第七届青年地学论坛

Rice ingestion has been recognized as an important route of dietary exposure to neurotoxic methylmercury (MeHg) that is usually synthesized in rice paddy soils. Although certain anaerobic microorganisms are known to convert inorganic mercury (Hg) to MeHg and the discovery of a two-gene cluster hgcAB has linked Hg methylation to several phylogenetically diverse groups of microorganisms, the prevalence and diversity of Hg methylators in microbial communities of rice paddy soils remain unclear; we also lack understanding of the role of non-Hg methylating communities in MeHg accumulation. Here, we collected 141 paddy soil samples from main rice-producing areas across China to identify the diversity of Hg methylator, and associations between bacterial community composition (including both Hg and non-Hg methylators), and Hg methylation efficiency (proxy as %MeHg). Results showed that %MeHg in the paddy soils varied from 0.005 to 2.838% at a national spatial scale, which can be explained by the variations of soil microbial community composition across different areas. We show that Hg methylating communities are diverse, with iron-reducing bacteria (i.e., Geobacter) and methanogens as the dominant taxa likely involved in Hg methylation in the soil. Phylogenetic analysis also uncovered some hgcAB sequences closely related to three novel Hg methylators, Geobacter anodireducens, Desulfuromonas sp. DDH964, and Desulfovibrio sp. J2 in these paddy soils. Our structure equation modeling suggested a much stronger link between bacterial community composition and %MeHg, compared to the abundance of methylating gene (hgcA) and edaphic properties. More importantly, random forest models suggested a more important role of non-Hg methylators in predicting variations of soil %MeHg than Hg methylators. Further analysis of microbial network revealed strong co-occurrence patterns between non-Hg and Hg methylators, suggesting their interactions may affect MeHg production in paddy soils. These findings shed new light on microbial community composition and major clades driving Hg methylation in rice paddy soils. Our work highlight the important role of both Hg and non-Hg methylating communities in predicting MeHg production in paddy fields.

土壤 微生物 汞