BER Research Highlights

Search Date: March 21, 2019

4 Records match the search term(s):


September 25, 2018

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

New paper lays foundation for future studies of the role of understudied microorganisms in methylmercury production.

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.

Contacts (BER PM)
David Lesmes and Paul Bayer
U.S. DOE
David.Lesmes@science.doe.gov, Paul.Bayer@science.doe.gov

(PI Contact)
Emily B. Graham
Research Scientist, Pacific Northwest National Laboratory
emily.graham@pnnl.gov

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.

Publications
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]

Related Links
Rarely Studied Microbes Associated With Mercury Toxicity in the Great Lakes

Topic Areas:

Division: SC-23 BER


September 18, 2018

Representing Microtopography Effects in Hydrology Models

A novel subgrid model improves the representation of hydrologic processes.

The Science
Microtopography is known to be an important control on surface water retention, evaporation, infiltration, and runoff generation.   Unfortunately, direct representation of microtopography effects in models of those processes is typically not feasible because of the high spatial and temporal resolution required. A subgrid model was developed to include microtopography effects in lower-resolution models, thus improving the representation of key hydrologic processes.

The Impact
The newly developed subgrid model is broadly applicable to disparate landscapes and significantly improves the representation of runoff generation and inundation compared with neglecting small-scale topography. The subgrid model enables process-resolving models of permafrost thermal hydrology to expand to catchment scales and decadal timeframes. 

Summary
Fine-scale simulations using high-resolution digital elevation models highlight the importance of microtopography and its effects on integrated hydrology in polygonal tundra, hummocky bogs, and hillslopes with incised rills. A subgrid model that modifies the flow and accumulation terms in lower-resolution models replicates the microtopography-resolving simulations at orders-of-magnitude smaller computation cost. The subgrid model makes it possible to incorporate thaw-induced dynamic topography in simulations addressing the evolution of carbon-rich Arctic tundra in a warming climate.

Contacts (BER PM)
David Lesmes and Daniel Stover
SC-23.1
David.Lesmes@science.doe.gov and Daniel.Stover@science.doe.gov

(PI Contact)
Ahmad Jan
Climate Change Science Institute, Oak Ridge National Laboratory
jana@ornl.gov

Funding
This work was supported by Interoperable Design of Extreme-scale Application Software (IDEAS) project and by the Next Generation Ecosystem Experiment (NGEE-Arctic) project.

Publications
Jan, A., E.T. Coon, J.D. Graham, and S.L. Painter, “A subgrid approach for modeling microtopography effects on overland flow.” Water Resources Research, 54(9), 6153-6167 (2018). [DOI:10.1029/2017WR021898]

Topic Areas:


August 14, 2018

A Simplified Way to Predict the Function of Microbial Communities

A pioneering study offers an easier approach to studying microbial functioning and could help scientists advance models of biogeochemical cycling.

The Science
In areas that flood frequently, microbial communities must adapt to repeated wet-dry cycles. Metabolic strategies help them survive, but these strategies can also influence nutrient cycling and atmospheric emissions from soils and sediments. An international team of scientists examined soils from rice paddies to understand how microbial communities function during floods. Their work suggests analyzing carbon that microbes extracted from water may prove critical to understanding and modeling these important communities.

The Impact
How microbes function in often-flooded soils has profound impacts on crop production, in part because they can deliver nutrients to plants and stabilize or release atmospheric emissions from soils. Understanding how microbial communities function in soils—before, during, and after flooding—can help scientists improve modeling and promote beneficial changes in those communities.

Summary
To understand how microbial activity varied in response to flooding, scientists studied three types of organic matter that are commonly found in three types of rice paddy soils: dried rice straw, charred rice straw, and cattle manure. Team members came from the SLAC National Accelerator Laboratory; Stanford University; Swedish University of Agricultural Sciences; University of California, Riverside; and EMSL, the Environmental Molecular Sciences Laboratory, a U.S. Department of Energy Office of Science user facility. While other studies used a similar approach to look at well-aerated, upland soil and simple carbon compounds, or single micro-organisms, none examined the full complexity of natural soil and carbon substrates during the transition from dry to flooded conditions. The team used EMSL’s Fourier-transform ion cyclotron resonance mass spectrometer to analyze dissolved carbon and then observed how microbial functioning changed. These pioneering experiments produced surprising results. Not only were researchers able to better understand how microbes breathed and obtained energy during flooded conditions, but they discovered that a focus on water-extractable carbon was sufficient to predict microbial respiration rates from diverse metabolic strategies. Though more in-depth studies will be important to reveal underlying functions, the insights gained from this study give scientists a proxy to begin modeling these complex interactions.

BER PM Contact
Paul Bayer, SC-23.1

PI Contact
Kristin Boye
Stanford University
kboye@slac.stanford.edu

Funding
This work was supported by the U.S. Department of Energy’s Office of Science (Office of Biological and Environmental Research), including support of the Environmental Molecular Sciences Laboratory (EMSL), a DOE Office of Science User Facility; SLAC National Accelerator Laboratory and the BER Subsurface Biogeochemical Research program; Swedish Foundation for International Cooperation in Research and Higher Education; Swedish Research Council for Environment, Agricultural Sciences, and Spatial Planning; and U.S. National Science Foundation.

Publication
Boye, K., A.H. Hermann, M.V. Schaefer, M.M. Tfaily, and S. Fendorf. “Discerning Microbially Mediated Processes During Redox Transitions in Flooded Soils Using Carbon and Energy Balances.” Frontiers in Environmental Science 6 Article 15 (2018). [DOI:10.3389/fenvs.2018.00015]

Related Links
A Simplified Way to Predict the Function of Microbial Communities EMSL science highlight

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Division:
SC-23 BER


January 27, 2018

Clarifying Rates of Methylmercury Production

New model provides more accurate rate constant estimates for mercury methylation and demethylation.

The Science
Using new experiments and re-analyses of previous experiments, a new two-site reversible sorption model was developed to describe the production of methylmercury over time. The new model takes into account competing processes and results in faster rates of production than previously estimated.

The Impact
Simulations of methylmercury production and transport demonstrate that methylmercury production is likely significantly larger than estimated by currently used models.

Summary
Mercury (Hg) is a toxic element that occurs naturally and as an anthropogenic pollutant in the environment. The neurotoxin monomethylmercury (MMHg) is a particular concern because it biomagnifies in aquatic environments and has adverse development effects on young children and developing embryos. MMHg is formed in the environment from inorganic Hg through the action of microorganisms in a process called Hg methylation. Because of its toxicity, there have been many attempts to measure Hg methylation and MMHg demethylation rates in various environmental settings with differing results. Even in laboratory experiments, rates for the methylation of Hg to MMHg often exhibit kinetics that are inconsistent with first-order kinetic models. In a new study, scientists from Oak Ridge National Laboratory used time-resolved measurements of filter-passing Hg and MMHg during methylation/demethylation assays, and they re-analyzed previous assays. Then they used a multi-site kinetic sorption model to show that competing kinetic sorption reactions can lead to apparent non-first-order kinetics in Hg methylation and MMHg demethylation. The new model can describe the range of behaviors for time-resolved methylation/demethylation data reported in the literature including those that exhibit non first-order kinetics. Additionally, the team showed that neglecting competing sorption processes can confound analyses of methylation/demethylation assays, resulting in rate constant estimates that are systematically biased low. Simulations of MMHg production and transport in a hypothetical periphyton biofilm bed illustrate the implications of the new model and demonstrate that methylmercury production may be significantly different than projected by single-rate first-order models.

Contacts (BER PM)
Paul Bayer
Paul.Bayer@science.doe.gov; 301-903-5324

(PI Contact)
Scott Brooks
brookssc@ornl.gov/ 865-574-6398

Funding
This work was funded by the U.S. Department of Energy, Office of Science, Biological and Environmental Research, Subsurface Biogeochemical Research Program and is a product of the Science Focus Area (SFA) at ORNL. The isotopes used in this research were supplied by the United States Department of Energy Office of Science by the Isotope Program in the Office of Nuclear Physics.

Publications Olsen, T.A., K. A. Muller, S. L. Painter, and S. C. Brooks. "Kinetics of Mercury Methylation Revisited" Environmental Science & Technology 52(4), 2063-2070 (2018). [DOI:10.1021/acs.est.7b05152]

Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER