Methylmercury (MeHg) is a bioaccumulative neurotoxin that is produced by certain sulphate reducers and other anaerobic microorganisms. We are using an integrative geochemical and molecular approach to study the relationship between microbial community composition and methylation potential in sulphate-impacted freshwater sediments on Minnesota’s Iron Range. First, we performed a detailed characterization of methylating communities in two sulphate-impacted lakes using metagenomics, rRNA methods, and hgcA sequencing. Then, we extended our results to a larger number of samples by combining methylation rate measurements with hgcAB sequencing across a sulfate gradient in experimental wetland sediment mesocosms. Methylation rates and MeHg accumulation were not clearly related to sulfate concentration or sulfide accumulation in either the lakes or the sulfate-amended mesocosm sediments. In the lakes, hgcA gene sequencing and metagenomics showed that methylating communities differ among the sites, and that the most abundant methylating organisms were from groups not previously associated with MeHg production or hgcAB genes. We will describe metabolic capabilities of these methylating populations based on genomic reconstruction from metagenomic datasets. In the sulfate-amended mesocosm sediments, methylmercury production rates varied over two orders of magnitude across the experiment but did not show a clear relationship with the level of sulfate amendment. In general, differences among microbial communities trended with the sulfate gradient. Certain sulfate-reducing Deltaproteobacteria increased with elevated sulfate amendment, while iron-reducers (Geobacteraceae) and methanogens (Methanomicrobia) decreased. Cell counts indicated approximately similar cell densities across all sulfate amendments, although richness decreased with increasing sulfate level. We will describe trends in methylating communities based on hgcAB sequencing from across the experiment, and show how different methylating communities compared in terms of methylation potential. These results expand our knowledge of the clades and environmental factors that affect MeHg production in sulfate-impacted freshwater ecosystems.

Investigating the relationship between mercury methylation and microbial community composition across sulfate gradients in Northern Minnesota

255th National Meeting (2018)

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