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Abstract Title: Inorganic mercury bioavailability and microbial methylation capacity constraints on in situ mercury methylation
Presenter Name: Benjamin Peterson
Company/Institution: University of Wisconsin at Madison
Session: Special Session - Meta-omic and geochemical approaches to linking microbial activity to biogeochemical mercury cycling
Day and Session: Friday 29th July - Session Two
Start Time: 09:30 UTC
Co-Authors: Benjamin Peterson,Brett Poulin,Katherine McMahon

Abstract Information :

The formation of methylmercury (MeHg) in the environment is a two-step process. First, microbes take up bioavailable inorganic divalent mercury (Hg(II)i), which is determined by the ligand complexation of Hg(II)i. Second, Hg(II)i is methylated intracellularly and depends on the methylation capacity of the microbial community, which is conferred by the hgcAB genes. Although each of these factors has been studied in isolation or in laboratory experiments, the interaction between them under environmentally relevant conditions remains poorly understood. We conducted a MeHg formation experiment using an enriched stable isotope tracer in which Hg(II)i was equilibrated with pore waters and used in MeHg formation assays in intact peat core collected from across an environmental sulfate gradient of the Florida Everglades. Shotgun metagenomic sequencing was performed to quantify and characterize the microbial community with the hgcA gene. This multi-pronged approach facilitated the isolation of the importance of Hg(II)i bioavailability and the methylation capacity of the microbes in the formation of MeHg across a range of sulfate concentrations. Geochemical analysis of the pore water identified the DOC quality and quantity as the primary constraint on Hg(II)i bioavailability. Metagenomic sequencing identified correspondence between the formation of MeHg in incubation assays and the relative hgcA abundance in the microbial community. Taxonomic classification of the hgcA genes and metabolic pathway analysis of hgcA-containing metagenome-assembled genomes identified no hgcA sequences associated with sulfate-reducing bacteria. These results demonstrate that the formation of MeHg in the environment depends on synergy between the bioavailability of Hg(II)i and hgcA abundance of the microbial community. This study expands our understanding of the geochemical and microbial constraints on MeHg formation under complex environmental conditions and provides an experimental framework for further mechanistic studies.

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