Biotic and Abiotic Controls Over Mercury Methylation in the St. Louis River Estuary: An Examination of Seasonal and Vegetative Influences

EPA Grant Number: F13E20911
Title: Biotic and Abiotic Controls Over Mercury Methylation in the St. Louis River Estuary: An Examination of Seasonal and Vegetative Influences
Investigators: Graham, Emily Bonnell
Institution: University of Colorado at Boulder
EPA Project Officer: Lee, Sonja
Project Period: August 1, 2014 through August 1, 2015
Project Amount: $84,000
RFA: STAR Graduate Fellowships (2013) RFA Text |  Recipients Lists
Research Category: Fellowship - Environmental Science , Academic Fellowships


Methylmercury is a potent neurotoxin that strongly bioaccumulates
in plant and animal tissue, exposing humans to high levels of toxicity through fish and rice consumption, a significant economic and human health concern in the Great Lakes region. Most environmental mercury contamination originates from fossil fuel combustion and mining activity and is deposited into ecosystems in a relatively harmless inorganic form. Microbial methylation within anoxic sediments, however, converts inorganic pollutants into methylmercury, dramatically increasing mercury’s bioavailability. Yet, the role of biotic and abiotic factors in mercury cycling is not well understood. This project will use a suite of molecular microbiological techniques to unravel the controls over mercury cycling along seasonal and spatial gradients within the Lake Superior National Estuarine Research Reserve (LSNERR) in Superior, Wisconsin.


Microcosm incubation experiments will be used to determine the con- trols over mercury methylation in the St. Louis River Estuary. Within these incubations, dissolved organic matter (DOM) concentrations and characteristics will be manipulated across vegetative gradients to exam- ine the impact of changing DOM source matter and loadings on mercury methylation while changes in microcosm chemistry are monitored using standard biogeochemical techniques. Next-generation sequencing of the 16S rRNA gene will elucidate changes in the relative abundance of key microbial community members that affect mercury methylation.

Expected Results:

The relative abundance and diversity of sulfate-reducing bacteria, iron-reducing bacteria and methanogens, as well as the concentrations, stoichiometries and speciation of sulfur, iron and carbon in St. Louis River sediments are expected correlate with rates of mercury methylation. Vegetated areas are expected to experience higher rates of mercury methylation than unvegetated zones due to differences in microbial communities and organic matter content and chemistry; however, the effect of DOM additions on mercury methylation and microbial commu- nity structure should be more pronounced in unvegetated microcosms, because mercury methylation in these microcosms should be carbon- limited. Correlations between microcosm chemistry, DOM characteris- tics, microbial communities and mercury methylation should help elucidate differences in the factors regulating mercury methylation across vegetative gradients.

Potential to Further Environmental/Human Health Protection

This project will aid the remediation of aquatic heavy metal pollution
in an environmentally sensitive and economically important area. The results of this study should add to mechanistic understandings of the geochemical and microbial factors that regulate mercury methylation and in turn, aid in preserving clean water, protecting human food supplies and mitigating exposure to mercury toxicity.

Supplemental Keywords:

microbiology, Great Lakes, mercury

Progress and Final Reports:

  • Final