U.S. Department of Energy

Pacific Northwest National Laboratory

Rarely Studied Microbes Associated With Mercury Toxicity in the Great Lakes

Vegetated wetland sites lead to much higher methylmercury production than unvegetated sites but unvegetated sites are much more susceptible to disturbance-induced methylation.

New paper lays foundation for future studies of the poorly understood organisms.

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 the Great Lakes region. It also provides evidence that enhanced toxicity in response to organic matter loading may be due to increases in a specific clade of understudied microorganisms in addition to heightened overall microbiome activity.

The Impact

The study shows the potential for methylmercury (MeHg) generation by poorly understood fermenting microorganisms that have not been historically considered to influence mercury toxicity; it provides a foundation for future work targeting these organisms; and it suggests mechanisms for improved mercury contamination monitoring. The study also records new insights into how dissolved organic matter (DOM) may influence microbiome structure and activity in two disparate sediment types, an influence that in turn impacts MeHg production in natural settings within the Great Lakes.


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 control of MeHg production through chemical interactions that change the bioavailability of mercury and by supporting the growth of microbiomes. 

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 illustrate a legacy of mercury contamination from shipping and industry. Their research revealed a greater relative capacity for mercury methylation in vegetated sediments compared to non-vegetated ones. However, it also showed that mercury cycling in nutrient-poor non-vegetated sediments are susceptible to DOM inputs in the form of plant leachate. With leachate added, these non-vegetated microcosms produced substantially more MeHg than unamended microcosms and also showed a marked increase in Clostridia.

Clostridiahave 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, DOM chemistry suggested an increase in fermentation related to MeHg production. Metagenomic analysis supported both an increase in Clostridiaand fermentation.

In total, the study’s observations provide a foundation for future work on the involvement of novel microorganisms in mercury methylation in environmental conditions. They also highlight the need to further study the microbial ecology of mercury methylation.


Graham, E. B., Gabor, R. S., Schooler, S., McKnight, D. M., Nemergut, D. R., and Knelman, J. E. (2018). “Oligotrophic wetland sediments susceptible to shifts in microbiomes and mercury cycling with dissolved organic matter addition.”PeerJ. 2018 Apr 3;6:e4575. doi: 10.7717/peerj.4575.

July 2018
| Pacific Northwest National Laboratory