Biogeochemical cycling of sulfur plays a critical role in regulating the Earth's surface redox budget and is tightly linked to the evolution of atmospheric oxygen. Microbial sulfate reduction, a microbial process that remineralizes about half of organic carbon in modern marine sediments, is the main process that fractionates sulfur isotope ratios of surface sulfur reservoirs but mechanisms that control the magnitude of sulfur isotope during microbial sulfate reduction are still not well understood. This uncertainty particularly applies to physiological conditions associated with high, greater than 50 per mil, fractionations. Because sulfur isotope fractionations between sulfate and sulfide exceeding 50 per mil become increasingly more common in the Neoproterozoic, this trend was tentatively attributed to the increase of atmospheric oxygen and oxidative recycling of sulfur. This proposal explores physiological conditions that lead to similarly large fractionations during microbial sulfate reduction alone by testing two main hypotheses: 1. Sulfate reducing microbes can produce large sulfur isotope effects (greater than 50 per mil) when growing slowly on recalcitrant organic substrates, either in pure cultures or in consortia. These substrates are not commonly used to grow sulfate reducers, but may include glucose, cellulose, lignin, and hydroxyhydroquinones. 2. The magnitude of sulfur isotope effects is a function of the intracellular coupling of carbon and sulfur metabolisms, with large sulfur isotope effects produced during non-stoichiometric sulfate reduction and in the absence of some enzymes that transfer reducing equivalents from carbon to sulfur. These hypotheses emerge from recent studies in the PIs' laboratories, in which a newly isolated sulfate reducing bacterium (DMSS-1) produced a wide range of isotope fractionation (7 to 66 permil) under electron donor-limited growth conditions in pure culture grown on glucose. Notably, the maximum measured sulfur isotope effect (66 permil) was ~20 permil larger than those observed previously in pure culture studies, but were well within the range of observed high natural fractionations. To test the first hypothesis, multiple sulfur isotope effects will be measured in batch and continuous cultures of previously isolated microbes that grow slowly and couple sulfate reduction with the oxidation of various monosaccharides, disaccharides and other organic substrates. The second hypothesis will be tested by characterizing the stoichiometry of growth of DMSS-1 on glucose during the production of large sulfur isotope effects, and by measuring multiple sulfur isotope effects produced in continuous cultures of Desulfovibrio vulgaris wild type and different mutants that lack enzymes involved in the transfer of reducing equivalents from lactate to sulfate. The results of the proposed work will directly test and constrain models of present and past sulfur cycles, oxygenation of the Earth, and the evolution of ocean chemistry. It will also contribute to a wide variety of biogeochemical problems, including the remediation and monitoring of mSR in contaminated aquifers, where mSR is an important process for degrading organic contaminants in groundwater. The proposed research will support two junior faculty members and one graduate student for two years. The proposed project contains multiple subprojects that will provide undergraduate research opportunities under the guidance of graduate students and PIs. PIs have hosted over ten undergraduate students, high school students, as well as K-12 science teachers from the Boston area as interns during the summer. Funding is requested for a 6-week summer stipend for a K-12 science teacher intern. The teachers, high school students and undergraduates will work on projects that provide a hands-on experience in various topics related to microbial diversity, geochemistry, Earth history and modern methods of linking genomic information to geochemical processes.
