Intellectual merit: The proposed research is based on recent achievements by the PIs and others, and seeks to expand the PIs? on-going studies of isotopic fractionation during microbial and abiotic Hg transformations. During the last five years the PIs have shown that: (i) Mass dependent isotopic fractionation (MDF) occurred during the microbial reduction of ionic mercury (Hg[II]) by several bacterial strains that possess the enzyme mercuric reductase and MDF also occurred during microbial methylmercury (MeHg) degradation; (ii) Photoreduction of Hg(II) and photodegradation of MeHg caused MDF as well as mass independent fractionation (MIF) of up to 2?. Varying amounts of MIF (denoted as Ä199Hg and Ä201Hg) recorded in freshwater and marine fish tissue suggested different extents of photo degradation of MeHg prior to its incorporation into the aquatic food web. These results along with large variations in natural samples documented by the Blum lab and others, strongly suggest that the isotopic composition of Hg has the potential for distinguishing between different sources of Hg(0) emissions and pathways of Hg(II) reduction and MeHg degradation. To date, the PIs? studies have focused on Hg redox transformations and MeHg degradation. Yet, the bioaccumulation of MeHg in aquatic food webs has a profound effect on human and ecosystem health and the examination of whether or not isotope fractionation occurs during methylation of Hg is, therefore, a high priority. If significant fractionation occurs during formation of MeHg and is modulated by different environmental conditions and by the nature of the methylating processes, tools for distinguishing sources and pathways of MeHg in the environment may become available and enhance the management of Hg contaminated ecosystems. The first objective of the proposed study is the examination of isotopic fractionation during Hg methylation by sulfate and iron reducing bacteria to test the hypothesis that microbial Hg methylation results in significant MDF, but not in MIF. The second objective is the investigation of how environmental variables, which define freshwater and marine environments, impact MIF and MDF during Hg redox transformations to test the hypothesis that photochemical reduction, oxidation and demethylation will imprint diagnostic MDF and MIF signatures on reaction substrates and products. Finally, Hg isotopic fractionation during transformation pathways mediated by an important component of aquatic ecosystems, phototrophic planktonic organisms, has not been examined to date. The third objective addresses this lack of knowledge by testing the hypothesis that intracellular Hg(II) reduction and MeHg degradation in phytoplankton incubated in the light will result in MDF and possibly MIF.
Broader Impact: The proposed research activity will continue to lay the groundwork for a new approach or the identification of sources, sinks, and pathways of Hg transformations in impacted ecosystems. This approach has the potential to significantly enhance understanding of Hg biogeochemistry on temporal and spatial scales ranging from molecular mechanisms, to ecosystems, to global cycles, and to the geological record. As ecosystem Hg contamination remains a major public health concern, this project will support implementation of sound environmental practices to reduce Hg contamination and exposure. The proposed research will train a postdoctoral fellow in the application of stable isotope-based approaches in geobiology and ecosystem processes. In addition, undergraduate and graduate students will be integrated into the project, exposing them to cutting edge concepts and technologies, which are at the interface between biology, geology and ecosystem sciences. It is at this interface that important paradigm-shifting, research advances are being made. Undergraduate students will assist with the analytical geochemistry as part of senior thesis research projects and PhD dissertation projects. Results will be published and disseminated broadly.
This project sought to quantify the discrimination among stable isotopes of mercury (Hg) during a variety of abiotic and biologically-driven processes in aquatic environments. We specifically focused on reactions of Hg driven by sunlight (photochemical reactions), the biological conversion of inorganic mercury (Hg(II)) to the neurotoxic form, monomethylmercury (MeHg), and the accumulation of MeHg in aquatic food webs. A new photochemical pathway, the reduction of intracellular Hg(II) and MeHg was examined in marine phytoplankton. Distinct patterns of isotopic discrimination were observed during photochemical reactions of Hg(II) and MeHg and during Hg methylation, but not during the transfer of MeHg from food to aquatic animals including fish. In addition, the intracellular reduction of MeHg in marine phytoplankton resulted in a similar pattern of discrimination against the odd isotopes of Hg (199Hg and 201Hg) beyond that expected based solely on isotopic mass (referred to as "mass-independent fractionation" or MIF) that is observed in marine fish. Thus the isotopic composition of Hg in fish is largely set at the base of aquatic food webs. Methods were developed to determine the isotopic composition of MeHg in aquatic organisms and marine sediments. These techniques will improve our ability to track MeHg from where it is formed in the environment into aquatic food webs and therefore improve environmental management strategies aimed at reducing exposures to wildlife and humans.
