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Oak Ridge National Laboratory SFA: Biogeochemical and Molecular Mechanisms Controlling Mercury Transformation in Contaminated Environments

Laboratory Research Manager: Liyuan Liang
Co-Investigators: S. Brooks, B. Gu, A. Palumbo, J. Smith

The Oak Ridge National Laboratory (ORNL) Science Focus Area (SFA) Program responds to Environmental Remediation Science Program (ERSP) needs by addressing the scientific issues that limit contaminant remediation at the Oak Ridge Reservation (ORR). Over the initial 5 to 10 year period, ORNL’s SFA will address significant knowledge gaps regarding biogeochemical transformations that govern mercury (Hg) speciation at the sediment-water interface, particularly, the processes controlling methyl mercury (MeHg) production at contaminated sites.

This program integrates geochemistry, microbiology, molecular biology and molecular simulations to understand Hg behavior in the field. This research is underpinned by ORNL’s strong core expertise in field-to-laboratory geochemistry and microbiology, ORNL’s world-class neutron sources and high-performance computing capabilities. It is anticipated that the results of the SFA will have direct applications to DOE complex and contaminated sites nationwide.

Microbial processes are largely responsible for the production of methyl mercury, a highly toxic form of Hg that accumulates in biota. These processes are fundamental to the oxidation-reduction and methylation-demethylation transformations that determine the fate of Hg in the environment. Biogeochemical factors controlling MeHg production at contaminated sites presents a serious knowledge gap at the Oak Ridge Reservation (ORR) and across the DOE complex. ORR field data show that reduction in Hg levels does not necessarily lead to decrease in aqueous MeHg or to its bioaccumulation in fish. This observation suggests that at the high Hg levels commonly present in highly contaminated source areas, either MeHg production is inhibited or demethylation significantly exceeds methylation. An integrated program that com-bines both abiotic and microbiologic studies is essential to determine the key biogeochemical controls on net MeHg production. ORNL´s SFA will examine (1) how abiotic reactions affect Hg speciation and complexation, (2) what products or forms of Hg are bioavailable for microbial methylation, (3) what levels of Hg to initiate microbial demethylation and how this process can be tuned to favor demethylation, (4) what are the dominant microbes that participate in methyla-tion or demethylation and how do geochemical conditions affect these transformations, and (5) how catalyzed chemical and photochemical reactions influence Hg speciation and transforma-tions in water. To resolve these questions, we have developed an integrated hypothesis-driven research program that focuses on both abiotic and microbial processes involved in Hg trans-formations. The program incorporates suggestions from a recent peer review. The primary objectives are to elucidate the rates, mechanisms and controls of abiotic and microbial processes affecting Hg speciation and transformation, resolve what critical Hg precursors are produced and subsequently methylated at the sediment-water interface, and develop and validate subcellular models to understand in detail the biochemical and biophysical mechanisms of transformation between Hg species and MeHg.

The ORNL SFA is an integrated multi-scale, multi-disciplinary, multi-institutional research program whose goals will be accomplished through a series of controlled laboratory, microcosm and field experiments. We will use state-of-the-art techniques that include advanced chemical, spectroscopic, synchrotron-based X-ray and neutron scattering methods to investigate funda-mental processes that control Hg speciation, bioavailability, and net MeHg production. Func-tional genomics will be used to identify key microbial groups and genetic controls on Hg methylation. Studies of biomolecular structure, function and dynamics will be combined with molecular simulations to advance detailed understanding of reaction pathways and key mechanisms of methylation and demethylation. The four principal research tasks are: 1. Site biogeochemical processes and microcosm studies; 2. Fundamental mechanisms and transforma-tions; 3. Microbial transformations and genetics; 4. Molecular structure, dynamics, and simula-tion. This research, in collaboration with other SBR investigators, is designed to make funda-mental scientific contributions to support the prediction and mitigation of mercury at DOE contaminated sites.