The Distribution and Abundance of Mercury Methylating Microorganisms in Mid-Atlantic Wetlands

Abstract:

The discovery of the genes responsible for microbial methylmercury production, hgcAB, has led to the identification of novel Hg methylators with diverse metabolisms including Fe and SO₄^2- reducing bacteria, syntrophs, and methanogens. We recently developed DNA probes for hgcA in each group of methylators: Deltaproteobacteria, Firmicutes, and Archaea [Christensen, 2015]. In this study, we use the probes to determine quantity and distribution of hgcA+ organisms in mid-Atlantic marshes and sediments, and in Hg-contaminated wetland soils. We also analyze hgcA distribution over a 28-day soil slurry experiment designed to evaluate the impact of activated carbon on Hg methylation and demethylation [Gilmour, 2015]. Initial soils show Deltaproteobacteria comprise most hgcA+ organisms. Methanogens encompass >45% of the remaining methylators. The addition of SO₄^2- to induce SO₄^2- reducing conditions in slurries caused the number of hgcA+ Deltaproteobacteria to increase and the number of hgcA+ methanogens to decrease to >32%. In soils and slurries, Firmicutes were below detection, suggesting our Firmicute primers are either unrepresentative in natural samples, or that hgcA+ Firmicutes are rare. This observation is interesting as Firmicutes include organisms with divergent metabolisms, and their role in environmental methylation is still unknown. Slurries also show no correlation between hgcA abundance and Hg concentrations. We now plan to explore how hgcA abundance relates to Hg-methylation and electron acceptor availability. Our results offer initial insights into the natural distribution of hgcA, supporting the idea that the distribution of different methylators is related to electron acceptors and redox chemistry.

Christensen, G., Wymore, AM, King, A, Pdar, M, Hurt Jur, RA, Santillan, EFU, Gilmour, CC, Brandt, CC, Brown, SD, Palumbo, AV, Elias, DA (2015), A Study of Mercury Methylation Genetics: Qualitative and Quantitative Analysis of hgcAB in Pure Culture, paper presented at American Geophysical Union, San Francisco, CA, USA.

Gilmour, C., Ghosh, U, Santillan, EFU, Soren, AB, Bell, T, Butera, D, McBurney, A (2015), Impacts of Activated Carbon Amendment on Hg Methylation, Demethylation and Microbial Activity in Marsh Soils, paper presented at American Geophysical Union, San Francisco, CA, USA.