Mercury has become a high profile contaminant across the world, with methylmercury being the species of most concern due to its high toxicity and enhanced bioaccumulation in aquatic and terrestrial food webs. Eating rice is one of the major routes of human MeHg exposure. As a special type of wetland with seasonal flooding, rice paddy is demonstrated to be a hotspot of Hg methylation, providing the major source of MeHg accumulation by rice during the growing period. Until now, information on the identity and significance of various microbial groups involved in mercury methylation in rice paddy soil is scarce. Here we used a combination of incubation experiments with specific metabolic inhibitors and/or stimulants and Hg stable isotopes, to unreveal the methylation processes and identify the microbial guilds involved in Hg formation in rice paddy soil from an abandoned Hg mining area (SK), an artificial Hg minging area (GX) and a background area (HX). The results showed that the Hg methylation in the water-soil interface was higher than in the rhizosphere soil in all the three sites. Hg methylation rate in the background rice paddy was much higher than in the other two Hg polluted sites. Sulfate-reducing bacteria and methanogens were indicated to be the main methylators in rice paddy, and methanogens were likely compete, i.e. for some important nutrient, with other methylators. This study will provide an in depth understanding of the diversity and distribution of microorganisms responsible for mercury methylation in natural environments, which in turn can be used for specific strategies for restoration and management of landscapes to limit mercury methylation

Key words: rice paddy, microorganism, mercury, methylmercury, mercury methylation