Unrecognized microbial sources of methyl mercury in freshwater lakes

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

The chemical methylmercury is toxic environmental pollutant that can be found in high concentrations in fish. This has led to numerous fish consumption advisories. While we know that this chemical can be produced by microorganisms that change mercury from a non-toxic form into a toxic form, there are many questions concerning where this occurs in the environment. The goal of this project is to identify which microorganisms can perform this reaction in lakes. The study will focus on high resolution mapping where these microorganisms are active in lakes that have fish with high levels of methylmercury. These maps will be compared with state-of-the-science metagenomics techniques link activity to methylmercury production. Water managers, regulatory agencies, Hmong subsistence fishers, and other stakeholders will be engaged to increase understanding of the potential threat methylmercury poses. Successful completion of this work will lead to a better understanding of mercy methylation and lead to more informed decision making to minimize human exposure to contaminated fish. Methylmercury (MeHg) is an environmental pollutant of major concern due to its role as a neurotoxin that bioaccumulates in aquatic food webs, leading to human exposure through fish consumption. After inorganic mercury enters ecosystems due mainly to human activity, microbes are responsible for transforming it into the much more toxic MeHg. Preliminary data suggests that mercury methylation genes are distributed within and across divergent microbial lineages, including those with few or no cultured representatives. The role of these novel organisms in methylation is almost completely unexplored. Filed studies will be performed in collaboration with the US Geological Survey at two contrasting field sites: Lake Mendota in Madison, WI and a series of reservoirs along the Snake River in Hell's Canyon, Idaho. Both are highly eutrophic with primary production being fueled by agricultural nutrient input. Work to date has shown that MeHg accumulates just below the thermocline at both sites. The proposed work will build on these observations by pursuing three specific aims: Aim 1 is to measure methylation and demethylation rates across spatial and temporal redox gradients in the water column using samples collected from the study sites. Aim 2 is to link methylator guilds to in situ biogeochemistry using metagenomics and metatranscriptomics using shotgun sequencing community DNA and cDNA and computational analysis to recover population genomes while linking gene expression to individual functional groups. Key populations will be quantified using qPCR to provide a more quantitative and replicated view of the methylator distributions in time and space. Aim 3 is to define metabolic constraints on methylation using mesocosm incubations with mercury isotopes and targeted amendments hypothesized to inhibit or promote methylation. Successfully establishing the presence of mercury methylation in the water column would be transformative to our understanding of MeHg dynamics, as our current understanding is that this normally occurs in anaerobic sediments. By providing detailed analysis of Hg methylation dynamics, new approaches to controlling methylation and/or preventing uptake in fish may be possible. Outreach to a local Hmong community of fishers will increase the potential benefits by increasing understanding of how MeHg is take up by fish. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.