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.
Mercury has been released to the environment in large quantities beginning during the Industrial Revolution. Because it has a common gaseous form in the atmosphere it spreads quickly throughout the atmosphere and has contaminated virtually every ecosystem globally. Mercury is a potent neurotoxin and is thus a threat to human and wildlife health. Microbial processes transform the widely distributed inorganic forms of mercury into a more toxic and bioaccumulative form of organic mercury known as methylmercury. Mercury has seven stable isotopes that react at different rates during chemical transformations depending on their mass and also on whether their nuclei possess nuclear spin. As a result, the ratios of stable mercury isotope vary in nature depending on the origin of a particular pool of mercury and on the history of which biogeochemical reactions it has been subjected to. The research supported by this grant was designed to investigate how various biotic and abiotic reactions affected the isotopic composition of the product form of mercury compared to the reactant form (aka isotope fractionation). This information is essential for a full interpretation of mercury isotope signatures found in the environment. Research was completed following three different experimental approaches. First, laboratory experiment were used to measure mercury isotope fractionation during carefully controlled biotic and abiotic experiments which yielded isotope fractionation factors that can now be used to interpret isotopic variability in natural systems. Second, the well-documented observation that methylmercury biomagnifies along food chains whereas inorganic mercury does not biomagnify, was used to isolate the isotopic composition of inorganic and organic forms of mercury in forest, freshwater and marine foodchains and thus determine fractionation associated with methylation. Third, we developed a new gas chromatography based analytical method for separating large enough quantities of methylmercury from complex matrices that we could directly measure the isotopic composition of organic and inorganic mercury in individual samples. The results of this research have been documented in eleven peer-reviewed research articles, two peer-reviewed review articles and six conference abstracts, several of which are currently being written up for submission as research articles. At the University of Michigan this grant has partially supported two PhD students (Kwon and Motta), one Postdoctoral Fellow (Tsui), and two undergraduate researchers (Chirby and Cooper). Results from our studies have already been cited many times in the literature and are being used to enhance our understanding of Hg in the environment and the application of mercury isotopes to studies that may ultimately help control Hg levels in aquatic and terrestrial animals as well as in humans.
