Mercury methylation in the soils and sediments of Three Gorges Reservoir Region

Abstract

Previous studies demonstrated that microorganisms had an important role in the mercury (Hg) methylation process in the water and sediments of Three Gorges Reservoir (TGR). The purpose of this research was to analyze the microbial methylation of Hg in the soils and sediments of the water-level-fluctuating zone (WLFZ) of TGR. Different types of soils, sediment (≤ 155 m), semi-inundated soil (≥ 155 m), and non-inundated soil (≥ 175 m) of the WLFZ of Shibao (S), Zhenxi (Z), and Tujing (T) were investigated. Real-time PCR, terminal restriction fragment length polymorphism (T-RFLP), cloning, sequencing, and phylogenetic analysis were used to analyze the abundance and diversity of dsrB, hgcA, and mcrA genes, and their relationship with the levels of total Hg (THg), MeHg, and several biogeochemical factors that probably affected microbial methylation reaction. THg concentrations in different soil types of the WLFZ of TGR did not show significant differences (p > 0.05) in site S, while there were significant differences (p < 0.05) of MeHg levels in different soil types of the three sites. Phylogenetic analyses found that the dominant groups of microorganisms with dsrB in the sediment and non-inundated soil in site S differed remarkably. Microorganisms that probably related with Hg methylation mainly distributed in the sediment, with δ-proteobacteria as the dominant class. Real-time PCR found that soil MeHg levels correlated positively with the resident quantities of microorganisms with dsrB. The abundance of dsrB was much higher than that of hgcA which may indicate that only a small part of sulfate-reducing bacteria (SRB) related with Hg methylation. Soil MeHg levels correlated positively with the resident quantities of microorganisms with the dsrB gene. The phylogenetic analysis indicated that SRB that probably related with Hg methylation distributed mainly in the sediment, rather than in the non-inundated soil.