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Mercury methylation trait dispersed across diverse anaerobic microbial guilds in a eutrophic sulfate-enriched lake
Citation:
Peterson, B., E. McDaniel, A. Schmidt, R. Lepak, S. Janssen, P. Tran, R. Marick, J. Ogorek, J. DeWild, D. Krabbenhoft, AND K. McMahon. Mercury methylation trait dispersed across diverse anaerobic microbial guilds in a eutrophic sulfate-enriched lake. ENVIRONMENTAL SCIENCE & TECHNOLOGY. American Chemical Society, Washington, DC, 54(24):15840–15851, (2020). https://doi.org/10.1101/2020.04.01.018762
Impact/Purpose:
Our study represents the research to understand whether the capability for mercury methylation extends beyond the commonly used laboratory microbes today. Here we find that the capability to methylate mercury to form methylmercury exists in a much broader diversity of microbes than previously realized and that the machinery to perform methylation is dominantly found in anaerobes. This is important because it reveals that mercury methylation potential extends well beyond the process of sulfate reduction and likely includes many other energetic approaches (like fermentation). We expect this work to lead to further exploration of the diverse anaerobic microbial guilds commonly found in lakes, rivers and the ocean to determine whether these microbes, in addition to having the capability to methylate, actually perform Hg methylation.
Description:
Mercury (Hg) methylation is a microbially mediated process that converts inorganic Hg into bioaccumulative, neurotoxic methylmercury (MeHg). The metabolic activity of these methylators is highly dependent on biogeochemical conditions, which subsequently will influence MeHg production. However, our understanding of the ecophysiology of methylators in natural ecosystems is still limited. We identified potential locations of MeHg production in the water column of a freshwater lake with elevated sulfate levels. At these sites, we used shotgun metagenomics to characterize microorganisms with the methylation gene hgcA. Putative methylators were dominated by hgcA sequences divergent from those in well-studied, confirmed methylators. Using genome-resolved metagenomics, we classified these hgcA+ organisms as Bacteroidetes and the recently described Kiritimatiellaeota. We identified sulfate-reducing genomes encoding hgcA, but these accounted for only 22% of hgcA+ genome coverage. Fermentative hgcA+ genomes were the most abundant, accounting for over half of the hgcA gene coverage, many of which also mediate the primary degradation of organic molecules. This work highlights the dispersal of the methylation genes across the microbial food web and suggests that primary degradation of large organic molecules and fermentation may play an important role in the production of MeHg in the water column of freshwater lakes.
URLs/Downloads:
DOI: Mercury methylation trait dispersed across diverse anaerobic microbial guilds in a eutrophic sulfate-enriched lake