Mercury and Sulfur Redox Cycling Affect Methylmercury Levels in Rice Paddy Soils across a Contamination Gradient

Methylmercury (MeHg)contamination in rice via paddy soils is anemerging global environmental issue. An understanding of mercury (Hg)transformation processes in paddy soils is urgently needed in orderto control Hg contamination of human food and related health impacts.Sulfur (S)-regulated Hg transformation is one important process thatcontrols Hg cycling in agricultural fields. In this study, Hg transformationprocesses, such as methylation, demethylation, oxidation, and reduction,and their responses to S input (sulfate and thiosulfate) in paddysoils with a Hg contamination gradient were elucidated simultaneouslyusing a multi-compound-specific isotope labeling technique (Hg-200(II), (MeHg)-Hg-198, and Hg-202(0)). In addition to Hg-II methylation and MeHg demethylation,this study revealed that microbially mediated reduction of Hg-II, methylation of Hg-0, and oxidative demethylation-reductionof MeHg occurred under dark conditions; these processes served totransform Hg between different species (Hg-0, Hg-II, and MeHg) in flooded paddy soils. Rapid redox recycling of Hg speciescontributed to Hg speciation resetting, which promoted the transformationbetween Hg-0 and MeHg by generating bioavailable Hg-II for fuel methylation. Sulfur input also likely affectedthe microbial community structure and functional profile of Hg-II methylators and, therefore, influenced Hg-II methylation.The findings of this study contribute to our understanding of Hg transformationprocesses in paddy soils and provide much-needed knowledge for assessingHg risks in hydrological fluctuation-regulated ecosystems. This study investigates mercury methylation,demethylation,reduction, and oxidation processes in paddy soils, with implicationsfor assessing MeHg exposure risks for human health.