Physiological data suggest that a thermodynamic hierarchy for the utilization of oxidants could be written for Shewanella putrefaciens alone from aerobic growth in the oxic zone to thiosulfate reduction at the interface between sulfide-free and sulfitic zones of suboxic/anoxic marine basins and porewaters. With its versatile carbon metabolism and facultative nature, S. putrefaciens and organisms like it may well out compete sulfate- reducing bacteria for the consumption of thiosulfate and other sulfur intermediates in marine anoxic basins and sedimentary porewaters. These organisms may play a substantial role in the control of sulfur speciation in the marine environment. By utilizing organic carbon and producing sulfide, these organisms would be intimately involved in the poise of the chemocline. This project will provide new, basic scientific data on the molecular genetics of a widespread and very versatile anaerobic- respiring organism. By isolating and sequencing genes that code for sulfur reductase, this project will be able to study the regulatory mechanisms of gene expression. Manipulation of the genes in vivo will lead to a better understanding of the relationships between the various sulfur reduction pathways in S. putrefaciens and possibly other marine eubacteria. Genetic manipulation could also aid in comparing the regulation of sulfur reductase activity to that of nitrate reductase or Fe reductase in S. putrefaciens. This work will lead to the collaborative development of a probe for nonsulfate-reducing bacteria which utilize sulfur intermediates in natural sediments. Such a probe along with data on the mechanism of sulfur intermediate reduction could be used to explore the potential contribution of nonsulfate-reducing bacteria such as S. putrefaciens to the geochemistry of S, Fe, and C in a variety of suboxic/anoxic marine environments where these sulfur species exist.
