An investigation of the isotope fractionation of mercury (Hg) by microbial and abiotic redox transformations is proposed herein. It is believed that this work fits appropriately within the Geosciences program: Research in Biogeosciences Opportunities in Geomicrobial Processes. Specifically, this research fits into Focus II of this program, which emphasizes developing new research techniques (including isotope proxies) to address questions generated at the intersection between biology and geology. Two of the PIs (Blum and Klaue) have worked for the past five years to develop an analytical method with high enough precision to allow measurement of natural variations in the isotopic composition of Hg. This group has demonstrated that they can routinely measure Hg isotope ratios to an accuracy of 0.02 permil per atomic mass unit (amu) and recent research by this group has explored Hg isotope variation in meteorites and ore deposits. A central question in the application of Hg isotopes to the study of Hg in the environment and as a biogeochemical proxy is the degree to which microbes fractionate Hg during various redox processes. To answer this question quantitatively requires a team with specialization in the microbiology of Hg transformations (PIs Barkay and Reinfelder at Rutgers), and specialization in the isotope geochemistry of Hg (PIs Blum and Klaue at Michigan). In preliminary experiments this team has demonstrated that a strain of Hg resistant Escherichia coli, which produce the mercuric reductase enzyme (MR), fractionate Hg isotopes during the reduction of Hg (II) to Hg(0). The magnitude of this fractionation is up to 1.5 permil per amu, (>50 times uncertainty) and it follows a Rayleigh law with a fractionation factor of 1.0006. These initial results give the PIs great optimism that Hg isotopes will provide a powerful new tracer of Hg redox transformation in the environment, and a proxy record of changes in Hg redox processes through geological time. The objective of this study is to conduct a series of carefully controlled experiments to investigate Hg isotope fractionation during each of the major microbial and abiotic redox transformations. The specific processes that will be investigated include: 1) The reduction of Hg (II) to Hg(0) by the bacterial MR, by light-dependent and independent algal processes, by Fe(II)- dependent reduction in thiobacilli, and by photoreduction. 2) The oxidation of Hg (0) to Hg(II) by bacterial enzymes whose primary cellular role is protection against oxygen damage. 3) The methylation of Hg (II) to MeHg by sulfate reducing bacteria (SRB). 4) The degradation of MeHg to Hg (II) and CH4 and CO2 by the reductive and oxidative pathways, respectively. This project will set the groundwork for Hg isotope systematics and the use of Hg isotopes in geology and biogeochemistry. Once developed, the Hg isotope tool will allow future studies that address issues such as: 1) the tracking of sources, pathways and sinks of various Hg species in the environmental and in geological deposits, 2) the use of Hg isotopes as a paleo-redox proxy in lacustrine and marine deposits, and 3) the study of the evolution of Hg-microbe interactions in sediments and sedimentary rocks. Intellectual Merit. The proposed research activity will lay the groundwork for a completely new methodology that has the potential to significantly enhance understanding of mercury biogeochemistry on scales ranging from microbial mechanisms to individual lakes to global cycles and finally to the geological record. The research team members have proven track records in studying Hg-microbe interactions (Rutgers PIs) and the isotope geochemistry and aquatic ecology of Hg (Michigan PIs) and are poised to make rapid advancements in this combined research area. Broader Impacts. The proposed research will integrate students from both Rutgers and the University of Michigan, giving them experience in advanced laboratory techniques and at the intellectual intersection between the biological and geological sciences where important, and sometimes paradigm- shifting, research advances are being made. A female PhD student at Rutgers, who has already completed the preliminary experiments, will work on the microbial experiments for her dissertation work and travel to Michigan to participate in the mass spectrometry and theoretical isotope geochemistry. An undergraduate student at Michigan will assist with the analytical geochemistry as part of a senior thesis research project. The cross-fertilization of methods and scientific approaches will be beneficial to all of the research participants and will likely lead to additional collaborations. Results will be published and disseminated broadly. As exposure to Hg remains a major public health concern this project will assist in the implementation of sound environmental practices to reduce Hg contamination and exposure.
