Coupled biogeochemical processes account for vertical heterogeneity of groundwater iodine in the Hetao Basin

Iodine concentration in groundwater exhibits vertical heterogeneity, even within the same aquifer, which is mainly caused by specific biogeochemical processes. However, the microbial contributions to the vertical change of groundwater iodine remain unclear. In this study, we built a monitoring network of 24 wells targeting the depths of 20, 35 and 50 m in a semi-confined aquifer in the Hetao Basin, China and used hydrogeochemistry, metagenomics and metatranscriptomics to elucidate microbial-mediated vertical variations of groundwater iodine. Hydrogeochemical results showed significantly higher concentrations of iodide and total iodine in groundwater samples at 50 m depth than those at 20 and 35 m depth. Together with the characteristics of sulfur isotope composition and hydrogeochemistry, the metagenomic and metatranscriptomic results indicated that active sulfur redox cycling drove iodide enrichment in groundwater at 50 m depth. Following microbial sulfate reduction, sulfide oxidation processes, especially microbial sulfide oxidation coupled with iodate reduction, play important roles in iodide enrichment. Given the prevalence of sulfur redox reactions in environments, the sulfur redox cycling for iodide mobilization revealed in this study, especially sulfide oxidation coupled with iodate reduction, may contribute to iodide enrichment not only in iodine-rich aquifers worldwide but also in global marine oxygen minimum zones.