Cadmium Elevates Methylmercury Levels in Rice Paddies via Microbial Adaptation and Biogeochemical Alterations

Methylmercury (MeHg) in rice poses significant health risks to populations with rice-based diets. While cadmium (Cd) contamination of paddy soils is widespread, its role in influencing MeHg accumulation in rice remains unclear. We combined a nationwide survey of 103 rice paddies with controlled pot and incubation experiments to examine how Cd affects MeHg in soils and rice grains. Soil geochemical parameters, microbial community composition, and horizontal gene transfer (HGT) of functional genes were analyzed to disentangle biological and geochemical mechanisms. Across field sites, Cd concentrations were positively associated with rice MeHg levels, independent of total Hg. Pot and incubation experiments confirmed that Cd exposure increased MeHg levels in soils and grains. This enhancement was mediated by both microbial and geochemical pathways: Cd reshaped microbial communities, promoted HGT that conferred Cd resistance to Hg-methylating bacteria, and altered soil redox potential and dissolved organic carbon, thereby creating conditions favorable for Hg methylation. Our findings reveal Cd as a previously overlooked driver of MeHg risk in rice agroecosystems. Given the co-occurrence of Cd and Hg pollution in global rice-growing regions, integrated management of multiple metals is needed to mitigate MeHg exposure through rice consumption.