Kinetic oxygen isotope effects during dissimilatory sulfate reduction : A combined theoretical and experimental approach

• Kinetic isotope effects related to the breaking of chemical bonds drive Sulfur isotope fraction at ion during dissimilatory sit I fate reduction (DSR), whereas oxygen isotope fractional ion during DSR is dominated by exchange between intercellular sulfur intermediates and water. We use a simplified biochemical model for DSR to explore how a kinetic oxygen isotope effect may be expressed. We then explore these relationships in light of evolving sulfur and Oxygen isotope compositions (delta S-34(SO4) and delta O-18(SO4)) during batch culture growth of twelve strains of surface-reducing bacteria. Cultured under conditions to optimize growth and with identical delta O-18(H2O) and initial delta O-18(SO4), all strains show 34 S enrichment, whereas only six strains show significant O-18 enrichment. The remaining six show no (or minimal) change in delta O-18(SO4) over the growth of the bacteria. We use these experimental and theoretical results to address three questions: (i) which Sulfur intermediates exchange oxygen isotopes with water, (ii) what is the kinetic oxygen isotope effect related to the reduction of adenosine phosphosulfate (APS) to sulfite (SO32-), (iii) does a kinetic oxygen isotope effect impact the apparent oxygen isotope equilibrium values? We conclude that oxygen isotope exchange between water and a sulfur intermediate likely occurs downstream of APS and that our data constrain the kinetic oxygen isotope fractionation for the reduction of APS to sulfite to be smaller than 47 parts per thousand. This small oxygen isotope effect impacts the apparent oxygen isotope equilibrium as controlled by the extent to which APS reduction is rate-limiting.

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