Subseafloor sulfate reduction: Investigation of an unrecognized pathway and quantification of low rates
Bacterially-catalyzed sulfate reduction is the dominant respiratory pathway in much of the deep subseafloor sediment, where it yields energy for a significant fraction of the Earth's total biomass. Sulfate reduction impacts the ecology of the deep biosphere, the biogeochemistry of sediment pore water and the global marine sulfur and carbon cycles. ^ Chemical concentration profiles, thermodynamics, and sulfate isotopic compositions are a few of the methods used to determine the presence of sulfate reduction, its biochemical mechanisms, and the rates at which it occurs. By employing these methods we show that a previously unrecognized sulfate reduction pathway is likely to occur in global marine sediments and that relatively low sulfate reduction rates, which are unquantifiable by standard methods, can be quantified. The latter is achieved by modeling oxygen isotopic compositions of sulfate with the rates of exchange of oxygen isotopes during sulfate reduction. Rates as low as 2.5 x 10-19 mol cell-1 yr -1 have been determined for sediments in the eastern equatorial Pacific. Sulfate reduction rates at this location are three or more orders of magnitude lower than rates in coastal sediments and demonstrate survival at metabolic extremes. ^ The oxygen isotope model and an analysis of carbon dioxide production in marine sediments were used to draw conclusions about the mechanism for isotopic exchange during sulfate reduction, which has potential consequences for the fate of chemical species and their respective cycles. We find that isotope exchange is (1) between adenosine phosphosulfate (APS) or sulfite and cytoplasmic water or (2) occurs during disproportionation of elemental sulfur to sulfate and sulfide. Concentrations of reduced iron are not consistent with the hypothesis that exchange occurs between water and sulfite with a subsequent re-oxidation of sulfite by iron (Fe3+). ^ In this dissertation we also conclude that the previously unsuspected process, sulfate-reducing ammonium oxidation, may remove a significant amount of biologically accessible nitrogen from the world ocean and recycle it back to the atmosphere. Furthermore, the oxygen isotope model may be particularly beneficial in determining sulfate reduction rates in sediment with low activity, which represents a significant portion of the subseafloor. Incorporating low rates into the current global average sulfate reduction rates would increase the accuracy of the global assessment and better constrain the sulfate budget of the ocean. ^
Heather Nicole Schrum,
"Subseafloor sulfate reduction: Investigation of an unrecognized pathway and quantification of low rates"
Dissertations and Master's Theses (Campus Access).