Determining the Reliability of Measuring Mercury Cycling Gene Abundance with Correlations with Mercury and Methylmercury Concentrations

Abstract

Methylmercury (MeHg) is a bioaccumulative toxic contaminant in many ecosystems, but factors governing its production are poorly understood. Recent work has shown that the anaerobic microbial conversion of mercury (Hg) to MeHg requires the Hg-methylation genes <i>hgcAB</i> and that these genes can be used as biomarkers in PCR-based estimators of Hg-methylator abundance. In an effort to determine reliable methods for assessing <i>hgcA</i> abundance and diversity and linking them to MeHg concentrations, multiple approaches were compared including metagenomic shotgun sequencing, 16S rRNA gene pyrosequencing and cloning/sequencing <i>hgcAB</i> gene products. Hg-methylator abundance was also determined by quantitative <i>hgcA</i> qPCR amplification and metaproteomics for comparison to the above measurements. Samples from eight sites were examined covering a range of total Hg (HgT; 0.03-14 mg kg^-1 dry wt. soil) and MeHg (0.05-27 μg kg^-1 dry wt. soil) concentrations. In the metagenome and amplicon sequencing of <i>hgcAB</i> diversity, the <i>Deltaproteobacteria</i> were the dominant Hg-methylators while <i>Firmicutes</i> and methanogenic <i>Archaea</i> were typically ∼50% less abundant. This was consistent with metaproteomics estimates where the <i>Deltaproteobacteria</i> were steadily higher. The 16S rRNA gene pyrosequencing did not have sufficient resolution to identify <i>hgcAB</i>⁺ species. Metagenomic and <i>hgcAB</i> results were similar for Hg-methylator diversity and clade-specific qPCR-based approaches for <i>hgcA</i> are only appropriate when comparing the abundance of a particular clade across various samples. Weak correlations between Hg-methylating bacteria and soil Hg concentrations were observed for similar environmental samples, but overall total Hg and MeHg concentrations poorly correlated with Hg-cycling genes.