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Metals Likely Promoted Protometabolism in Early Ocean Alkaline Hydrothermal Systems


Metals Likely Promoted Protometabolism in Early Ocean Alkaline Hydrothermal Systems

Norio Kitadai et al. Sci Adv.


One of the most plausible scenarios of the origin of life assumes the preceding prebiotic autotrophic metabolism in sulfide-rich hydrothermal vent environments. However, geochemical mechanisms to harness the reductive power provided by hydrothermal systems remain to be elucidated. Here, we show that, under a geoelectrochemical condition realizable in the early ocean hydrothermal systems, several metal sulfides (FeS, Ag2S, CuS, and PbS) undergo hour- to day-scale conversion to the corresponding metals at ≤-0.7 V (versus the standard hydrogen electrode). The electrochemically produced FeS-Fe0 assemblage promoted various reactions including certain steps in the reductive tricarboxylic acid cycle with efficiencies far superior to those due to pure FeS. The threshold potential is readily generated in the H2-rich alkaline hydrothermal systems that were probably ubiquitous on the Hadean seafloor. Thus, widespread metal production and metal-sustained primordial metabolism were likely to occur as a natural consequence of the active hydrothermal processes on the Hadean Earth.


Fig. 1
Fig. 1. Geoelectrochemical metal production in the early ocean alkaline hydrothermal systems.
(A) At the vent-seawater interface, metal sulfides precipitated through mixing between the ancient seawater rich in metal cations (for example, Fe2+) and the alkaline hydrothermal fluid containing HS were exposed to a negative electric potential and were electroreduced to the corresponding metals (for example, Fe0) with the reactivity depending on the potential and the nature of sulfides. (B to E) X-ray diffraction (XRD) patterns of FeS, Ag2S, CuS, and PbS before and after the electrolysis, respectively. The small peaks noted by asterisks (*) in (B) represent NaCl signals. The XRD data for the other sulfides are presented in fig. S3. The potential/pH diagrams of the relevant metal-sulfide systems are shown in the left columns. The colors represent the thermodynamically predicted stability regions of metals (red) and sulfides (green, orange, and blue refer to the sulfides with the metal/sulfur ratio of 1, >1, and <1, respectively) in the aqueous condition examined in the present study.
Fig. 2
Fig. 2. Summary of the electroreduction behaviors of metal sulfides under a simulated early ocean condition.
(A) A circle located at a potential and a metal indicates that a detectable amount of the metal was generated during the 7-day electrolysis with the potential. Analogously, crosses show that no metal production was observed in our experiment. The color of the cross denotes the dominant sulfide stoichiometry seen after the 7-day electrolysis (see Fig. 1 legend for the color convention). The numbers indicate the duration in hours of experiments required by the complete sulfide-to-metal conversions. (B) A redox calculation for 1 mmol kg−1 H2 [H2(aq) → 2H+ + 2e] as a function of temperature and pH indicates a geoelectrochemically feasible potential range of 0 to −1.1 V versus SHE.
Fig. 3
Fig. 3. Nonenzymatic reactions in the presence of as-prepared FeS (blue) and the FeS electrochemically reduced at −0.7 V (versus SHE) for 7 days (FeS_PERM) (red).
The yields were calculated relative to the initial amounts of starting materials of respective reactions. A full dataset for the identified and quantified products are presented in fig. S9 and tabulated in table S1 together with the results under the following conditions: with H2 gas, with FeCl2, with pure Fe0, and without reductant. The right diagram shows the reactions examined in the present study (a to g) and those demonstrated previously by simulating hydrothermal vent environments on the early Earth [CO2 → CO in (6) and CO → C2 and C3 compounds in (7, 9)].

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