Abstract
Methanogens are putatively ancestral autotrophs that reduce CO2 with H2 to form biomass using a membrane-bound, proton-motive Fe(Ni)S protein called the energy-converting hydrogenase (Ech). At the origin of life, geologically sustained H+ gradients across inorganic barriers containing Fe(Ni)S minerals could theoretically have driven CO2 reduction by H2 through vectorial chemistry in a similar way to Ech. pH modulation of the redox potentials of H2, CO2 and Fe(Ni)S minerals could in principle enable an otherwise endergonic reaction. Here, we analyse whether vectorial electrochemistry can facilitate the reduction of CO2 by H2 under alkaline hydrothermal conditions using a microfluidic reactor. We present pilot data showing that steep pH gradients of approximately 5 pH units can be sustained over greater than 5 h across Fe(Ni)S barriers, with H+-flux across the barrier about two million-fold faster than OH–-flux. This high flux produces a calculated 3-pH unit-gradient (equating to 180 mV) across single approximately 25-nm Fe(Ni)S nanocrystals, which is close to that required to reduce CO2. However, the poor solubility of H2 at atmospheric pressure limits CO2 reduction by H2, explaining why organic synthesis has so far proved elusive in our reactor. Higher H2 concentration will be needed in future to facilitate CO2 reduction through prebiotic vectorial electrochemistry.
Highlights
Methanogens are putatively ancestral autotrophs that reduce CO2 with H2 to form biomass using a membrane-bound, proton-motive Fe(Ni)S protein called the energy-converting hydrogenase (Ech)
The question becomes: how can a growing, replicating system get better at making copies of itself, with increasingly complex prebiotic chemistry eventually giving rise to nucleotides and genetic information? This question is much easier to answer if the catalysts that drive growth (CO2 fixation) are inherited by daughter cells [42]
Phylogenetics and comparative physiology suggest that last universal common ancestor (LUCA) was an anaerobic autotroph that grew from the reaction between H2 and CO2 via some form of the acetyl CoA pathway [1,13–16,59] feeding into an incomplete reductive Krebs cycle [5,29,31]
Summary
The origin of life has not been considered a question in biology until recently— prebiotic chemistry, by definition, took place before biology began. In turn, makes sense of the known process of CO2 fixation in anaerobic prokaryotes, which use inorganic mineral-like structures such as Fe(Ni)S clusters to fix CO2 These clusters are small, can self-assemble from ions in solution (through chelation) [43,44], can be oxidized and reduced in turn [45], and may be inherited directly, for example, in association with cell membranes [42]. These positive feedbacks mean that the more organics that are formed, the more Fe(Ni)S clusters are chelated, and the more likely they are to associate with the membrane, driving synthesis of more organics as a ‘proto-Ech’ Such clusters are homologous with living systems [3,14], form spontaneously inside protocells [43,44], are heritable and evolvable [42]—for example, being chelated initially by amino acids [44,46], later by short non-coded polypeptide nests [54], and genetically encoded proteins [55]—can be oxidized and reduced in turn [20,47], and confer a direct selective advantage to protocells even before the emergence of genetic heredity [42]. How did such systems arise? Their reliance on mineral-like Fe(Ni)S clusters, hydrothermally sustained proton gradients and the abundant gases H2 and CO2, which both have pH-dependent reduction potentials, implies that a wholly inorganic form of vectorial electrochemistry could have driven CO2 fixation to generate organics at the origin of life [13,18,36,40,41,53,56–58]
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