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 (34S/32S) and minor (33S/32S, 36S/32S) 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 34S/32S (hereafter, 34εDsrAB) to be 15.3 ± 2‰, 2σ. The accompanying minor isotope effect in 33S, described as 33λDsrAB, 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 34εDsrAB. The minor isotope fractionation observed from DsrAB is consistent with a kinetic or equilibrium effect. Our in vitro constraints on the magnitude of 34εDsrAB is similar to the median value of experimental observations compiled from all known published work, where 34εr−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 (34εSO4−H2S = 17.3 ± 1.5‰, 2σ) and in modern marine sediments (34εSO4−H2S = 17.3 ± 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.

Experimental estimation of the bisulfite isomer quotient as a function of temperature: Implications for sulfur isotope fractionations in aqueous sulfite solutions

“…It has long been recognized that the isotope fractionations associated with each of the individual steps within the metabolism represent important constraints to include in models that attempt to connect the experimentally observed net fractionations associated with the metabolism to the underpinning intra-cellular enzyme-mediated steps (e.g., Rees, 1973;Brunner and Bernasconi, 2005;Johnston et al, 2007;Bradley et al, 2011;Wing and Halevy, 2014), but the isotope effects associated with enzyme-mediated transformations are among the least well-constrained and are often incompletely treated in current models. Recently, Leavitt et al (2015) reported the first experimental constraints on the sulfur isotope fractionations associated with sulfite reduction via dissimilatory sulfite reductase, which were performed in vitro utilizing Dsr with the DsrC subunit inactive (representing solely DsrAB) isolated from the sulfate reducer Desulfovibrio vulgaris (strain Hildenborough). The solution conditions for these in vitro experiments were carefully chosen to be similar to in vivo (i.e., intracellular) conditions of the organism in proximity to its optimal growth temperature (D. vulgaris: pH = 7.1, T = 20 or 31°C; Leavitt et al, 2015).…”

“…Recently, Leavitt et al (2015) reported the first experimental constraints on the sulfur isotope fractionations associated with sulfite reduction via dissimilatory sulfite reductase, which were performed in vitro utilizing Dsr with the DsrC subunit inactive (representing solely DsrAB) isolated from the sulfate reducer Desulfovibrio vulgaris (strain Hildenborough). The solution conditions for these in vitro experiments were carefully chosen to be similar to in vivo (i.e., intracellular) conditions of the organism in proximity to its optimal growth temperature (D. vulgaris: pH = 7.1, T = 20 or 31°C; Leavitt et al, 2015). Leavitt et al (2015) were able to constrain a fractionation factor associated with sulfite reduction by DsrAB isolated from D. vulgaris of about 34 e sulfite-sulfur = 15 ± 2‰ from these experiments.…”

“…In addition to stimulating a global oxidative sulfur cycle (Jørgensen, 1977(Jørgensen, , 1982Jørgensen et al, 1990;Canfield and Teske, 1996;Jørgensen and Nelson, 2004), aqueous sulfide (H 2 S/HS À ) produced by DSR provides a primary source of reduced sulfur for authigenic pyrite (FeS 2 ) formation in marine sedimentary environments (e.g., Rickard, 2014). DSR has long been known to produce relatively large and variable sulfur isotope fractionations between extracellular sulfate and sulfide (e.g., Kaplan and Rittenberg, 1964;Sim et al, 2011aSim et al, , 2011bLeavitt et al, 2013Leavitt et al, , 2015 that can exert primary controls on the sulfur isotopic composition of geologically-preserved sulfur minerals such as authigenic pyrite and sulfate minerals, which in turn have been used to search for the earliest signs of the metabolism in the ancient rock record (e.g., Shen et al, 2001Shen et al, , 2009Shen and Buick, 2004;Ueno et al, 2008;Wacey et al, 2011; see also Johnston, 2011). Such variability is generally understood to arise from factors that affect the overall expression of step-wise isotope fractionations associated with a series of intracellular enzymemediated reactions that centrally involve the production and consumption of sulfite compounds (Parey et al, 2013;Wing and Halevy, 2014;Leavitt et al, 2015).…”

“…DSR has long been known to produce relatively large and variable sulfur isotope fractionations between extracellular sulfate and sulfide (e.g., Kaplan and Rittenberg, 1964;Sim et al, 2011aSim et al, , 2011bLeavitt et al, 2013Leavitt et al, , 2015 that can exert primary controls on the sulfur isotopic composition of geologically-preserved sulfur minerals such as authigenic pyrite and sulfate minerals, which in turn have been used to search for the earliest signs of the metabolism in the ancient rock record (e.g., Shen et al, 2001Shen et al, , 2009Shen and Buick, 2004;Ueno et al, 2008;Wacey et al, 2011; see also Johnston, 2011). Such variability is generally understood to arise from factors that affect the overall expression of step-wise isotope fractionations associated with a series of intracellular enzymemediated reactions that centrally involve the production and consumption of sulfite compounds (Parey et al, 2013;Wing and Halevy, 2014;Leavitt et al, 2015). Our understanding of the precise mechanisms and isotope effects associated with the enzyme-mediated transformation of sulfite compounds within sulfate reducing microorganisms will require a detailed understanding of the molecular composition of sulfite compounds in aqueous solution.…”

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, 2015;Santos et al, 2015;Sim et al, 2013), but studies approaching the problem from the opposite direction, interrogating entire communities, are lacking.…”