| Summary: | In PNAS, Wing and Halevy present a new model that quantitatively describes the magnitude of sulfur isotope fractionation produced by dissimilatory microbial sulfate reduction (MSR). MSR is a major player in the global biogeochemical cycles and is responsible for the respiration of up to 30% of organic matter in marine sediments. This metabolism produces large isotope effects, in which the product, sulfide, is depleted in the heavy isotopes ([superscript 33]S, [superscript 34]S, and [superscript 36]S) relative to the most abundant isotope [superscript 32]S (3), enriching modern seawater sulfate in [superscript 34]S by about 21‰ (parts per thousand) compared with mantle sulfur. Sedimentary sulfur minerals preserve a record of this effect and are used to track changes in the sulfur isotope composition of seawater and the biogeochemical sulfur, carbon, and oxygen cycles through geologic time (4). Such reconstructions require an understanding of factors that control the magnitude of sulfur isotope effects and dictate the fractionation of sulfur isotopes by sulfate reducers under a range of growth conditions.
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