Sulfide Metabolism and Toxicity in Chlorobaculum Tepidum

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Hydrogen sulfide, the most reduced species of sulfur known, is an important intermediate in the global sulfur cycle, and is also both a potent toxin to and signaling molecule for humans. The research that will be supported by this grant will increase our understanding of a key enzyme that oxidizes sulfide either as a key part of energy metabolism and for detoxification, sulfide:quinone oxidoreductase (SQR). Genes encoding homologs of SQR are found in organisms from all three domains of life ranging from small genome size bacteria and archaea to humans, but few members of this enzyme family have been studied in molecular detail. This study will utilize a model system, Chlorobaculum tepidum, which is the only organism known to contain three genes that encode SQR homologs. Prelimary data indicate that all three genes are expressed and that two of these genes encode active SQRs while the activity of the third gene product remains cryptic. C. tepidum is an anoxygenic, phototrophic bacterium that utilizes sulfide and other reduced sulfur compounds as electron donors for photosynthesis under anaerobic conditions. Anaerobic sulfur oxidizers like C. tepidum help to maintain low sulfide fluxes from anaerobic environments like sediments and hydrothermal systems thereby helping to maintain the aerobic biosphere. The specific goals of this research are to understand the roles of the C. tepidum SQR homologs in the context of the biology of this organism and help to define specific relationships between sequence and functional diversity in the SQR family. All three of the homologs are from clades of SQR that have not previously been studied in biochemical detail. SQRs from two other strains of green sulfur bacteria that are thought to represent low sulfide and high sulfide adapted ecotypes relative to C. tepidum will also be examined. Broader Impacts. A powerful combination of electrochemical, biochemical, and molecular genetic techniques will be employed in the course of this program that will provide highly interdisciplinary training for the undergraduate and graduate students involved. The laboratory has a very successful history of mentoring undergraduates who present data at national meetings and serve as co-authors on published papers. Plans are also in place to disseminate information to the general public and to K-12 educators through numerous ongoing outreach programs at the University of Delaware. These activities include helping the public construct their own microbial gardens and view samples on a phase contrast microscope and providing materials and experiments for incorporating microbiology into K-12 science curriculum.