Leavitt William D, Bradley Alexander S, Santos André A, Pereira Inês A C, Johnston David T
Department of Earth and Planetary Sciences, Harvard UniversityCambridge, MA, USA; Department of Earth and Planetary Sciences, Washington University in St. LouisSt. Louis, MO, USA.
Department of Earth and Planetary Sciences, Washington University in St. Louis St. Louis, MO, USA.
Front Microbiol. 2015 Dec 24;6:1392. doi: 10.3389/fmicb.2015.01392. eCollection 2015.
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, [Formula: see text]) to be 15.3 ± 2‰, 2σ. The accompanying minor isotope effect in (33)S, described as [Formula: see text], 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)εDsrAB. The minor isotope fractionation observed from DsrAB is consistent with a kinetic or equilibrium effect. Our in vitro constraints on the magnitude of [Formula: see text] 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 ([Formula: see text] 17.3 ± 1.5‰, 2σ) and in modern marine sediments ([Formula: see text] 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.
环境硫同位素记录的精确解读需要依据硫循环中主要的同位素分馏过程——微生物硫酸盐还原作用（MSR），对硫同位素分馏的生化控制进行定量理解。在此，我们首次直接观测了硫酸盐还原菌能量代谢过程中的核心酶——异化亚硫酸盐还原酶（DsrAB）所产生的主要硫同位素分馏（(34)S/(32)S）以及次要硫同位素分馏（(33)S/(32)S、(36)S/(32)S）。体外亚硫酸盐还原实验结果使我们能够计算出体外DsrAB的(34)S/(32)S同位素效应（以下简称[公式：见原文]）为15.3 ± 2‰，2σ。伴随的(33)S次要同位素效应，即[公式：见原文]，经计算为0.5150 ± 0.0012，2σ。这些观测结果有助于严格评估与异化MSR途径相关的同位素分馏，以及控制硫酸盐还原菌自然群落分馏总体程度的环境变量。DsrAB在亚硫酸盐还原过程中引发的同位素效应是先前间接估计值的0.3 - 0.6倍，先前的间接估计值范围为(34)εDsrAB为25‰至5‰。从DsrAB观测到的次要同位素分馏与动力学或平衡效应一致。我们对[公式：见原文]大小的体外限制与所有已知已发表工作汇编的实验观测中值相似，其中(34)ε r-p = 16.1‰（r-p表示反应物与产物，n = 648）。该值与实验室恒化器实验（[公式：见原文] 17.3 ± 1.5‰，2σ）和现代海洋沉积物（[公式：见原文] 17.3 ± 3.8‰）中高硫酸盐还原速率下的MSR值紧密匹配。针对特定酶促过程的直接同位素后果进行研究是为重新解释现代和古代环境中的生物地球化学和地球生物学硫同位素记录奠定生化基础的关键一步。
