Changes in Microbial Community Structure in an Experimental Peatland Amended with Sulphate: Implications for Methylmercury Production

Abstract Anthropogenic changes in sulphate deposition affect vast areas of peatland, potentially altering the structure and function of their microbial communities, but little is known about the duration, magnitude, or character of these effects. We paired 16S tag-encoded pyrosequencing with experimental changes in sulphate regime to assess the community responses of the microflora of a sub-boreal Minnesota peatland, and focused on the production of methylmercury (MeHg), a bioaccumulative neurotoxin, as a sulphate-influenced microbial community function. An experimental treatment was amended with sulphate at a rate of 32 kg/ha from 2001 to 2008, alongside an unamended control. In 2006, sulphate additions ceased in a portion of the experimental area creating a 3-year recovery treatment. All three sulphate regimes were applied to the ombrotrophic central bog area and to the slightly more minerotrophic lagg areas of the peatland, for a total of six treatments. The structure of three communities (the total community, Deltaproteobacteria only, and only operational taxonomic units with a significant correlation with MeHg) was assessed using PERMANOVA analysis of weighted UNIFRAC matrices. With one exception, all communities retrieved from experimental treatments differed from those found in control and recovery treatments, which were either indistinguishable or very similar to one another, indicating that increased long-term sulphate deposition had altered the peatland microflora. MeHg accumulation followed a broadly similar pattern, suggesting that sulphate may affect MeHg production through changes in the microbial community structure as well as via changes in the abundance or activity of specific phylotypes. We identified 182 new potential mercury methylators, notably representatives of the Verrucomicrobia, Elusimicrobia, and Myxococcales. The relatively rapid return of the recovery treatment communities to a structural and functional profile more similar to the control than the experimental treatments indicates that the peatland microflora was not irreversibly altered by sulphate.