Wednesday, October 16, 2013
Environmental Science and Engineering Seminar
What controls sulfur isotope fractionation during microbial sulfate reduction?
Min Sub Sim, Postdoctoral Fellow, Geological & Planetary Sciences, Caltech
Microbial sulfate reduction (MSR) is an anaerobic metabolism responsible for roughly 50% of the remineralization of organic matter in marine sediments. This process produces significant sulfur isotope effects, where the product sulfide is depleted in heavy isotopes with respect to the starting sulfate. While the fractionation of sulfur isotopes has been used as diagnostic of MSR, a mechanistic understanding of factors that control the range of fractionation is still lacking. Here we demonstrate how the physiology of sulfate reducing microbes affects the fractionation of quadruple sulfur isotopes. For example, depending on the organic electron donor, sulfur isotope enrichment factors and triple isotope fractionation coefficients vary between 6 to 66 ‰, and 0.5090 to 0.5144, respectively. These values greatly exceed those previously attributed to MSR. Sulfur isotope effect also increases when electrons are diverted from the respiratory chain to nitrogen fixation or fermentation. Our findings question some fundamental assumptions related to the interpretations of naturally-occurring sulfur isotope fractionation, and allow the interpretation with a stronger microbiological perspective.