Carbon reduction powered by natural electrochemical gradients under submarine hydrothermal vent conditions

Energy metabolism at the emergence of life has been the topic of intense theoretical and experimental study. Alkaline hydrothermal vents (AHVs) may have facilitated energy-transfer and carbon-fixation at life’s emergence, via a primordial chemiosmotic mechanism. Specifically, pH separation across vent walls could have been the forerunner to pH separation across microbial membranes, with electron-conductive inorganic barriers containing [Ni-]FeS minerals mimicking the active sites of metalloenzymes in potentially ancient biological reductive acetyl-CoA Wood-Ljungdahl (WL) pathway, reverse Krebs and other metabolic pathways. We previously demonstrated pH gradient dependent reduction of CO2 to formate by H2 in AHV interface conditions. Here, we address the same problem of CO2 reduction, however using a macroscale reactor that allowed us to use minerals synthesized via protocols analogous to the alkaline vent mount formation. This reactor also allowed us to probe more variables, and explore longer experimentation time-frames. Thus, this work aimed to investigate the effect of the different aspects of the gradientrich hydrothermal-vent interface on the observed CO2 electrochemical reduction- such as different minerals, temperature gradients, as well as the flow of electrons under passive vs. induced currents and potentials. Using experimental simulations and electrochemistry techniques, We detected two key steps of the WL pathway (CO2 to formic acid, and the formation acetic acid) as well as reactions in the reverse Krebs cycle (fumarate to succinate). Both Ni in the mineral, and temperature as a property, were shown to affect the formation of formate. Extremely small currents were enough to efficiently carry out CO2 reduction. This work develops benchmarks to electrochemically explore the model of protometabolisms in the vent-ocean interface and opens to future analyses on the catalytic or electrocatalytic properties of Fe-[Ni-]S minerals as precursors of metalloenzymes.