Rarely Studied Microbes Associated With Production of Toxic Methylmercury in Great Lakes Estuary

The Science

The bioaccumulation of mercury in plant and animal tissue is strongly linked to mercury methylation in sediments, and poses a significant environmental and human health concern in freshwater wetlands of the Great Lakes region. A study led by Emily Graham, a research scientist at Pacific Northwest National Laboratory, shows the influence of wetland vegetation in regulating mercury toxicity in a Great Lakes estuary. It also provides evidence that enhanced production of methylmercury in vegetated areas of the estuary is associated with degradation of dissolved organic matter, a shift in the microbial community towards fermentative microbes, and changes in the microbiome structure toward Clostridia species.

The Impact

This study shows the potential for methylmercury (MeHg) generation by understudied fermenting microorganisms that have not been historically considered to influence mercury toxicity. It also shows that dissolved organic matter (DOM) may influence microbiome structure and activity in vegetated areas of the estuary. Together, these findings provide scientists with a greater understanding of environmental conditions that lead to methylmercury production and offers a way to improve monitoring for mercury contamination in estuaries within the Great Lakes.

Summary

Inorganic mercury in wetlands becomes toxic methylmercury (MeHg) due to a primarily microbial process known as mercury methylation. Dissolved organic matter (DOM) is a strong regulator of MeHg production because its chemical interactions change the bioavailability of mercury and support the growth of specific types of microbial communities.

In this study, the team used anoxic microcosms with sediments from geochemically disparate vegetated and non-vegetated wetland environments. Sediments were from nearshore areas of Lake Superior’s St. Louis River Estuary, where sediments contain a legacy of mercury contamination from shipping and industry. The team’s research revealed a greater relative capacity for mercury methylation in vegetated sediments compared to non-vegetated ones. However, they also showed that mercury cycling in nutrient-poor non-vegetated sediments is susceptible to DOM inputs in the form of plant leachate. With leachate added, these non-vegetated microcosms produced substantially more MeHg than un-amended microcosms and also showed a marked increase in species of bacterial Clostridia.

Clostridia have the genetic potential to methylate mercury but have not been considered among the primary microbes responsible for mercury toxicity. These microbes ferment recalcitrant organic matter, and in addition to their increased abundance, an analysis of their metabolism suggested an increase in fermentation related to MeHg production. Metagenomic analysis supported both an increase in Clostridia and fermentation.
In total, the study’s observations provide a foundation for future work on the involvement of these understudied microorganisms in mercury methylation in estuaries of the Great Lakes. They also highlight the need to further study the microbial ecology of mercury methylation.

Principal Investigator(s)

Emily B. Graham
Pacific Northwest National Laboratory
[email protected]

Funding

This work was supported by EPA STAR and NOAA NERRS fellowships to Emily B. Graham and a JGI CSP grant to Diana R. Nemergut. The first author also was supported in part by DOE, Office of Biological and Environmental Research (BER), as part of Subsurface Biogeochemical Research Program’s Scientific Focus Area (SFA) at PNNL.

References

Graham, E. B., R.S. Gabor, S. Schooler, D.M. McKnight, D.R. Nemergut, and J.E. Knelman. “Oligotrophic wetland sediments susceptible to shifts in microbiomes and mercury cycling with dissolved organic matter addition.” PeerJ. 6:e4575. (2018). [DOI:10.7717/peerj.4575].