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Enzymatic controls on sulfur isotope fractionation: implications for isotope biosignatures

Final Activity Report Summary - ECoSIF (Enzymatic Controls on Sulfur Isotope Fractionation: Implications for Isotope Biosignatures)

The sequential reduction of sulphate to sulphide during sulphate reduction by sulphate reducing bacteria leads to a mass-dependent fractionation of sulfur isotopes. As a result, the produced sulphide becomes enriched in lighter isotopes relative to sulphate. This difference in the isotopic composition has been used as an indicator of biological sulphate reduction in sedimentary sulphide minerals and oceanic sulphate preserved in the geologic record. Indeed, sulfur isotope evidence suggests that biological sulphate-reduction dates back to several billion years, making it one of the earliest metabolisms on Earth. Interpretation and identification of biological sulfur fractionation in the geologic record depends on the extent to which we understand the physiological and environmental factors controlling fractionation. It is well documented that reaction networks of biological sulphate reduction control sulfur isotopic fractionations. The extent of fractionation depends on physiological differences between organisms, as well as on environmental variables that influence the rates of sulphate-reduction, such as temperature and substrate concentration. Two key enzymes involved in sulphate reduction, namely APS reductase and dissimilatory sulphite reductase, impart fractionations during sulphate reduction. Precise fractionation factors associated with these enzyme-catalysed reductions however have never been well characterised for any organism.

In this project we examined how enzymes of selected sulphate-reducers fractionated sulfur isotopes. Using crude extracts of desulfovibrio vulgaris we measured the fractionation during the reduction of sulphite to sulphide by the dissimilatory sulphite reductase (dsr) under different temperatures and with both organic and inorganic substrates. It was observed that temperature had a greater influence on the magnitude of fractionation than the substrate type. At 37 °C, H2 and formate produced fractionations of 18 and 15 ‰ respectively. At 25 °C the magnitude of fractionation increased to 25 ‰. This was coupled to a reduction in the rate of sulphite reduction. The results from these experiments were anticipated to contribute to our understanding of the factors that influence the fractionation of sulfur isotopes during biological sulphate reduction, and thus improve interpretation and identification of biological sulfur fractionation in the geologic record.