Redox potential and C/N ratio predict the structural shift of mercury methylating microbe communities in a subalpine Sphagnum peatland

Abstract

Subalpine peatlands are a critical sink of mercury (Hg) and source of methylmercury (MeHg). However, little is known about the identity of Hg methylators or the geochemical factors controlling their distribution. Here, high-throughput sequencing of 16S rRNA and hgcA genes were used to investigate the community composition of Hg methylating microbes and their relating geochemical factors in Dajiuhu, which is a subalpine Sphagnum peatland. The results showed that MeHg content in the soils varied from 0.9 to 13.3 ng·g−1, while the ratio of MeHg to total Hg varied between 0.98% and 8.13%. The majority of the hgcA sequences clustered with Geobacteraceae, followed by Methanoregulaceae. The ratio of hgcA abundance and quantitative PCR (qPCR) gene copy numbers (hgcA/qPCR) showed that iron-reducing bacteria (FeRB) have the highest Hg methylator proportion within it, followed by methanogens. Although a high abundance of sulfate-reducing bacteria (SRB) was observed based on 16S rRNA, the abundance that contained hgcA sequences was low. Notably, as the MeHg concentration increased, the community structure of Hg methylating microbes shifted from FeRB dominance to FeRB and methanogen dominance, which was predicted by the redox potential (Eh) and C/N value.