Campbell Brian J, Sessions Alex L, Fox Daniel N, Paul Blair G, Qin Qianhui, Kellermann Matthias Y, Valentine David L
Department of Earth Science, University of California, Santa Barbara, Santa Barbara, CA, United States.
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, United States.
Front Microbiol. 2017 Oct 4;8:1886. doi: 10.3389/fmicb.2017.01886. eCollection 2017.
Fatty acids produced by H-metabolizing bacteria are sometimes observed to be more D-depleted than those of photoautotrophic organisms, a trait that has been suggested as diagnostic for chemoautotrophic bacteria. The biochemical reasons for such a depletion are not known, but are often assumed to involve the strong D-depletion of H. Here, we cultivated the bacterium H16 (formerly H16) under aerobic, H-consuming, chemoautotrophic conditions and measured the isotopic compositions of its fatty acids. In parallel with the wild type, two mutants of this strain, each lacking one of two key hydrogenase enzymes, were also grown and measured. In all three strains, fractionations between fatty acids and water ranged from -173‰ to -235‰, and averaged -217‰, -196‰, and -226‰, respectively, for the wild type, SH mutant, and MBH mutant. There was a modest increase in δD as a result of loss of the soluble hydrogenase enzyme. Fractionation curves for all three strains were constructed by growing parallel cultures in waters with δD values of approximately -25‰, 520‰, and 1100‰. These curves indicate that at least 90% of the hydrogen in fatty acids is derived from water, not H. Published details of the biochemistry of the soluble and membrane-bound hydrogenases confirm that these enzymes transfer electrons rather than intact hydride (H) ions, providing no direct mechanism to connect the isotopic composition of H to that of lipids. Multiple lines of evidence thus agree that in this organism, and presumably others like it, environmental H plays little or no direct role in controlling lipid δD values. The observed fractionations must instead result from isotope effects in the reduction of NAD(P)H by reductases with flavin prosthetic groups, which transfer two electrons and acquire H (or D) from solution. Parallels to NADPH reduction in photosynthesis may explain why D/H fractionations in are nearly identical to those in many photoautotrophic algae and bacteria. We conclude that strong D-depletion is not a diagnostic feature of chemoautotrophy.
由利用氢代谢的细菌产生的脂肪酸有时被观察到比光合自养生物的脂肪酸具有更高的氘贫化程度，这一特征被认为是化学自养细菌的诊断依据。这种贫化的生化原因尚不清楚，但通常认为与氢的强烈氘贫化有关。在此，我们在需氧、消耗氢的化学自养条件下培养了细菌H16（以前称为H16），并测量了其脂肪酸的同位素组成。与野生型同时，该菌株的两个突变体，每个突变体都缺失两种关键氢化酶中的一种，也进行了培养和测量。在所有三个菌株中，脂肪酸与水之间的分馏范围为-173‰至-235‰，野生型、SH突变体和MBH突变体的平均值分别为-217‰、-196‰和-226‰。由于可溶性氢化酶的缺失，δD有适度增加。通过在δD值约为-25‰、520‰和1100‰的水中培养平行培养物，构建了所有三个菌株的分馏曲线。这些曲线表明，脂肪酸中至少90%的氢来自水，而非氢。已发表的关于可溶性和膜结合氢化酶生物化学的详细信息证实，这些酶转移电子而非完整的氢化物（H）离子，没有提供将氢的同位素组成与脂质的同位素组成联系起来的直接机制。因此，多条证据一致表明，在这种生物以及可能类似的其他生物中，环境氢在控制脂质δD值方面几乎不起直接作用。相反，观察到的分馏必定是由带有黄素辅基的还原酶还原NAD(P)H时的同位素效应导致的，这些还原酶转移两个电子并从溶液中获取H（或D）。光合作用中与NADPH还原的相似之处可能解释了为什么[此处原文缺失相关内容]中的D/H分馏与许多光合自养藻类和细菌中的几乎相同。我们得出结论，强烈的氘贫化不是化学自养的诊断特征。
