Wing Boswell A, Halevy Itay
Department of Earth and Planetary Sciences and GEOTOP, McGill University, Montréal, QC, Canada H3A 0E8; and
Department of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot 76100, Israel
Proc Natl Acad Sci U S A. 2014 Dec 23;111(51):18116-25. doi: 10.1073/pnas.1407502111. Epub 2014 Oct 31.
We present a quantitative model for sulfur isotope fractionation accompanying bacterial and archaeal dissimilatory sulfate respiration. By incorporating independently available biochemical data, the model can reproduce a large number of recent experimental fractionation measurements with only three free parameters: (i) the sulfur isotope selectivity of sulfate uptake into the cytoplasm, (ii) the ratio of reduced to oxidized electron carriers supporting the respiration pathway, and (iii) the ratio of in vitro to in vivo levels of respiratory enzyme activity. Fractionation is influenced by all steps in the dissimilatory pathway, which means that environmental sulfate and sulfide levels control sulfur isotope fractionation through the proximate influence of intracellular metabolites. Although sulfur isotope fractionation is a phenotypic trait that appears to be strain specific, we show that it converges on near-thermodynamic behavior, even at micromolar sulfate levels, as long as intracellular sulfate reduction rates are low enough (<<1 fmol H2S⋅cell(-1)⋅d(-1)).
我们提出了一个用于伴随细菌和古菌异化硫酸盐呼吸作用的硫同位素分馏定量模型。通过纳入独立可得的生化数据，该模型仅用三个自由参数就能重现大量近期的实验分馏测量结果：（i）硫酸盐摄入细胞质的硫同位素选择性，（ii）支持呼吸途径的还原态与氧化态电子载体的比例，以及（iii）呼吸酶活性的体外与体内水平之比。分馏受异化途径中的所有步骤影响，这意味着环境硫酸盐和硫化物水平通过细胞内代谢物的直接影响来控制硫同位素分馏。尽管硫同位素分馏是一个似乎具有菌株特异性的表型特征，但我们表明，只要细胞内硫酸盐还原速率足够低（<<1 fmol H2S·细胞⁻¹·天⁻¹），即使在微摩尔硫酸盐水平下，它也会趋近于近热力学行为。
