Sulfur and oxygen isotope insight into microbial sulfur metabolism (Doctoral dissertation, University of Cambridge)

Dissimilatory microbial sulfate reduction (MSR) is a process where microbes
utilize sulfate as an electron acceptor to oxidize organic matter in anoxic
environments. In modern marine sediments, MSR is responsible for over half of the anoxic oxidation of organic matter. In addition, the anaerobic oxidation of methane (AOM) is coupled largely to MSR in marine sediments, in a process called sulfatedriven AOM, preventing the Earth’s oceans from becoming a major source of this potent greenhouse gas to the surface. The aim of this thesis was to elucidate the pathways of MSR coupled to organic matter oxidation and AOM by using a largely geochemical approach; specifically the chemical and isotope (C, S, O) variation in pure-culture sulfate reducing bacteria and sedimentary pore fluid profiles. I use this data to better understand how sulfate is involved in different diagenetic processes. The most powerful tool that used was the
combined measurement and modeling of sulfur and oxygen isotopes in sulfate
(δ18OSO4 and δ34SSO4, respectively), which enabled me to model how sulfate is
recycled within pure cultures as well as the natural environment. First I explore the combined multiple sulfur (33S /32S, 34S/32S) and oxygen (18O/16O) isotope fractionation in pure cultures of a marine Desulfovibrio sp. DMSS-1
grown on different organic substrates. The use of multiple isotopes allows me to
conclude that reversibility of each step during MSR in my experiment is correlates with the cell-specific rate sulfate reduction rate. I suggest that in environmental settings where the availability of the electron donor can change dramatically there may be more changes in the microbial mechanism of MSR that can be more pronounced. In the second half of this thesis I explore MSR in marine and marginal marine environments and the consumption of sulfate through sulfate-driven AOM and organoclastic MSR. I find that in environments where methane is in excess there is a lower limit of the slope between δ18OSO4 and δ34SSO4 that results in what I call a distinct isotopic signature. This isotope signature differs to that when sulfate is reduced by either organic matter oxidation or by the slower, diffusive flux of methane within marine sediments. I suggest that this signature likely results from negligible reoxidation of sulfur species when the electron donor is abundant.