Water Resources Research Act Program

Details for Project ID 2019WI001G

Microbial drivers of mercury methylation in freshwater eutrophic systems

Institute: Wisconsin
USGS Grant Number: G20AP00001
Year Established: 2019 Start Date: 2019-11-01 End Date: 2022-10-31
Total Federal Funds: $221,160 Total Non-Federal Funds: $221,160

Principal Investigators: Katherine D. McMahon

Project Summary: Methylmercury (MeHg) production in aquatic systems leads to mercury (Hg) accumulation in aquatic food webs, which can pose a threat to humans and other animals consuming the fish. Human impacts such as eutrophication and impoundments create anoxic environments in which the MeHg-producing organisms can thrive. These organisms are metabolically diverse, and their metabolism links biogeochemical cycling to the production of MeHg. Despite decades of research, our understanding of these microbial communities and their metabolic activity in the environment is still very incomplete. However, the recent discovery of the methylation gene cluster hgcAB provides a molecular marker to identify potential methylating organisms in environmental samples. Current applications of this discovery depend mostly on using molecular probes to identify potential methylators. While this improved our understanding of the environmentally relevant methylators, the approach has major limitations. Emerging technologies in the field of microbial ecology, such as metagenomics and metatranscriptomics, make it much easier to identify methylators and functionally characterize the in situ microbial community. Additionally, stableisotope probing can be used to identify organisms responding to particular carbon amendments. Paired with MeHg-production assays, this is a powerful tool for identifying shifts in microbial activity that might underlie changes in MeHg production potential. The proposed project aims to deploy these tools in a system of eutrophic hydroelectric reservoirs where the USGS and Idaho Power Company are conducting a project to comprehensively study the full Hg cycle. Our objectives are as follows: Objective 1: Identify and characterize Hg-methylating communities in the water column of a eutrophic impoundment. We will use metagenomics to recover genomes from methylators and their community partners. Objective 2: Identify active hgcAB+ organisms and their active metabolic pathways in situ. We will use metatranscriptomics to determine which methylating populations are most active, and which metabolic traits they are expressing. Objective 3: Characterize whole microbial community metabolism across redox gradients through RNA expression analysis. We will further analyze the metatranscriptomes to learn about the other microbial community members and their expressed functions in situ. Objective 4: Conduct mesocosm incubations with amendments to track the flow of carbon through methylating communities. We will work with our USGS collaborators to design incubation experiments aiming to stimulate or inhibit key organisms in the community, and examine the effect on methylation and demethylation rates. DNA Stable Isotope Probing will be used to track carbon incorporation into microbial biomass. This work is an exciting step forward in our ability to identify linkages between biogeochemical processes, such as complex carbon degradation and terminal electron acceptor respiration, and the production of MeHg in situ by using advanced molecular insights into microbial metabolism.