Sulfur Isotope Effects of Dissimilatory Sulfite Reductase

Abstract

The precise interpretation of environmental sulfur isotope records requires a quantitative understanding of the biochemical controls on sulfur isotope fractionation by the principle isotope-fractionating process within the S cycle, microbial sulfate reduction (MSR). Here we provide the only direct observation of the major ( 34 S/ 32 S) and minor ( 33 S/ 32 S, 36 S/ 32 S) sulfur isotope fractionations imparted by a central enzyme in the energy metabolism of sulfate reducers, dissimilatory sulfite reductase (DsrAB). Results from in vitro sulfite reduction experiments allow us to calculate the in vitro DsrAB isotope effect in 34 S/ 32 S (hereafter, 34 ε ‰ DsrAB ) to be 15.3 ± 2 , 2σ. The accompanying minor isotope effect in 33 S, described as 33 λ DsrAB , is calculated to be 0.5150 ± 0.0012, 2σ. These observations facilitate a rigorous evaluation of the isotopic fractionation associated with the dissimilatory MSR pathway, as well as of the environmental variables that govern the overall magnitude of fractionation by natural communities of sulfate reducers. The isotope effect induced by DsrAB upon sulfite reduction is a factor of 0.3-0.6 times prior indirect estimates, which have ranged from 25 to 53‰ in 34 ε DsrAB . The minor isotope fractionation observed from DsrAB is consistent with a kinetic or equilibrium effect. Our in vitro constraints on the magnitude of 34 ε DsrAB is similar to the median value of experimental observations compiled from all known published work, where 34 ε r−p = 16.1 (r-p indicates reactant vs. product, n = 648). This value closely matches those of MSR operating at high sulfate reduction rates in both laboratory chemostat experiments ( 34 ε SO4− = H2S17.3 ± 1.5 , 2σ) and in modern marine sediments ( 34 ε SO4− = H2S17.3 ± 3.8 ). Targeting the direct isotopic consequences of a specific enzymatic processes is a fundamental step toward a biochemical foundation for reinterpreting the biogeochemical and geobiological sulfur isotope records in modern and ancient environments.

Quantification and isotopic analysis of intracellular sulfur metabolites in the dissimilatory sulfate reduction pathway

“…Here we describe tools to monitor the concentrations and isotopic compositions of intracellular sulfur metabolites. After the pioneering study of dissimilatory sulfite reductase (Leavitt et al, 2015), kinetic isotope effects imparted by other key enzymes are currently also under investigation. Recently, Santos et al (2015) revealed the role of DsrC, a small subunit of dissimilatory sulfite reductase, and proposed a new biochemical mechanism behind sulfite respiration.…”

“…After the pioneering study of dissimilatory sulfite reductase (Leavitt et al, 2015), kinetic isotope effects imparted by other key enzymes are currently also under investigation. Recently, Santos et al (2015) revealed the role of DsrC, a small subunit of dissimilatory sulfite reductase, and proposed a new biochemical mechanism behind sulfite respiration. All these advances in molecular biology, biochemistry, and isotope biogeochemistry have enabled investigations of microbial sulfur isotope fractionation at a sub-cellular level.…”

“…(C) Recent advances in understanding the dissimilatory sulfite reduction and the associated sulfur isotope effect. Santos et al (2015) revealed the role of DsrC, a small subunit of dissimilatory sulfite reductase, and Leavitt et al (2015) conducted the first enzyme-specific sulfur isotope experiments, showing that the 34 S/ 32 S fractionation during the first step of sulfite reduction was15‰. Influence of the electron donating reactions for APS and sulfite reduction on the relative abundance of APS and sulfite.…”

“…First, according to the Rees model and its later versions (Rees, 1973;Brunner and Bernasconi, 2005;Wing and Halevy, 2014), sulfur isotope fractionation between intracellular sulfate and sulfide would increase up to the equilibrium value (Sim et al, 2011b) with the reversibility reaching near unity at the thermodynamic limit of microbial growth. Alternatively, the important enzymes in dissimilatory sulfate reduction pathway may act as a coherent respiratory complex, including a recently-characterized dissimilatory sulfite reduction system (Santos et al, 2015), where substrates are channeled through the multi-enzyme complex. Compared with reactions involving free intermediates, a substrate-channeled reaction would maintain a relatively constant isotope effect with the reversibility being more tightly controlled.…”

Diversity decoupled from sulfur isotope fractionation in a sulfate‐reducing microbial community

“…This potentially confounds environmental interpretations of the sulfur isotope rock record, given the multitude of contemporary microbes capable of MSR and the lack of techniques to establish which, if any, of these genotypes were present and active at the time an isotope fractionation signal was generated. In the last decade, research has shifted toward exploring physiological contributions of specific model species to the extent of isotope fractionation (Leavitt, Bradley, Santos, Pereira, & Johnston, ; Santos et al, ; Sim et al, ), but studies approaching the problem from the opposite direction, interrogating entire communities, are lacking.…”