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Enzymatic controls on sulfur isotope fractionation: implications for isotope biosignatures


The sequential reduction of sulfate to sulfide during dissimilatory sulfate reduction leads to a mass-dependant fractionation of sulfur isotopes. As a result, sulfide becomes enriched in lighter isotopes relative to sulfate. This difference in the isotopic composition has been used as an indicator of biological sulfate reduction in sedimentary sulfides and oceanic sulfate preserved in the geologic record. Indeed, sulfur isotope evidence suggests that sulfate-reduction dates back to the Archaean, making it one of the earliest metabolisms on Earth. Interpretation and identification of biological sulfur fractionation in the geologic record depends on how well we understand the physiological and environmental factors controlling fractionation. It is well documented that biological reaction networks of sulfate reduction, control isotopic fractionations of multiple sulfur isotopes. The extent of fractionation depends on physiological differences between organisms, as well as environmental variables that influence sulfate-reduction rates. Two key conserved enzymes involved in the reduction of sulfate to sulfite (APS reductase), and sulfite to sulfide (dissimilatory sulfite reductase), impart significant fractionations during sulfate reduction. Precise fractionation f actors associated with these enzyme-catalyzed reductions however, have never been well characterized for any organism. We propose to examine how purified enzymes of selected sulfate-reducers fractionate multiple sulfur isotopes. Temperature-dependant activity assays will be used to determine fractionation patterns over the full range of activity of each enzyme. Isotopic abundances for sulfate, sulfite and sulfide will be determined for multiple sulfur isotopes. The results from these experiments will place crucial constraints on current fractionation models and improve interpretation and identification of biological sulfur fractionation in the geologic record.

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