Habitat type and location and the temperature, pH, and dissolved H2 concentrations in specified environments. from Bioenergetic constraints on the origin of autotrophic metabolism

Autotrophs form the base of all complex food webs and seemingly have done so since early in Earth history. Phylogenetic evidence suggests that early autotrophs were anaerobic, used CO2 as both an oxidant and carbon source, were dependent on H2 as an electron donor, and used iron-sulfur proteins (termed ferredoxins) as a primary electron carrier. However, the reduction potential of H2 is not typically low enough to efficiently reduce ferredoxin. Instead, in modern strictly anaerobic and H2-dependent autotrophs, ferredoxin reduction is accomplished using one of several recently evolved enzymatic mechanisms, including electron bifurcating and coupled ion translocating mechanisms. These observations raise the intriguing question of why anaerobic autotrophs adopted ferredoxins as central electron carriers only to have to evolve complex machinery to reduce them. Here, we report calculated reduction potentials for H2 as a function of observed environmental H2 concentration, pH and temperature. Results suggest that a combination of alkaline pH and high H2 concentration yield H2 reduction potentials low enough to efficiently reduce ferredoxins. Hyperalkaline, H2 rich environments have existed in discrete locations throughout Earth history where ultramafic minerals are undergoing hydration through the process of serpentinization. These results suggest that serpentinizing systems, which would have been common on early Earth, naturally produced conditions conducive to the emergence of H2-dependent autotrophic life. The primitive process of hydrogenotrophic methanogenesis is used to examine potential changes in methanogenesis and Fd reduction pathways as these organisms diversified away from serpentinizing environments.This article is part of the theme issue ‘Serpentinite in the earth system’.自养生物（Autotrophs）构成所有复杂食物网的基础，且这一生态功能自地球早期历史以来便已延续至今。系统发育学证据表明，早期自养生物为厌氧型生物，以二氧化碳（CO₂）作为氧化剂与碳源，依赖氢气（H₂）作为电子供体，并以铁硫蛋白（iron-sulfur proteins，即铁氧还蛋白（ferredoxins））作为主要电子载体。然而，氢气的还原电位通常不足以高效还原铁氧还蛋白。与之相反，在现代严格厌氧且依赖氢气的自养生物中，铁氧还蛋白的还原需通过多种新近演化出的酶促机制完成，包括电子分岔（electron bifurcating）与偶联离子转运（coupled ion translocating）机制。这些观察结果引出了一个耐人寻味的问题：厌氧自养生物为何会将铁氧还蛋白选为核心电子载体，却又不得不演化出复杂的机制系统以完成其还原？本研究报道了基于实测环境氢气浓度、pH值与温度计算得到的氢气还原电位。研究结果显示，碱性pH环境与高氢气浓度的组合，可使氢气还原电位降低至足以高效还原铁氧还蛋白的水平。在地球历史的各个时期，超镁铁质矿物通过蛇纹岩化作用（serpentinization）发生水合反应的离散区域中，曾存在过高碱性、富氢气的环境。上述结果表明，在早期地球本就普遍分布的蛇纹岩化系统，自然形成了有利于依赖氢气的自养生命诞生的环境条件。本研究以原始氢营养型产甲烷作用为模型，探讨了当这些自养生物脱离蛇纹岩化环境并发生演化分化时，产甲烷作用与铁氧还蛋白（Fd）还原通路可能发生的变化。本文属于“地球系统中的蛇纹岩（Serpentinite in the earth system）”专题栏目的一部分。