National Institute of Oceanography, Israel

Bacterial sulfate reduction (BSR) is responsible for the majority of organic matter oxidation in marine sediments and therefore is a key player in the global carbon cycle. The biochemical pathway of sulfate reduction occurs in several reversible steps; each of these steps has an associated isotope fractionation. The ratio between the forward and backward fluxes at each step, the relationship of the individual fluxes to the overall rate of BSR and the resultant expressed sulfur and oxygen isotope fractionation during BSR in natural environments remains enigmatic. The aim of this study is to further our understanding of BSR through analysis and subsequent modelling of sulfur and oxygen isotopes in pore fluid sulfate from ODP-acquired deep sea sediments, the shallow Eastern Mediterranean, and the Yarqon estuary (Israel). Our data demonstrates a correlation between the net rate of BSR and the slope of the relative evolution of oxygen and sulfur isotopes (delta (super 18) O (sub (SO4)) vs. delta (super 34) S (sub (SO4)) ) in the residual sulfate pool. We combine these results with literature data to show that this correlation scales over many orders of magnitude (for rate of BSR). Our model, combined with previously published pure culture data [1], suggests that the critical parameter for the relative evolution of oxygen and sulfur isotopes during BSR in natural environments is the rate of intracellular sulfite oxidation. We find that the lower the net rate of BSR, the steeper the slope (delta (super 18) O (sub (SO4)) vs. delta (super 34) S (sub (SO4)) ) and the more intense sulfite oxidation.