Mercury Methylation in Alaskan Peatlands Spanning a Large Range of Trophic Structure

Abstract:

The process of mercury (Hg) methylation has long been recognized as a key area of research in order to understand spatial and temporal variability of toxic methylmercury (MeHg) on the landscape. Numerous factors affect MeHg production, the most important generally falling into those that affect inorganic Hg(II) bioavailability (e.g., Hg(II) concentration and ligand composition), and those that affect microbial community composition and activity. The principal goal of this project is to decipher the details of MeHg production in Alaskan peatlands exhibiting a range of trophic status, including those lacking in electron acceptors that support the traditional respiratory pathway of MeHg production (e.g., sulfate reduction). MeHg production is carried out by a diverse group of microorganisms that possess the gene cluster (hgcAB), including the well-studied sulfate and iron- reducing bacteria (SRB and FeRB). However, less well known bacteria also possess the hgcAB genes, including: syntrophs, methanogens, acetogens, and fermenters. Methylation and demethylation activities were determined by injecting trace levels of the stable isotope (¹⁹⁸Hg and Me²⁰⁴Hg) into intact peat cores. In addition, the short-lived radioisotope ¹⁹⁷Hg was used in laboratory incubations. Laboratory studies also included assays for changes in diagnostic gas concentrations (CH₄, CO₂, H₂) and LMW organic acids (formate, acetate, propionate, butyrate) to infer specific microbial processes, and the use of genomics to confirm microbial assemblages and the presence/absence of hgcAB genes. Overall, we observed Hg methylation rates were greatest at minerotrophic sites with active syntrophy and methanogenesis. Methylation and demethylation rates corresponded significantly across sites. There was no evidence of SO₄^- reduction in these samples, and addition of SO₄^- did not stimulate methylation suggesting that methylation was conducted by SRB that were metabolizing syntrophically and/or by fermentation.