulfate-reducing bacteria (SRB) have demonstrated the ability for As methylation under various environmental conditions. However, the capacity of SRB to methylate As specially in groundwater remains unverified. In this study, the predominant biogeochemical processes contribute to the enrichment of methylated arsenic (MeAs) in alluvial-lacustrine aquifers have been investigated through a combination of hydrogeochemical monitoring and incubation experiments. Field investigations in the Jianghan Plain demonstrated that MeAs concentrations in groundwater ranged from 0.34 to 444 μg/L, which are significantly higher than those reported in other region globally. The results suggested that a strongly reducing and neutral environment with elevated level of As(III) and dissolved organic carbon (DOC) facilitated the accumulation of MeAs in groundwaters. Sulfate reduction emerged as an important promoter of As methylation in groundwater, with Desulfovibrio potentially identified as the key SRB genus through high-throughput sequencing of dsrB gene. Moreover, the incubation experiments showed the As methylation efficiency was up to 22.8 % with As(III) being a critical substrate in the aquifer systems from the Jianghan Plain, while such a lower efficiency compared with paddy soil environments likely attributable to the limited available organic matter and the distinct microbial communities. This study provides novel insights on As methylation mechanisms and theoretical support for in-situ remediation of As-contaminated aquifers.