Revisiting the dissimilatory sulfate reduction pathway

Abstract

Sulfur isotopes in the geological record integrate a combination of biological and diagenetic influences, but a key control on the ratio of sulfur isotopes in sedimentary materials is the magnitude of isotope fractionation imparted during dissimilatory sulfate reduction. This fractionation is controlled by the flux of sulfur through the network of chemical reactions involved in sulfate reduction and by the isotope effect associated with each of these chemical reactions. Despite its importance, the network of reactions constituting sulfate reduction is not fully understood, with two principle networks underpinning most isotope models. In this study, we build on biochemical data and recently solved crystal structures of enzymes to propose a revised network topology for the flow of sulfur through the sulfate reduction metabolism. This network is highly branched and under certain conditions produces results consistent with the observations that motivated previous sulfate reduction models. Our revised network suggests that there are two main paths to sulfide production: one that involves the production of thionate intermediates, and one that does not. We suggest that a key factor in determining sulfur isotope fractionation associated with sulfate reduction is the ratio of the rate at which electrons are supplied to subunits of Dsr vs. the rate of sulfite delivery to the active site of Dsr. This reaction network may help geochemists to better understand the relationship between the physiology of sulfate reduction and the isotopic record it produces.

Intracellular metabolite levels shape sulfur isotope fractionation during microbial sulfate respiration

“…3 C, F, and I). This finding is in marked contrast to the wide range of 33 S-32 S and 34 S-32 S fractionations that are predicted by phenomenological models of multiple S-isotope fractionation during microbial sulfate reduction (8,14,17). As explored below, this behavior has its roots in the similar ½MK red =½MK ox and 34 « kin uptake values assigned to the sulfate reducers examined here.…”

“…In parallel with experimental studies, theoretical work has built a broad foundation for understanding the net sulfur isotope fractionation expressed during sulfate respiration (13)(14)(15)(16)(17). These efforts initially dealt with sulfur flow through simplified metabolic networks (13) (Fig.…”

“…These efforts initially dealt with sulfur flow through simplified metabolic networks (13) (Fig. 1A) and have expanded to incorporate, for example, electron supply to the reaction cycles of individual respiratory enzymes (17). The reversibility of an individual enzymatic reaction is a central theoretical concept behind these approaches, as it carries the isotopic memory of downstream steps in the pathway (Fig.…”

High-sulfur in beef cattle diets: A review

“…In assimilatory reduction, bacteria reduce sulfate to hydrogen sulfide (H 2 S) which is then utilized to produce S containing amino acids or co-factors such as biotin and pantothenic acid; while in dissimilatory reduction the bacteria reduce sulfate and produce H 2 S as an end product of their metabolism (Bradley et al, 2011). These dissimilatory sulfate reducing 6 bacteria (SRB) utilize anaerobic respiration pathways for their bioenergetic processes, and depend on the dissimilatory sulfite reductase enzyme activity to do so (Bradley et al, 2011).…”

Sulfur and Oxygen Isotope Fractionation During Bacterial Sulfur Disproportionation Under Anaerobic Haloalkaline Conditions

“…In biochemical pathways, the flow of substrates and hence, the magnitude of isotope fractionation is usually controlled by various environmental and physiological factors; more complicating, many reactions are reversible and characterized by considerable backward reactions. For example, the magnitude of sulfur isotope fractionation associated with dissimilatory sulfate reduction in a single sulfate reducer depends largely on the cell specific sulfate reduction rate and corresponding growth rate: the lower these rates (due to limited available energy), the higher the sulfur isotope fractionation, and vice versa (Bradley et al 2011;Sim et al 2011;Wing and Halvey 2014). Upon low energy conditions, the enzymes of the sulfate reduction pathway operate maximally reversible leading to near equilibrium conditions resulting in maximal sulfur isotope fractionation .…”