What Controls the Sulfur Isotope Fractionation during Dissimilatory Sulfate Reduction?

Sulfate often behaves conservatively in the oxygenated environments but serves as an electron acceptor for microbial respiration in a wide range of natural and engineered systems where oxygen is depleted. As a ubiquitous anaerobic dissimilatory pathway, therefore, microbial reduction of sulfate to sulfide has been of continuing interest in the field of microbiology, ecology, biochemistry, and geochemistry. Stable isotopes of sulfur are an effective tool for tracking this catabolic process as microorganisms discriminate strongly against heavy isotopes when cleaving the sulfur-oxygen bond. Along with its high preservation potential in environmental archives, a wide variation in the sulfur isotope effects can provide insights into the physiology of sulfate reducing microorganisms across temporal and spatial barriers. A vast array of parameters, including phylogeny, temperature, respiration rate, and availability of sulfate, electron donor, and other essential nutrients, has been explored as a possible determinant of the magnitude of isotope fractionation, and there is now a broad consensus that the relative availability of sulfate and electron donors primarily controls the magnitude of fractionation. As the ratio shifts toward sulfate, the sulfur isotope fractionation increases. The results of conceptual models, centered on the reversibility of each enzymatic step in the dissimilatory sulfate reduction pathway, are in qualitative agreement with the observations, although the underlying intracellular mechanisms that translate the external stimuli into the isotopic phenotype remain largely unexplored experimentally. This minireview offers a snapshot of our current understanding of the sulfur isotope effects during dissimilatory sulfate reduction as well as their potential quantitative applications. It emphasizes the importance of sulfate respiration as a model system for the isotopic investigation of other respiratory pathways that utilize oxyanions as terminal electron acceptors.