The alkaline-hydrothermal-vent hypothesis proposes that Hadean serpentinization-driven vents acted as inorganic electrochemical reactors in which natural proton gradients across thin Fe(Ni)S mineral barriers drove the reduction of CO2_2 by H2_2 toward the organic precursors of life. Whether its chemical preconditions could actually have been met remains unquantified. This problem asks for a quantitative feasibility determination of the three linked feedstock-and-flux conditions that the hypothesis and its methanotrophy-based variant require: (1) whether abiotic geochemistry in Hadean alkaline vents can supply methanethiol (CH3_3SH) at rates sufficient for primordial methyl synthesis, given that methanethiol in modern vent fluids appears largely biogenic; (2) whether the ferruginous Hadean ocean held nitrogen-oxide oxidants (NO, NO2−_2^-, NO3−_3^-) at the low-micromolar levels that methanotrophy-based versions of the scenario require, despite reduction by abundant Fe2+^{2+} and ultraviolet photolysis; and (3) whether vent-interface conditions can achieve net CO2_2-reduction yields and product diversity (formate, acetate, up to fatty-acid and amino-acid precursors) that, combined with concentration mechanisms such as thermophoresis, reach the thresholds needed for spontaneous protocell self-assembly. The three conditions serve one objective — establishing or refuting the quantitative chemical feasibility of the vent scenario — with the oxidant condition bearing specifically on the methanotrophy-based variant.

Background

Life's core carbon and energy metabolism is chemiosmotic: essentially all cells capture energy from electrochemical ion gradients across membranes, and the only carbon-fixation pathway that is both energetically profitable overall and present in both bacteria and archaea is the Wood–Ljungdahl (acetyl-CoA) pathway, which reduces CO2_2 with H2_2 using iron–nickel–sulfur (Fe(Ni)S) enzymes. The alkaline-vent origin-of-life hypothesis (Russell, Martin, Lane and colleagues; synthesized in Sojo et al. 2016) proposes that this metabolism was prefigured by vent geochemistry. Serpentinization — the reaction of seawater with ultramafic seafloor rock — generates warm, strongly alkaline (pH 9–11), H2_2-rich fluids, as at the modern Lost City hydrothermal field. On the Hadean Earth (the eon older than about 4.0 billion years, from which no intact crust survives) these fluids met a mildly acidic (pH 5–7), CO2_2-rich, anoxic and ferruginous ocean (i.e. loaded with dissolved Fe2+^{2+}). Across the thin inorganic walls of vent micropores, mineral Fe(Ni)S precipitates (mackinawite and greigite, structural relatives of biological Fe–S clusters) would separate the two fluids, so that the pH contrast — several units — produced natural proton gradients with the magnitude and polarity of the proton-motive force that modern cells generate by active ion pumping. Because the H2_2/H+^+ redox couple becomes more reducing at high pH while CO2_2 becomes easier to reduce at low pH, this 'vectorial' arrangement makes the overall reduction of CO2_2 by H2_2 to formaldehyde-level organics thermodynamically favorable — a pH contrast of more than three units contributes on the order of 180 mV of thermodynamic drive (Yamaguchi et al. 2014; Hudson et al. 2020) — even though the reaction is kinetically sluggish.

Methyl-group chemistry and oxidant availability are the two chemical cruxes of specific variants of the hypothesis. Methanethiol (CH3_3SH, the simplest thiol, i.e. the sulfur analogue of methanol) is invoked as the abiotic carrier of methyl groups for acetyl-type synthesis, analogous to the methylated cofactors of the modern pathway; laboratory syntheses under simulated hydrothermal conditions have long been known, but measurements in modern mid-ocean-ridge vent fluids (Reeves et al. 2014) attribute the observed CH3_3SH mainly to thermal decomposition of pre-existing, probably microbial, organic matter, leaving abiotic supply unquantified. Separately, 'methanotrophy-first' formulations (Russell, Nitschke and colleagues) — versions of the scenario in which the first metabolism extracted energy from methane oxidation — require dissolved nitrogen oxides (NO, NO2−_2^-, NO3−_3^-) at low-micromolar concentrations (Russell and Nitschke 2017; Wong et al. 2017) as high-potential electron acceptors to pull the reduction of CO2_2 and the oxidation of methane or methyl sulfides; yet well-known inorganic chemistry, such as the reduction of nitrite and nitrate by iron sulfide and ferrous iron, implies that an anoxic, Fe2+^{2+}-rich ocean should have consumed these oxidants rapidly.

Experimental tests of the central step have so far been carried out mostly in benchtop reactors: electrochemical cells with Fe(Ni)S electrodes and 'origin-of-life reactors' that precipitate thin Fe(Ni)S membranes between simulated ocean and vent fluids. These produce only low yields of simple organics (micromolar formaldehyde and related products), which has been attributed to low H2_2 partial pressure at atmospheric conditions, the instability of formaldehyde in the acidic phase, and the competing reduction of H+^+ back to H2_2; and the possible role of Mg2+^{2+} ions in promoting CO2_2 reduction over H+^+ reduction remains unexamined. Modern vent fluids do show unambiguous abiotic CO2_2 reduction to formate and methane (McDermott et al. 2015), so abiotic carbon reduction at vents is real; the open question is whether its rates, product spectrum, and feedstock fluxes suffice for the prebiotic chemistry the hypothesis needs. Thermophoresis — the migration of molecules along temperature gradients, which can concentrate organics in microporous vent structures by orders of magnitude — has been proposed to bridge the gap between micromolar synthesis and the higher concentrations needed for membrane-forming amphiphiles to self-assemble into protocells (simple cell-like vesicles), but this too awaits realistic coupled demonstration.

Problem Statement

Determine, quantitatively, whether the chemical preconditions of the alkaline-hydrothermal-vent origin-of-life scenario could have been met on the Hadean seafloor, by resolving the three linked feedstock-and-flux gaps that the source hypothesis leaves unquantified. These three questions serve one objective: establishing or refuting the quantitative chemical feasibility of the vent scenario, from its core carbon-fixation chemistry through to the sufficiency of its products for protocell assembly.

(1) Methyl supply: can abiotic geochemistry in and around serpentinizing alkaline vents produce methanethiol (CH3_3SH) at rates, concentrations, and Hadean flux sufficient to drive primordial methyl synthesis under Hadean-plausible vent conditions (temperature, pressure, pH, H2_2, CO2_2 and H2_2S activities, catalyst mineralogy), or is the abiotic contribution bounded below the levels required by methyl synthesis, discriminating abiotic production from the thermal-decomposition-of-biological-organics source identified in modern vent fluids.

(2) Oxidant supply: did the ferruginous Hadean ocean contain dissolved nitrogen-oxide oxidants (NO, NO2−_2^-, NO3−_3^-) at the low-micromolar levels that methanotrophy-based versions of the scenario require, despite their sinks — reduction by abundant Fe2+^{2+}, ultraviolet photolysis, hydrolysis, and hydrothermal circulation — over an explicitly stated, independently justified range of plausible Hadean atmospheric and oceanic conditions, or is the requirement unmeetable anywhere in that range?

(3) Net organic synthesis and sufficiency: can vent-interface conditions — natural pH gradients across Fe(Ni)S precipitates at realistic H2_2 activity and pressure — reduce CO2_2 to organics at net rates and yields, and with sufficient product diversity (formate, acetate, and intermediates up to fatty-acid and amino-acid precursors), that, combined with physical concentration mechanisms such as thermophoresis in vent micropores, they reach the concentration thresholds required for spontaneous protocell self-assembly?

The phylogenetic questions raised alongside these in the same source (the testability of the proposed Ech-direction and independent-pumping hypotheses — Ech being a membrane-bound energy-conserving hydrogenase, an iron–sulfur enzyme complex that couples hydrogen chemistry to proton translocation across membranes — and the deep archaea–bacteria rooting of the tree of life) concern historical reconstruction rather than chemical feasibility and are not part of this problem.

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