Methylmercury (MeHg) contamination in rice via paddy soils is an emerging global environmental issue. An understanding of mercury (Hg) transformation processes in paddy soils is urgently needed in order to control Hg contamination of human food and related health impacts. Sulfur (S)-regulated Hg transformation is one important process that controls Hg cycling in agricultural fields.
Recently, the research group led by Prof. FENG Xinbin from the Institute of Geochemistry, Chinese Academy of Sciences (CAS) has successfully traced Hg transformation processes in paddy soil, including methylation, demethylation, oxidation, and reduction, by using a multi-compound-specific isotope labeling technique (²⁰⁰Hg^II, Me¹⁹⁸Hg, and ²⁰²Hg⁰). Responses of Hg transformation to sulfur input (sulfate and thiosulfate) were also elucidated.
The study was published in Environmental Science & Technology on 17th May.
This study revealed that microbially mediated reduction of Hg^II, methylation of Hg⁰, and oxidative demethylation-reduction of MeHg occurred under dark conditions; these processes served to transform Hg between different species (Hg⁰, Hg^II, and MeHg) in flooded paddy soils. Rapid redox recycling of Hg species contributed to Hg speciation resetting, which promoted the transformation between Hg⁰ and MeHg by generating bioavailable Hg^II for fuel methylation. Sulfur input also likely affected the 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 transformation processes in paddy soils and provide much-needed knowledge for assessing Hg risks in hydrological fluctuation-regulated ecosystems.
This work was supported by the National Natural Science Foundation of China, the CAS “Light of West China” program, Guizhou Provincial 2020 Science and Technology Subsidies, and the Guizhou Provincial Natural Science Foundation.

Schematic diagram showing Hg transformation in paddy soils. (Image by IGCAS)

Contact:
MENG Bo
Institute of Geochemistry, Chinese Academy of Sciences
Email: mengbo@vip.skleg.cn

(By Prof. FENG Xinbin’ s group)