Overlooked Role of Putative Non-Hg Methylators in Predicting Methylmercury Production in Paddy Soils

Rice
ingestion has been recognized as an important route of dietary
exposure to neurotoxic methylmercury (MeHg) that is commonly synthesized
in rice paddy soils. Although Hg methylators are known to regulate
soil MeHg formation, the effect of non-Hg methylating communities
on MeHg production remains unclear. Here, we collected 141 paddy soil
samples from main rice-producing areas across China to identify associations
between bacterial community composition (including both Hg and putative
non-Hg methylators) and MeHg production. Results showed that the MeHg
content in the paddy soils varied from 0.11 to 8.36 ng g–1 at a national spatial scale, which could be due to the shifts of
soil microbial community composition across different areas. Our structure
equation modeling suggested a strong link between bacterial community
composition and MeHg content and %MeHg. More importantly, random forest
analyses suggested a more significant role of putative non-Hg methylators
than Hg methylators in predicting variations of soil MeHg content.
The relative abundance of putative non-Hg methylators such as unclassified Xanthomonadales and Chitinophagaceae were
strongly correlated with soil
MeHg contents. Further, microbial network analysis revealed strong
co-occurrence patterns between the putative non-Hg and Hg methylators.
These findings highlight an overlooked role of non-Hg methylating
communities in predicting MeHg production in paddy soils.