Multiple stable isotopes as a tool for studying the mechanism of bacterial sulfate reduction

Student thesis: MSc Thesis

Abstract

Dissimilatory bacterial sulfate reduction (BSR) is a gained energy process, where prokaryotes utilize sulfate as an electron acceptor to oxidize organic matter in anoxic environments. In marine sediment, BSR is responsible for over half of the oxidation of organic matter. In addition, previous studies have suggested that sulfate is coupled to iron through different processes. Furthermore, the anaerobic oxidation of methane (AOM) is coupled mostly to bacterial sulfate reduction (BSR) in marine sediments, preventing the Earth's vast oceans from becoming a major source of this potent greenhouse gas. However, despite its important role, the mechanism of BSR and how it is connected to other biogeochemical is still not clearly understood.
The aim of this study was to follow the pathways of BSR in sediments by using geochemical approach, specifically chemical and isotope (C,S,O) profiles of pore fluids and water column, in order to understand how sulfate is incorporated in different diagenetic processes. The most powerful tool that used was the combined measurement and modeling of δ18OSO4 δ34SSO4  which enabled to determine how sulfate is recycled. Sulfur isotope fractionation is a function of each step of the sulfate reduction and the forward and backward fluxes. On the other hand, The oxygen isotope curve shows not only kinetic fractionation effect but also a component of the ambient water isotopic composition.
Data from the shallow Eastern Mediterranean on  and the Yarqon estuary, shows
correlation between the net rate of BSR and the slope of the relative evolution of oxygen and sulfur isotopes (δ18OSO4 δ34SSO4) and the more intense sulfite oxidation.
Another goal of this study was to investigate the possibility that microorganisms use the sulfur intermediate in the iron reduction zone as an electron shuttling to increase the availability of iron. I observed in one pore fluids profile at the Mediterranean sediments and two water column profiles from Lake Kinneret an increase in δ18OSO4 while the δ34SSO4 and sulfate concentration remained untouched. This implies that sulfate is incorporated (with net zero sulfate reduction ) during bacterial iron reduction. Although the specific mechanism requires further investigation, I offer that sulfate is reduced to unknown species and then is fully oxidation back to sulfate pool via iron reduction.
The last part of this study dealt with the coupling between methane and sulfate in the AOM zone. A pore fluids profile from the Yarqon estuary shows that the AOM at this occurs in a spread zone, similar to pore fluids profiles that have been observed in seeps. I suggest that the unique conditions of Yarqon estuary results in limitations of sulfate in this zone. The cross plot of δ18OSO4 vs.  δ34SSO4 indicates a change in the BSR mechanism at this zone and or suggests that more sulfate is being recycled at this zone. This can be, for example, through sulfide oxidation back to sulfate, most likely via iron reduction, increasing the supply of sulfate at this zone' which may be limiting factor for AOM. 
 
Date of Award2011
Original languageEnglish
SupervisorOrit Sivan (Supervisor), Alexandra V. Turchyn (Supervisor) & Barak Herut (Supervisor)

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