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

Abstract

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.

CO2reduction driven by a pH gradient

“…The two fluids were introduced at the tips of a Y-shaped borosilicate microfluidic reactor (Figure 1B). Ambient pressures of H 2 and CO 2 have proven insufficient to drive CO 2 reduction (24, 25), so instead of attempting to dissolve either gas by bubbling prior to the reaction, here we used gas pressure-driven microfluidic pumps. The alkaline fluid was pushed with H 2 at 1.5 bar, with the ocean analog being pushed with CO 2 at the same pressure (see reactor schematic in Figure 1B and photograph in Figure S1).…”

“…Following formation of the precipitate, and to prevent further precipitation from clogging the microfluidic channels (25), in the second (post-precipitation) stage we switched the ocean fluid to pure deaerated H 2 O, pushed by CO 2 (Figure 1B, right). The vent simulant was left as before (Na 2 S, K 2 HPO 4 , and Na 2 Si 3 O 7 , pushed by H 2 ).…”

“…We have used 1.5 bar pressures of both H 2 and CO 2 , which are nominally not too dissimilar from the ambient-pressure bubbling employed in similar earlier work that did not yield any detectable reduced carbon (24, 25). In those experiments, the two gases were dissolved into the fluids prior to the reactions.…”

“…In those experiments, the two gases were dissolved into the fluids prior to the reactions. Recent calculations suggest that atmospheric pressures of H 2 and CO 2 are only marginally insufficient to drive the reaction to formate (25). We therefore infer that the positive results reported in this work are most plausibly due to the advantages of the gas-driven system used here, which likely result in higher dissolved gas concentrations than the atmospheric-pressure bubbling performed previously (24, 25).…”

“…Recent calculations suggest that atmospheric pressures of H 2 and CO 2 are only marginally insufficient to drive the reaction to formate (25). We therefore infer that the positive results reported in this work are most plausibly due to the advantages of the gas-driven system used here, which likely result in higher dissolved gas concentrations than the atmospheric-pressure bubbling performed previously (24, 25). Chiefly, the ability to keep the gases consistently within the system, as well as more reliably maintaining the fluids anoxic, may suffice as an explanation for the positive results observed here in contrast with previous attempts.…”