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.