Wil Leavitt (Dept. of Earth & Planetary Sciences/Johnston Lab)
The sedimentary sulfur isotope record is an integrator of biochemical processes, the most quantitatively important of which is microbial sulfate reduction (MSR). Interpretations of the sedimentary sulfur isotope record rely largely on our understanding of the fractionation associated with MSR. This has been empirically determined by numerous cellular-scale studies [1, 2, 3]. Still, a mechanistic understanding for the controls on this fractionation has proven elusive. Moreover, metabolic isotope models of MSR underpin our interpretations of the modern and ancient sulfur isotope records, yet such models require we know the magnitude of fractionation at each node (i.e. enzymatic step) within the metabolic network [4, 5]—to-date these remain underdetermined in the literature. We present data from experimental work with the purified dissimilatory sulfite reductase (Dsr), from which we can measure and calculate the enzyme-specific isotope fractionation factors (34aDsr and 33aDsr) . In all cases, elemental and isotopic mass balance was satisfied, and individual sulfur pools were isolated from the bulk solution by sequential precipitation and extraction. These are the first enzyme-level constraints on isotope fractionation during MSR, and serve as a template for evaluating the other prominent enzymatic reduction steps within this metabolic process. This work also provides a set of fundamental boundary conditions for the metabolic fractionation models of MSR [2, 5]. By taking an enzyme-level approach to understanding isotopic fractionation in MSR, we begin to provide the most fundamental constraints on the biogeochemical fractionation signatures . Akin to the early carbon isotope work on RuBisCO and its importance to our understanding of the carbon cycle , the work presented herein will help to unlock the secrets of the sulfur cycle and ultimately allow for the full isotopic interpretation of Earth’s sulfur isotope records.
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