Possible mechanisms of CO₂ reduction by H₂ via prebiotic vectorial electrochemistry

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.