Wenk Christine B, Wing Boswell A, Halevy Itay
Department of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot, Israel.
Department of Geological Sciences, University of Colorado, Boulder, CO, USA.
ISME J. 2018 Feb;12(2):495-507. doi: 10.1038/ismej.2017.185. Epub 2017 Oct 31.
Dissimilatory sulfate reduction (DSR) has been a key process influencing the global carbon cycle, atmospheric composition and climate for much of Earth's history, yet the energy metabolism of sulfate-reducing microbes remains poorly understood. Many organisms, particularly sulfate reducers, live in low-energy environments and metabolize at very low rates, requiring specific physiological adaptations. We identify one such potential adaptation-the electron carriers selected for survival under energy-limited conditions. Employing a quantitative biochemical-isotopic model, we find that the large S isotope fractionations (>55‰) observed in a wide range of natural environments and culture experiments at low respiration rates are only possible when the standard-state Gibbs free energy (ΔG'°) of all steps during DSR is more positive than -10 kJ mol. This implies that at low respiration rates, only electron carriers with modestly negative reduction potentials are involved, such as menaquinone, rubredoxin, rubrerythrin or some flavodoxins. Furthermore, the constraints from S isotope fractionation imply that ferredoxins with a strongly negative reduction potential cannot be the direct electron donor to S intermediates at low respiration rates. Although most sulfate reducers have the genetic potential to express a variety of electron carriers, our results suggest that a key physiological adaptation of sulfate reducers to low-energy environments is to use electron carriers with modestly negative reduction potentials.
异化硫酸盐还原（DSR）在地球历史的大部分时间里一直是影响全球碳循环、大气组成和气候的关键过程，然而，硫酸盐还原微生物的能量代谢仍知之甚少。许多生物，特别是硫酸盐还原菌，生活在低能量环境中，代谢速率极低，需要特定的生理适应性。我们确定了一种这样的潜在适应性——在能量受限条件下选择用于生存的电子载体。利用定量生化同位素模型，我们发现，在广泛的自然环境和低呼吸速率的培养实验中观察到的大的硫同位素分馏（>55‰），只有当DSR过程中所有步骤的标准态吉布斯自由能（ΔG'°）比-10 kJ mol更正时才有可能。这意味着在低呼吸速率下，仅涉及具有适度负还原电位的电子载体，如甲萘醌、红素氧还蛋白、红氧还蛋白或一些黄素氧还蛋白。此外，硫同位素分馏的限制意味着，在低呼吸速率下，具有强负还原电位的铁氧还蛋白不能作为硫中间体的直接电子供体。尽管大多数硫酸盐还原菌具有表达多种电子载体的遗传潜力，但我们的结果表明，硫酸盐还原菌对低能量环境的关键生理适应性是使用具有适度负还原电位的电子载体。
