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. 2018 Apr 4;4(4):eaao7265.
doi: 10.1126/sciadv.aao7265. eCollection 2018 Apr.

Geoelectrochemical CO Production: Implications for the Autotrophic Origin of Life

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Free PMC article

Geoelectrochemical CO Production: Implications for the Autotrophic Origin of Life

Norio Kitadai et al. Sci Adv. .
Free PMC article

Abstract

Wächtershäuser's proposal of the autotrophic origin of life theory and subsequent laboratory demonstrations of relevant organic reactions have opened a new gate for the exploration of the origin of life. However, this scenario remains controversial because, at present, it requires a high pressure of CO as a source of carbon and reducing energy, although CO must have been a trace C species on the Hadean Earth. We show that, simulating a geoelectrochemical environment in deep-sea hydrothermal fields, CO production with up to ~40% Faraday efficiency was attainable on CdS in CO2-saturated NaCl solution at ≤-1 V (versus the standard hydrogen electrode). The threshold potential is readily generated in the H2-rich, high-temperature, and alkaline hydrothermal vents that were probably widespread on the early komatiitic and basaltic ocean crust. Thus, Wächtershäuser's scenario starting from CO2 was likely to be realized in the Hadean ocean hydrothermal systems.

Figures

Fig. 1
Fig. 1. Electrochemical CO2 reduction on various metal sulfides in 0.1 M NaCl saturated with 1 atm CO2 at room temperature (~25°C).
(A and B) FEs for CO2 reduction to (A) CO (CO2 + 2H+ + 2e → CO + H2O) and (B) HCOO (CO2 + 2H+ + 2e → HCOO) at –0.8, –1.0, and –1.2 V versus the SHE. (C) CO production efficiencies on CdS and Ag2S as a function of electric potential. Error bars correspond to the overall reproducibility of the experimental data that was evaluated from three independent experiments.
Fig. 2
Fig. 2. Thermodynamically predicted electric potentials of fluids from deep-sea hydrothermal systems.
Redox potentials of (A) H2S (1 mmol kg–1), (B) H2S (10 mmol kg–1), (C) H2 (1 mmol kg–1), and (D) H2 (10 mmol kg–1) as a function of pH at 25°, 100°, 200°, and 300°C calculated assuming the half reactions of S + 2H+ (or H+) + 2e → H2S (or HS) [for (A) and (B)] and 2H+ + 2e → H2 [for (C) and (D)]. S (solid sulfur) was chosen as the H2S (or HS) oxidation product because the H2S/S redox couple provides a major potential control on the sulfide-rich hydrothermal vent environments (19). See the Supplementary Materials for the calculation procedure.
Fig. 3
Fig. 3. Abiotic carbon fixation in the primitive hydrothermal system.
(A) The hot and active mantle convection in the primitive Earth induced a lot of massive upwelling of komatiite/basaltic melts beneath the ocean floor (30), (B) where interactions between the solidified (ultra)mafic rocks and the CO2-rich seawater with different water/rock ratios led to a variety of end-member fluid chemistry, including the H2-rich, high-temperature (T), and alkaline type and the metal-rich, high-temperature, and acidic (or neutral) ones (31, 32). (C) On the ocean floor, mixing of the hydrothermal fluids and seawater generated sulfide-rich chimneys (22), and the potential gradient across the chimney drove a continuous electron flow. The electric potential at the chimney-seawater interface could reach less than –1 V (versus SHE) in alkaline hydrothermal vent environments. The low potential, in the presence of sulfides rich in Cd2+ and Ag+, allowed the electrochemical CO2 reduction to CO with the FE as high as dozens of percent, together with H2 evolution. The produced CO served as a driving force for the subsequent abiotic organic synthesis that preceded the origin of life as schematically indicated in the figure.

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