Sulfate reduction a form of energy metabolism that has existed on Earth for billions of years. It is a metabolic strategy of many Bacteria and some Archaea. Sulfate reduction operates using sulfate to oxidize hydrogen or organic material, thereby producing sulfide. Sulfur has several stable isotopes, including 32S, 33S, and 34S - and the process of sulfate reduction doesn’t convert them all at the same rate, slightly favoring the lighter isotopes such as 32S. This results in sulfide having a lower 34S/32S ratio than sulfate. The process of creating this difference is an isotope fractionation.
We are interested in understanding the biological and environmental controls on the magnitude of this isotope fractionation. Because both sulfate and sulfide can be preserved in sedimentary rocks - for example, sulfate in gypsum and sulfate in sulfide - we can measure how the average isotope fractionation has changed over Earth history. We’d like to better understand how to interpret this, and so are conducting a range of experiments with sulfate reducing cultures and enzymes.