- We present new sulfur and oxygen isotope data in sulfate (δ34SSO4δ34SSO4 and δ18OSO4δ18OSO4, respectively), from globally distributed marine and estuary pore fluids. We use this data with a model of the biochemical steps involved in bacterial sulfate reduction (BSR) to explore how the slope on a δ18OSO4δ18OSO4 vs. δ34SSO4δ34SSO4 plot relates to the net sulfate reduction rate (nSRR) across a diverse range of natural environments. Our data demonstrate a correlation between the nSRR and the slope of the relative evolution of oxygen and sulfur isotopes (δ18OSO4δ18OSO4 vs. δ34SSO4δ34SSO4) in the residual sulfate pool, such that higher nSRR results in a lower slope (sulfur isotopes increase faster relative to oxygen isotopes). We combine these results with previously published literature data to show that this correlation scales over many orders of magnitude of nSRR. Our model of the mechanism of BSR indicates that the critical parameter for the relative evolution of oxygen and sulfur isotopes in sulfate during BSR in natural environments is the rate of intracellular sulfite oxidation. In environments where sulfate reduction is fast, such as estuaries and marginal marine environments, this sulfite reoxidation is minimal, and the δ18OSO4δ18OSO4 increases more slowly relative to the δ34SSO4δ34SSO4. In contrast, in environments where sulfate reduction is very slow, such as deep sea sediments, our model suggests sulfite reoxidation is far more extensive, with as much as 99% of the sulfate being thus recycled; in these environments the δ18OSO4δ18OSO4 increases much more rapidly relative to the δ34SSO4δ34SSO4. We speculate that the recycling of sulfite plays a physiological role during BSR, helping maintain microbial activity where the availability of the electron donor (e.g. available organic matter) is low.