Methylmercury (MeHg) is a bioaccumulative neurotoxin produced by certain sulfate-reducing bacteria and other anaerobic microorganisms. Because microorganisms differ in their capacity to methylate mercury, the abundance and distribution of methylating populations may determine MeHg production in the environment. We compared rates of MeHg production and the distribution of hgcAB genes in epilimnetic sediments from a freshwater lake that were experimentally amended with sulfate levels from 7 to 300 mg L-1. The most abundant hgcAB sequences were associated with clades of Methanomicrobia, sulfate-reducing Deltaproteobacteria, Spirochaetes, and unknown environmental sequences. The hgcAB+ communities from higher sulfate amendments were less diverse and had relatively more Deltaproteobacteria, whereas the communities from lower amendments were more diverse with a larger proportion of hgcAB sequences affiliated with other clades. Potential methylation rate constants varied 52-fold across the experiment. Both potential methylation rate constants and % MeHg were the highest in sediments from the lowest sulfate amendments, which had the most diverse hgcAB+ communities and relatively fewer hgcAB genes from clades associated with sulfate reduction. Although pore water sulfide concentration covaried with hgcAB diversity across our experimental sulfate gradient, major changes in the community of hgcAB+ organisms occurred prior to a significant buildup of sulfide in pore waters. Our results indicate that methylating communities dominated by diverse anaerobic microorganisms that do not reduce sulfate can produce MeHg as effectively as communities dominated by sulfate-reducing populations.
Bibliographical noteFunding Information:
We thank C. Thunder, S. Lafond-Hudson, and M. Samuelson for assistance with sample collection and H. Huang for providing analytical support for mercury analyses. Thanks to C. Nguyen, L. Thomas, the University of Minnesota College of Science and Engineering Systems staff, and the Minnesota Supercomputing Institute for computing support and the use of facilities. Special thanks to the staff at the University of Minnesota Genomics Center (UMGC) for advice and assistance with high-throughput sequencing of custom hgcAB amplicon libraries. We acknowledge the University of Minnesota Undergraduate Research Fellowship Program (UROP) for supporting G.M.W. This work was supported by the Minnesota Pollution Control Agency (MPCA), as well as the University of Minnesota MnDRIVE initiative and the University of Minnesota BioTechnology Institute. The DNA sequencing was funded by a grant from the University of Minnesota Duluth’s Natural Resources Research Institute Permanent University Trust Fund and by a MnDRIVE-supported research grant from the University of Minnesota Duluth’s Swenson College of Science and Engineering.
© 2020 American Chemical Society.
PubMed: MeSH publication types
- Journal Article
- Research Support, Non-U.S. Gov't