Possible mechanisms of CO(2) reduction by H(2) via prebiotic vectorial electrochemistry

Methanogens are putatively ancestral autotrophs that reduce CO(2) with H(2) 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 CO(2) reduction by H(2) through vectorial chemistry in a similar way to Ech. pH modulation of the redox potentials of H(2), CO(2) and Fe(Ni)S minerals could in principle enable an otherwise endergonic reaction. Here, we analyse whether vectorial electrochemistry can facilitate the reduction of CO(2) by H(2) 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 CO(2). However, the poor solubility of H(2) at atmospheric pressure limits CO(2) reduction by H(2), explaining why organic synthesis has so far proved elusive in our reactor. Higher H(2) concentration will be needed in future to facilitate CO(2) reduction through prebiotic vectorial electrochemistry.