Production of Methylmercury in Peatlands Following Permafrost Thaw Increases along a Trophic Gradient

Permafrost thaw in peatlands risks increasing the production and mobilization of methylmercury (MeHg), a bioaccumulative neurotoxin that poses a health hazard to humans. We studied 12 peatlands on a trophic gradient in northwestern Canada, including permafrost peat plateaus and thawed bogs and fens, to determine the effects of thaw on MeHg production from measures of soil and porewater MeHg and in situ methylation assays. The production of MeHg was greater in thawed peatlands, especially rich fens, as indicated by higher potential rates of microbial methylation of inorganic mercury (Hg) to MeHg and higher soil %MeHg (MeHg:total Hg). Soil %MeHg was 0.1% in permafrost peat plateaus, 0.7% in bogs, and 2.0% in fens. Microbial analysis indicated three putative methylators (two methanogens and one novel bacteria) as most influential to the community composition, although their abundances were not consistently highest in fens. Fens had a greater range of porewater MeHg concentrations than bogs, potentially due to hydrological flushing, controls on MeHg solubility, or redox disequilibria in fens. MeHg in porewater was strongly associated with dissolved organic matter that had a low aromaticity and a low oxygen-to-carbon ratio. Regionally upscaling our results suggested that the expansion of bogs and fens due to thawing may increase the landscape-scale potential for MeHg production by 65% by 2100, representing a substantial risk to downstream aquatic ecosystems.