Fe2+ disproportionation within iron-rich alkaline vent analogues reveals proto-bioenergetic systems

Fe2+ disproportionation within iron-rich alkaline vent analogues reveals proto-bioenergetic systems | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Fe 2+ disproportionation within iron-rich alkaline vent analogues reveals proto-bioenergetic systems Chloé Truong, Nil Gaudu, Orion Farr, Adriana Clouet, Daniel Ferry, and 6 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7036221/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Alkaline hydrothermal vents are plausible environments for emergence of life on Earth. By means of a simplified analogical reconstruction of the vent-ocean interface of these systems reproducing early Earth conditions, we show that iron (oxy-hydr)oxide minerals may have carried out proto-bioenergetic processes driven by pH and redox gradients. The initial pH gradient precipitates the iron (oxy-hydr)oxide mineral barriers (magnetite, green rust and amakinite) and yields reducing conditions, enabling the production of metallic iron at room temperature via the disproportionation of Fe 2+ to Fe 3+ and Fe 0 . The association of Fe 0 with magnetite suggests the coupling of Fe 3+ / H 2 co-production by amakinite oxidation with the thermodynamically unfavorable reduction of Fe 2+ to Fe 0 . This abiotic disproportionation process coupling exergonic and endergonic reactions resembles a proto-bioenergetic mechanism increasing the non-equilibrium reduction state of the system and offers an interesting analogue of the electronic bifurcation reaction, fundamental to the thermodynamic requirements of life. Geology Thermodynamics and statistical mechanics Biophysics bioenergetics iron minerals redox alkaline vents early Earth endergonic thermodynamic origin of life. Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 INTRODUCTION Since their discovery in the early 2000’s, hydrothermal vents resulting from serpentinization processes have added new perspectives concerning the question of the transitions from geochemical to biochemical entities (Kelley et al., 2005; Russel, 2003; Russel et al., 2010). These systems arise from the circulation of high temperature/high pressure water through deep ultramafic rocks, yielding fluids enriched in dissolved H 2 and CH 4 (McCollom and Seewald, 2001). This results in highly reducing alkaline (pH 9–11), moderate temperature (30–90°C) effluents which, upon contact with seawater, precipitate chimneys with high content in carbonate minerals (Ludwig et al., 2006). In addition to the mentioned reductants H 2 and CH 4 , AHV-effluents contain various organic compounds, derived principally from CO 2 reduction at high temperature and high pressure in the subsurface (Horita and Berndt, 1999; McCollom and Seewald, 2006; Seewald et al., 2006; Proskurowski et al., 2008). Concomitantly, serpentinization processes occurring in the subsurface of the vents are linked to the formation of metal alloys, commonly awaruite (FeNi 3 ), and even pure native iron (Fe 0 ) (Chamberlain et al., 1965; Frost and Beard, 2007) which, in reaction with aqueous CO 2 , can also induce the formation of formate and acetate (He et al., 2010; Hudson et al., 2020; De Graaf et al., 2023). Such a strong potential for abiotic organic synthesis crystallized hypotheses on the emergence of life in hydrothermal serpentinization systems (Russell et al., 1997; Russell et al., 2006; Camprubi et al., 2017). In modern oceans, AHV are predominantly made up of brucite Mg(OH) 2 and calcium carbonates CaCO 3 , mainly aragonite and calcite (Ludwig et al., 2006; Okumura et al., 2016). A type of chimney dominated by brucite (up to 99 wt %) harboring a porous structure (~ 50%) has been discovered at the Shinkai Seep Field (Okumura et al., 2016). The brucite crystals are oriented by the circulation of the effluent, leading to a stratified structure which results in the formation of micropores and nanopores in the mineral walls (Lee et al., 2024). The micro- and nanoconfined structures of such AHVs provide natural high-surface-area chemical reactors with more efficient reaction rates, selectivity and chemical complexity compared to bulk systems (Muñoz-Santiburcio and Marx, 2017; Lee et al., 2024). Due to a greater abundance of Mg-Fe rich ultramafic rocks, e.g. komatiites, in the early Earth than at present, Archean serpentinization processes were likely to produce higher amount of H 2 than currently, although the qualitative composition of the fluid was expected to be similar to nowadays (Leong et al., 2021). Modern and Archaean alkaline chimneys also feature distinct mineralogical assemblages due to main differences in the vent-ocean interface. The primitive ocean was likely more acidic by 0.5 to 3 pH units compared to present one and, most importantly, devoid of dissolved O 2 ( e.g. Halevy et al., 2017). Fe(II) was also present in high concentrations in ancient anoxic oceans, making an abundant presence of iron (oxy-hydr)oxides (together with carbonates and silicates) in early Earth AHVs plausible (Russel et al., 2006; Kotopoulou et al., 2020; Johnson et al., 2024). Such reactive minerals are of prime interest for chemical reactions at the vent-ocean interface. For example, they can mediate the oxidation of methane to methanol and yield the formation of various organic compounds (Barge et al., 2019; Farr et al., 2023). Iron (oxy-hydr)oxides, in particular layered double hydroxides (LDH) such as green rust, are frequently discussed as ingredients in putative proto-bioenergetic systems. Since they tend to deprotonate in response to oxidation reactions while conserving their crystalline structure, LDH are capable of conformational changes during oxidation-reduction and protonation-deprotonation oscillations (Génin al., 2006; 2008). This conformational flexibility makes them potential non-enzymatic catalysts for primitive anabolic reactions (Russell, 2018), especially since these minerals share structural similarities with metal-cluster-containing enzymes involved in current bioenergetic processes (Duval et al., 2019). In order to better understand the formation of iron-rich early Earth mineral structures at the vent-ocean interface, we produced experimental analogues of such alkaline chimneys in the laboratory, focusing on the precipitation of the reactive minerals iron (oxy-hydr)oxides. The analogues were formed at the interface between an alkaline solution and an acidic solution with high iron concentration (Fig. 1 ). These early Earth AHV analogues were allowed to precipitate under strictly anaerobic conditions for different periods of time (5h, 24h, 48h, 72h and 10 days) and were then analyzed combining multi-scale approaches. Mineral phases were identified in bulk using X-ray diffraction (XRD), then analogues with preserved integrity were studied using scanning electron microscopy (SEM) coupled to energy dispersive X-ray spectroscopy (EDXS) to obtain information concerning the structural assembly of minerals. Ultrathin sections were prepared from selected areas for nanoscale analysis using transmission electron microscopy (TEM). RESULTS AND DISCUSSION Growth of the alkaline vent analogues over time Within the first hour, Fe 3+ -rich phases such as magnetite (Fe II Fe III 2 O 4 ) are the prime minerals to form (Fig. 2 ). Fe 3+ precipitation is favored in Fe 2+ /Fe 3+ solution at the direct interface between the alkaline fluid and the acidic iron-rich solution. Hydroxylation of Fe 2+ occurs around pH 7 to 9 while it ranges from pH 1 to 5 for Fe 3+ (Jolivet et al., 2006). As ferric aquo complexes are more acidic than ferrous ones (Jolivet et al., 2004), the formation of Fe 3+ -rich phases suggests that the crystallization front at the direct interface of the acidic and basic solutions is exposed to acidic rather than basic conditions. Ferric complexes occur very rapidly to first form oligomers, then nuclei and eventually particles (Jolivet et al., 2004; 2006; Zhu et al., 2016), likely within the first minutes of the mineral barrier formation or possibly even right after adding the two solutions to the respective sides of the dialysis membrane. Magnetite thus results from the adsorption of Fe 2+ onto an amorphous or poorly crystalline Fe 3+ -rich precursor such as nanoparticulate ferrihydrite or an Fe 2+ /Fe 3+ gel (Sugimoto and Matijevic, 1979; Combes et al., 1988; Jolivet et al., 2006). As observed in SEM, the alternation of nanocrystalline magnetite and a more homogeneous material deposited in laminae also indicates the presence of two distinct mineral phases. Together, these mineral phases deposited directly onto the dialysis membrane form a thin laminar mineral structure about ten micrometers thick (Fig. 3 A and B). The broad XRD peaks observed at 1h suggest nanocrystalline or poorly crystallized magnetite, evolving toward better ordered or bigger crystals after 5h (Fig. 2 ). After 5 hours, the contact area with the dialysis membrane has a similar laminar texture as in the 1h sample (Fig. 3 D, in blue), suggesting that no transformation of minerals occurs over this time interval. In the parts furthest from the dialysis membrane, i.e. furthest from the basic solution, other regions feature larger magnetite crystals assembled in a disorderly pattern (Fig. 3 E, in cyan). Over time, at 5h (Fig. 3 C), the crystallinity and the abundance of magnetite thus increases, but magnetite remains the only mineral phase detected by XRD (Fig. 2 ). Starting at about 24h, the layered double hydroxide (LDH) green rust ([Fe II (6−x) Fe III x (OH 12 ] x + [ x/n A n− · y H 2 O] x− with A = Cl − or CO 3 2− ) precipitates, containing chloride ions in the interlayers (Fig. 2 ). Green rust can be formed abiotically by several processes, for example through the partial air oxidation of amakinite Fe(OH) 2 ( e.g. Refait et al., 1999), the partial reduction of ferrihydrite (Hansen, 1989) or by co-precipitation of Fe 2+ and Fe 3+ in solution under anoxic conditions (e.g. Ruby et al., 2006). Here, green rust is found to be formed prior to Fe(OH) 2 , ruling out the Fe(OH) 2 oxidation pathway. The co-precipitation pathway represents the most straightforward rationalization of the observations of the present study. Concomitant with the formation of green rust, we observe the formation of a new laminar region alternating between two mineral phases (Fig. 3 F and G, in yellow), magnetite with a rounded morphology and green rust platelets (Fig. 3 H, I and J). The iron oxyhydroxide formation is determined by both the pH and the E h , as described in the Pourbaix diagram of iron ( e.g. Jolivet et al, 2004; Génin et al., 2006). The alternation of magnetite and green rust thus indicates local variation of pH and/or E h at the crystallization front. While magnetite and green rust can form in the same pH range, i.e. 8.5 to 12 (Jolivet et al., 2004), their E h range is more constrained: green rust being formed in more reductive conditions, i.e. at lower redox potential than magnetite, due to a greater amount of Fe 2+ in the crystal lattice (Génin et al., 2006; Ruby et al., 2006). Formation of green rust may thus be explained by the local reservoir of iron (enrichment of Fe 2+ initially added in solution) at the crystallization front. We propose that magnetite and possibly ferrihydrite massive precipitation in the first hours of the experiment result in a depletion of Fe 3+ at the solidification front, thus driving an enrichment of Fe 2+ which allows green rust to form preferentially over magnetite. In turn, this preferential formation of green rust leads to a relative depletion of Fe 2+ at the crystallization front, driving an enrichment of Fe 3+ explaining alternances with magnetite. Starting at about 48h and until 10 days (Fig. 4 ), all the assemblages described above (blue, cyan and yellow, see Fig. 5 ) harbor the same characteristics (Fig. 4 A, B, E and G, Fig. 5 , Fig. S2, Fig. S3), corroborating the hypothesis that the mineral phases, in particular the alternating layers of magnetite and green rust, are formed directly at the crystallization front and undergo little or no subsequent transformation. After 48h, hydroxychloride green rust evolves into the more stable hydroxycarbonate green rust, as evidenced by a shift of the (0 0 3) XRD diffraction peak from 7.82 Å to 7.59 Å (Fig. 2 , Table S1). Iron hydroxide amakinite Fe II (OH) 2 precipitates (Fig. 2 ), corresponding to the formation of a new region with a homogeneous texture marked by fractures (Fig. 4 A, C, E and G, Fig. 5 A). The precipitation of Fe(OH) 2 indicates a strong increase of the pH and a decrease of the E h at the crystallization front (Génin et al., 2006) (Fig. 5 A). Concomitantly, the precipitation of Fe(OH) 2 is related to the formation of metallic iron “nails”, observed from 48h onwards (Fig. 2 , Fig. 4 D, F and H). De facto, the pH and redox conditions allowing the formation of Fe(OH) 2 and Fe 0 are very close (Van Genuchten et al., 2018) (Fig. 5 A). From 48h to 10 days, the system does not significantly evolve, except from further growth of the metallic iron nails and from increasing abundance of magnetite surrounding Fe 0 (Fig. 4 D, F and H). In the iron-rich acidic solution remains, pH remains at a value of 2 (Fig. 5 B) whereas the ternary system Fe 0 /Fe(OH) 2 /GR has an alkaline equilibrium pH value around 8 (Refait et al. 2002; Génin et al., 2006) (Fig. 5 A). This discontinuity can also be applied to the Eh values. The Fe 2+ /Fe 3+ solution has a midpoint potential around + 0.68 V while the ternary system Fe 0 /Fe(OH) 2 /GR has an equilibrium potential around − 0.54 V (Refait et al., 2002; Génin et al., 2006) (Fig. 5 A). Overall, the pH gradient induces the speciation of Fe 3+ and Fe 2+ through their mobilization by mineral phases, from the most oxidized to the most reduced iron mineral phases. In other words, the initial pH gradient mediates a redox gradient and leads to a strong reductive effect on the (oxy-hydr)oxides mineral formation. The preferential precipitation of Fe(III) (oxy-hydr)oxides due to the pH gradient leads to the formation of Fe(OH) 2 (amakinite or ferroan brucite) at a well-defined stage and therefore at a well-defined spatial location in the AHV analogue. Thus, the formation of the mineral phases is in opposition to the imposed pH and redox constraint, which is likely due to the OH − having a diffusion rate higher than the iron complexation. This can be seen through the increase of the pH at the crystallization front, enabling the precipitation of stable mineral phases at very alkaline pH. Otherwise, only ferric iron complexes, stable at lower pH values, would be observed. Formation of Fe 0 , a thermodynamic conundrum While the alternating sequence of iron oxide/hydroxide minerals from magnetite through green rust and on to amakinite is straightforwardly rationalized by considering local pH-values and precipitation-induced variations of soluble Fe 2+ /Fe 3+ ion concentrations as described above, the formation of Fe 0 is thermodynamically puzzling. Only Fe ions in the 2 + and 3 + oxidation states were present in the starting solution and both halves of the reaction vessel were hermetically sealed. The bulk ambient redox potential therefore is exclusively determined by the initial Fe 2+ /Fe 3+ ratio, and while local potentials can be lower than that of the bulk when predominantly Fe 2+ is present (together with appropriate pH-values resulting in the formation of amakinite), there is no simple exergonic reaction scheme that would account for the reduction of Fe 2+ (and, even less, Fe 3+ ) to metallic iron. Fe 0 is out-of-thermodynamic-equilibrium with the redox potential of the solution and its generation therefore corresponds to an endergonic reaction. A possible lead to solve this conundrum may be provided by the observation that the regions made up from metallic iron have formed within the (strongly reduced) amakinite-rich precipitation front but are coated by a layer of (less reduced) iron-oxides (Fig. 4 D, F and H, Fig. 5 , Fig. 6 A, Fig. S4). Ultrathin FIB sections prepared from a metallic iron nail show that these iron oxides are closely associated to the entire surface of the nails (Fig. 6 B and E, Fig. S4). Estimation of oxygen percentage performed via EDXS confirms that the surrounding minerals are rich in oxygen (> 30%) (Fig. 6 C and D), identified as magnetite given the 4.8 A° interplanar distance of the (1 1 1) lattice plane (Fig. 6 E and F). Such a pattern is characteristic of an iron disproportionation reaction (O’Neill et al., 1993; Frost et al., 2004), that is, the production of compounds part of which is in a more oxidized state and another part in a more reduced one as compared with the starting material (Yakovlev et al., 2009). Here, Fe 2+ would thus be disproportionated into Fe 3+ (more oxidized state) and Fe 0 (more reduced state). Such a type of reaction is reminiscent of the dismutation of wüstite (FeO) minerals (Eq. 1 ) (Yakovlev et al., 2009) which is thermodynamically possible. $$\:4\:FeO\to\:\:{Fe}^{0}+\:{Fe}_{3}{O}_{4}$$ 1 This mechanism has been experimentally observed at high temperatures and high pressures, i.e. under the conditions of the mantle (Frost et al., 2004), as well as at lower temperatures, yet still up to 300°C (Pouyan et al., 1983). In these cases, disproportionation can be explained by the stability domain of wüstite, which decreases concomitantly with temperature and pressure, while the stability field of metallic iron and magnetite increases at lower temperature and pressure (Pouyan et al., 1983; Yakovlev et al., 2009). In our case, all three mineral phases involved, namely amakinite Fe(OH) 2 , magnetite Fe 3 O 4 and metallic iron Fe 0 are stable at low temperatures and in the same range of redox conditions (Moody, 1976; Revie and Uhlig, 2008). Still, Fe 2+ from amakinite has to be oxidized into Fe 3+ , and Fe 0 has to be obtained from Fe 2+ (or Fe 3+ , which is thermodynamically less favorable). Moreover, these two redox reactions likely are causally related, since the formation of metallic iron is seen to be structurally related to the formation of magnetite. In the temporal evolution of the analogue described here, increasingly reduced mineral phases bear witness of a progressive and local reduction of the fluid at the crystallization front (Fig. 5 ), likely explained by an enrichment of Fe 2+ in the local reservoir of iron. In such acidic environments, free protons are abundant and able to accept electrons from Fe 2+ , thus representing the most likely oxidant for Fe(OH) 2 minerals, deprotonating as a function of the oxidation (Eq. 2 ) (Génin et al., 2006) and forming Fe 3+ and H 2 (Eq. 3 , Fig. S4). Oxidation and deprotonation of amakinite is better known as the Schikorr reaction, where ferrous hydroxide Fe(OH) 2 is dismutated into magnetite and H 2 , even at low temperature (below 100°C) (Eq. 4 ) (Schikorr, 1929; Neubeck et al., 2014). $$\:3\:{Fe\left(OH\right)}_{2}\:\rightleftarrows\:\:{Fe}_{3}{O}_{4}+2\:{H}_{2}O\:+\:2\:{H}^{+}\:+\:2\:{e}^{-}$$ 2 $$\:3\:{Fe\left(OH\right)}_{2}\:\rightleftarrows\:\:{Fe}_{3}{O}_{4}+\:{H}_{2}\:+\:2\:{H}_{2}O\:$$ 3 An endergonic reaction performed by a proto-bioenergetic system By thermodynamically modeling the Schikorr reaction with production of H 2 , knowing Eq. 4 , it appears that this reaction has a ΔG of + 10 kJ/mol, resulting in an equilibrium H 2 pressure of 0.13 bar. However, in the experiments of Carlin et al., (2024) at 100°C, the H 2 pressures controlled by kinetics were measured at a pressure of 0.003 bar. By taking this value of 0.003 which is likely an overestimation in our conditions, we obtained a ΔG of -5 kJ/mol, meaning the ΔG of the Schikorr reaction remains largely negative in experiments at ambient temperature. Thus, the Schikorr reaction in our early Earth AHV analogue is a highly exergonic reaction. On the other hand, the amakinite dismutation (or Fe 2+ disproportionation) into magnetite and metallic iron described in Eq. 4 leads to a highly positive ΔG of + 35 kJ/mol. Another possibility would be to consider H 2 as a sufficiently powerful reductant to reduce Fe(OH) 2 to Fe 0 , oxidizing the H 2 to H 2 O (Eq. 5 ). However, for the reaction (Eq. 5 ) to occur, the H-H bond dissociation requires the astonishing ΔG of + 435 kJ/mol at ambient temperature (Herzberg and Shoosmith, 1959), making the direct amakinite dismutation (Eq. 4 ) more likely. In any case, the observation of metallic iron undoubtedly means that the system of AHV analogues presented here performs an endergonic reaction. $$\:4\:{Fe\left(OH\right)}_{2}\:\to\:\:{Fe}_{3}{O}_{4}+\:Fe\:+\:4\:{H}_{2}O\:$$ 4 $$\:{Fe\left(OH\right)}_{2}\:+\:{H}_{2}\:\to\:\:Fe\:+\:2\:{H}_{2}O$$ 5 We speculate that during the Schikorr reaction (Eq. 3 ), a portion of the electrons and protons respectively produced by the oxidation of Fe 2+ to Fe 3+ and the associated the dehydroxylation of OH groups associated with Fe 2+ (Eq. 2 ), are decoupled in a mineral system dominated by the layer double hydroxide amakinite (or ferroan brucite). This allows the migration of proton and hydroxide ions in a pH gradient. As a result, the electrons remaining almost on site in Fe(OH) 2 have a very reducing electrochemical potential and are able to reduce Fe(II) to Fe(0). By allowing some electrons to carry out a positive ΔG reaction (the endergonic reduction of Fe 2+ into Fe 0 ) relying on a negative ΔG redox reaction (the exergonic oxidation of Fe 2+ into Fe 3+ coupled to H 2 production), this early Earth AHV analogue therefore generates local ambient potentials that are thermodynamically out-of-equilibrium (within regions containing Fe 0 ) while taking the entire system closer to equilibrium, the latter process making this dismutation process thermodynamics-compliant. It is precisely this type of process which is the hallmark of all living entities. Life converts environmental free energy into intracellular disequilibria ( i.e. low entropy) which subsequently drive the quasi-totality of all metabolic pathways (Branscomb and Russel, 2013; Branscomb, 2023; Nitschke et al., 2024). The mechanism performing this conversion of free energy in living organisms is called bioenergetics. The two fundamental intracellular disequilibria built up by bioenergetic processes are far-from-equilibrium ratios of phosphates to polyphosphates and reducing potentials exceeding those of the environment (Nitschke, 2022). The latter disequilibrium is achieved by several bioenergetic mechanisms, one of which is called electron bifurcation (Buckel and Thauer, 2018), a process which disproportionates a 2-electron redox compound into a more reducing redox center on one side and a less reducing one on the other side. Fe 0 has been previously mentioned as an indispensable precursor for the electronic bifurcation process (Brabender et al., 2024). Accordingly, the disproportionation mechanism itself of amakinite into magnetite and metallic iron represents a tantalizing analogue in a purely abiotic system of the electron bifurcation reaction, fundamental to bioenergetics and hence to life’s thermodynamic prerequisites. CONCLUSION The formation of iron oxyhydroxides at the interface between an alkaline solution and an acidic solution with high iron concentrations results in a spatially highly structured barrier of iron oxyhydroxide minerals. The initial pH conditions imposed by the two solutions induce a speciation of Fe 3+ and Fe 2+ mobilized by the mineral phases and lead to a strong reductive effect. The formation of the mineral phases is in complete opposition with the imposed pH and redox constraints: the precipitation of minerals favored in acidic conditions, such as Fe(III) complexes, occurred at the direct interface with the basic solution (directly onto the dialysis membrane), while the precipitation of minerals requiring more basic conditions, such as amakinite, occurred at the direct interface with the acidic solution. We interpret this phenomenon as the result of the OH − diffusion rate on iron complexes. Ultimately, this strong pH constraint into AHV analogues leads to the formation of metallic iron Fe 0 due to the disproportionation of Fe 2+ (from amakinite minerals) into Fe 3+ (magnetite) and Fe 0 (metallic iron). The dismutation of amakinite at ambient temperature implies the coupling of the exergonic Schikorr reaction with the endergonic reduction of Fe 2+ into Fe 0 , a process reminiscent of the bioenergetic mechanism of electron bifurcation. The system we describe here thus generates a local decrease in entropy in the form of thermodynamic out-of-equilibrium ambient redox potentials mediated by the disproportionation of Fe 2+ ions. Such local lowering entropy driven by bulk free energy is the thermodynamic prerequisite for allowing the emergence of life in the framework of thermodynamics. METHODS Precipitation protocol for the iron-rich early Earth alkaline vents analogue The experimental setup for the AHV analogue precipitation is based on Barge et al., (2014). The experiments were conducted in strict anoxia under N 2 atmosphere in a Jacomex™ glove box (< 10 ppm O 2 ). We used a piece of dialysis membrane (Spectra-Por® Membrane MWCO: 3,500) placed at the interface of two glass vials (DEK Research™) hermetically sealed at the center. Two solutions were prepared: a basic solution (pH 14) composed of 0.27 M NaOH and 0.033 M Na 2 CO 3 35mL of degassed water; and an acidic solution (pH 2) composed of 0.06 M FeCl 3 •6H2O and 0.12 M FeCl 2 •4H 2 O into 35mL of degassed water. Both solutions were added simultaneously on either side of the dialysis membrane and let to incubate at room temperature for different durations (1h, 5h, 24h, 48h, 72h, 96h and 10d). The mineral membrane formed on the iron-rich acid solution side, directly onto the dialysis membrane (Fig. 1 ), was then dried in the glovebox and prepared for the various analyses. X-ray diffraction (XRD) Sample preparations and measurements were conducted under N 2 atmosphere in strictly anoxic conditions (< 10 ppm O 2 ). AHV analogues were crushed in an agate mortar and resuspended into degassed pure ethanol before being deposited on a zero-background Si wafer. The wafer was inserted in a custom-built anoxic sample chamber equipped with a Kapton R window. The sealed chamber was then removed from the glove-box and XRD patterns were collected on a XPert Pro Panalytical™ diffractometer at the IMPMC diffractometry platform. Data were collected using Co Kα radiation in continuous scan mode with an equivalent 0.033° 2θ step counting 1 hour (2 scans of 30 min) per sample over the 5–80° 2θ range. Minerals were identified using XPert HighScore Plus software, PDF4 and COD databases: iron hydroxychloride green rust (PDF4 ID: 00-040-0127), iron hydroxycarbonate green rust (PDF4 ID: 00-052-0163), magnetite (PDF4 ID: 01-076-1849), amakinite (COD ID: 00-900-9104), halite (PDF4 ID: 00-005-0628) and metallic iron α-Fe (PDF4 ID: 01-087-0722). Scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDXS) The whole mineral membranes were inserted into Epoxy Araldite 2020 resin directly in the glovebox to limit oxygen exposure. Once within the resin, the samples were taken out of the glovebox and placed in the oven at 35°C to accelerate the polymerization of the resin. The samples were then cut in half using a wire saw and polished, half on the top and half on the edge, with 6, 3 and 1µm diamond suspensions and an alcohol-based lubricant to limit oxygen exposure during the preparation. The samples were carbon-coated immediately after polishing and investigated using a Scanning Electron Microscope (SEM) coupled with Energy-Dispersive X-ray Spectroscopy (EDXS). SEM-EDXS data were collected at the CINaM microscopy platform, using a JEOL JSM-7900F equipped with a QUANTAX XFlash® Flat QUAD annular four-channel silicon drift (Bruker) for EDXS analysis. Images were collected using an acceleration voltage of 15 kV at a working distance of 11 mm, while EDXS analysis were conducted using a tension of 6 kV. Ultrathin sections by focused ion beam (FIB) Ultrathin FIB sections (20 µm * 5 µm * 100 nm) were extracted at Eurofins Biophy Research from one analogue of alkaline vents formed after 72h, using Ga milling with a SEM-FIB TESCAN CLARA. Two FIB sections were extracted from a selected region at the interface between the metallic iron and surrounding minerals, i.e. magnetite. A layer of Pt was deposited on the sample surface to protect it from Ga sputtering. Milling was performed gradually at low Ga currents (500 pA, 250 pA and 50 pA) with a final milling step at 20 pA to minimize common artefacts including local Ga implantation, mixing of components, or redeposition of the sputtered material onto the sample surface (Bernard et al., 2009; Schiffbauer and Xiao, 2009). Transmission electron microscopy (TEM) Nanoscale characterization of the minerals surrounding the metallic iron was performed on FIB sections at the CINaM microscopy platform. TEM data were collected using a JEOL JEM-2010 (LaB6) operating at 200 kV and equipped with a QUANTAX XFlash® Flat QUAD annular four-channel silicon drift SDD detector and a GATAN Ultrascan 1000XP camera. Mineral identification was achieved using selected-area electron diffraction (SAED) and energy-dispersive X-ray spectroscopy (EDXS). CHESS thermodynamic modeling Thermodynamic modeling was performed using CHESS code 4.0.3. (Van der Lee and Windt, 2002) with Thermodem database (Blanc et al., 2012; Marty et al., 2014). A precipitation medium was modelled at 25°C containing for 1 litre of solution: 0.27 M NaOH, 0.06 M (aq) Fe 3+ , 0.12 M (aq) Fe 2+ . The partial pressure of H 2 was fixed at 0.003 bar based on Carlin et al., (2024) experiments. Declarations ACKNOWLEDGEMENTS We warmly thank Isabelle Pinet (BIP) for her administrative support, Damien Chaudansson and Alexandre Altié (CINaM) for their valuable help with SEM-EDXS, TEM and samples preparation for the microscopy, Amandine David (Eurofins Scientific) for the production of the FIB sections, Ludovic Delbes (IMPMC) and Vasile Heresanu (CINaM) for their help with XRD. This work was supported by the French Agence Nationale pour la Recherche (grant no. ANR-22-CE30-0035-01). References Barge, L. M., Doloboff, I. J., Russell, M. J., VanderVelde, D., White, L. M., Stucky, G. D., Baum, M. M., Zeytounian, J., Kidd, R., & Kanik, I. (2013). Pyrophosphate synthesis in iron mineral films and membranes simulating prebiotic submarine hydrothermal precipitates. Geochimica et Cosmochimica Acta, 128, 1‑12. https://doi.org/10.1016/j.gca.2013.12.006 Barge, L. M., Flores, E., Baum, M. M., VanderVelde, D. G., & Russell, M. J. (2019). Redox and pH gradients drive amino acid synthesis in iron oxyhydroxide mineral systems. Proceedings Of The National Academy Of Sciences, 116(11), 4828‑4833. https://doi.org/10.1073/pnas.1812098116 Bernard, S., Benzerara, K., Beyssac, O., Brown, G., Stamm, L. G., & Duringer, P. (2008). Ultrastructural and chemical study of modern and fossil sporoderms by Scanning Transmission X-ray Microscopy (STXM). Review Of Palaeobotany And Palynology, 156(1‑2), 248‑261. https://doi.org/10.1016/j.revpalbo.2008.09.002 Blanc, P., Lassin, A., Piantone, P., Azaroual, M., Jacquemet, N., Fabbri, A., & Gaucher, E. (2012). Thermoddem : A geochemical database focused on low temperature water/rock interactions and waste materials. Applied Geochemistry, 27(10), 2107‑2116. https://doi.org/10.1016/j.apgeochem.2012.06.002 Brabender, M., Pereira, D. P. H., Mrnjavac, N., Schlikker, M. L., Kimura, Z., Sucharitakul, J., Kleinermanns, K., Tüysüz, H., Buckel, W., Preiner, M., & Martin, W. F. (2024). Ferredoxin reduction by hydrogen with iron functions as an evolutionary precursor of flavin-based electron bifurcation. Proceedings Of The National Academy Of Sciences, 121(13). https://doi.org/10.1073/pnas.2318969121 Branscomb, E., & Russell, M. J. (2013). Turnstiles and bifurcators: The disequilibrium converting engines that put metabolism on the road. Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1827(2), 62‑78. https://doi.org/10.1016/j.bbabio.2012.10.003 Branscomb, E. (2023). Boltzmann’s casino and the unbridgeable chasm in emergence of life research. arXiv (Cornell University). https://doi.org/10.48550/arxiv.2312.00932 Buckel, W., & Thauer, R. K. (2018). Flavin-Based Electron Bifurcation, A New Mechanism of Biological Energy Coupling. Chemical Reviews, 118(7), 3862‑3886. https://doi.org/10.1021/acs.chemrev.7b00707 Camprubi, E., Jordan, S. F., Vasiliadou, R., & Lane, N. (2017c). Iron catalysis at the origin of life. IUBMB Life, 69(6), 373‑381. https://doi.org/10.1002/iub.1632 Carlin, W., Malvoisin, B., Brunet, F., Lanson, B., Findling, N., Lanson, M., Fargetton, T., Jeannin, L., & Lhote, O. (2024). Kinetics of low-temperature H2 production in ultramafic rocks by ferroan brucite oxidation. Geochemical Perspectives Letters, 29, 27‑32. https://doi.org/10.7185/geochemlet.2408 Combes, J. M., Manceau, A., Calas, G., & Bottero, J. Y. (1989). Formation of ferric oxides from aqueous solutions: A polyhedral approach by X-ray absorption spectroscopy: I. Hydrolysis and formation of ferric gels. Geochimica et Cosmochimica Acta, 53(3), 583-594. https://doi.org/10.1016/0016-7037(89)90001-X De Graaf, R., De Decker, Y., Sojo, V., & Hudson, R. (2023). Quantifying Catalysis at the Origin of Life**. Chemistry - A European Journal, 29(53). https://doi.org/10.1002/chem.202301447 Duval, S., Baymann, F., Schoepp-Cothenet, B., Trolard, F., Bourrié, G., Grauby, O., Branscomb, E., Russell, M. J., & Nitschke, W. (2019). Fougerite: the not so simple progenitor of the first cells. Interface Focus, 9(6), 20190063. https://doi.org/10.1098/rsfs.2019.0063 Farr, O., Gaudu, N., Danger, G., Russell, M. J., Ferry, D., Nitschke, W., & Duval, S. (2023). Methanol on the rocks: green rust transformation promotes the oxidation of methane. Journal of the Royal Society Interface, 20(206), 20230386. Frost, D. J., Liebske, C., Langenhorst, F., McCammon, C. A., Trønnes, R. G., & Rubie, D. C. (2004). Experimental evidence for the existence of iron-rich metal in the Earth’s lower mantle. Nature, 428(6981), 409‑412. https://doi.org/10.1038/nature02413 Frost, B. R., & Beard, J. S. (2007). On Silica Activity and Serpentinization. Journal Of Petrology, 48(7), 1351‑1368. https://doi.org/10.1093/petrology/egm021 Génin, J. R., Ruby, C., Géhin, A., & Refait, P. (2006). Synthesis of green rusts by oxidation of Fe(OH)2, their products of oxidation and reduction of ferric oxyhydroxides ; –pH Pourbaix diagrams. Comptes Rendus Géoscience, 338(6‑7), 433‑446. https://doi.org/10.1016/j.crte.2006.04.004 Génin, J. R., Renard, A., & Ruby, C. (2008). Fougerite FeII − III oxyhydroxycarbonate in environmental chemistry and nitrate reduction. Hyperfine Interactions, 186(1‑3), 31‑37. https://doi.org/10.1007/s10751-008-9837-z Halevy, I., & Bachan, A. (2017). The geologic history of seawater pH. Science, 355(6329), 1069-1071. Hansen, H. C. B. (1989). Composition, stabilization, and light absorption of Fe(II)Fe(III) hydroxy-carbonate (‘green rust’). Clay Minerals, 24(4), 663‑669. https://doi.org/10.1180/claymin.1989.024.4.08 He, C., Tian, G., Liu, Z., & Feng, S. (2010). A Mild Hydrothermal Route to Fix Carbon Dioxide to Simple Carboxylic Acids. Organic Letters, 12(4), 649‑651. https://doi.org/10.1021/ol9025414 Herzberg, G., & Shoosmith, J. (1959). Spectrum and Structure of the Free Methylene Radical. Nature, 183(4678), 1801‑1802. https://doi.org/10.1038/1831801a0 Horita, J., Berndt, M. E. (1999). Abiogenic Methane Formation and Isotopic Fractionation Under Hydrothermal Conditions. Science, 285(5430), 1055‑1057. https://doi.org/10.1126/science.285.5430.1055 Hudson, R., De Graaf, R., Rodin, M. S., Ohno, A., Lane, N., McGlynn, S. E., Yamada, Y. M. A., Nakamura, R., Barge, L. M., Braun, D., & Sojo, V. (2020c). CO 2 reduction driven by a pH gradient. Proceedings Of The National Academy Of Sciences, 117(37), 22873‑22879. https://doi.org/10.1073/pnas.2002659117 Johnson, J. E., Present, T. M., & Valentine, J. S. (2024). Iron : Life’s primeval transition metal. Proceedings Of The National Academy Of Sciences, 121(38). https://doi.org/10.1073/pnas.2318692121 Jolivet, J., Chanéac, C., & Tronc, E. (2004). Iron oxide chemistry. From molecular clusters to extended solid networks. Chemical Communications, 5, 477‑483. https://doi.org/10.1039/b304532n Jolivet, J., Tronc, E., & Chanéac, C. (2006). Iron oxides : From molecular clusters to solid. A nice example of chemical versatility. Comptes Rendus Géoscience, 338(6‑7), 488‑497. https://doi.org/10.1016/j.crte.2006.04.014 Kelley, D. S., Karson, J. A., Früh-Green, G. L., Yoerger, D. R., Shank, T. M., Butterfield, D. A., Hayes, J. M., Schrenk, M. O., Olson, E. J., Proskurowski, G., Jakuba, M., Bradley, A., Larson, B., Ludwig, K., Glickson, D., Buckman, K., Bradley, A. S., Brazelton, W. J., Roe, K.,. . . Sylva, S. P. (2005). A Serpentinite-Hosted ecosystem : the Lost City hydrothermal Field. Science, 307(5714), 1428‑1434. https://doi.org/10.1126/science.1102556 Kotopoulou, E., Lopez‐Haro, M., Gamez, J. J. C., & García‐Ruiz, J. M. (2020). Nanoscale Anatomy of Iron‐Silica Self‐Organized Membranes : Implications for Prebiotic Chemistry. Angewandte Chemie International Edition, 60(3), 1396‑1402. https://doi.org/10.1002/anie.202012059 Lee, H., Okumura, T., Ooka, H., Adachi, K., Hikima, T., Hirata, K., Kawano, Y., Matsuura, H., Yamamoto, M., Yamamoto, M., Yamaguchi, A., Lee, J., Takahashi, H., Nam, K. T., Ohara, Y., Hashizume, D., McGlynn, S. E., & Nakamura, R. (2024). Osmotic energy conversion in serpentinite-hosted deep-sea hydrothermal vents. Nature Communications, 15(1). https://doi.org/10.1038/s41467-024-52332-3 Leong, J. A. M., Ely, T., & Shock, E. L. (2021). Decreasing extents of Archean serpentinization contributed to the rise of an oxidized atmosphere. Nature Communications, 12(1). https://doi.org/10.1038/s41467-021-27589-7 Li, Y. (2022). Minerals as Prebiotic Catalysts for Chemical Evolution towards the Origin of Life. Dans IntechOpen eBooks. https://doi.org/10.5772/intechopen.102389 Ludwig, K. A., Kelley, D. S., Butterfield, D. A., Nelson, B. K., & Früh-Green, G. (2006). Formation and evolution of carbonate chimneys at the Lost City Hydrothermal Field. Geochimica et Cosmochimica Acta, 70(14), 3625‑3645. https://doi.org/10.1016/j.gca.2006.04.016 Marty, N. C., Claret, F., Lassin, A., Tremosa, J., Blanc, P., Madé, B., Giffaut, E., Cochepin, B., & Tournassat, C. (2014). A database of dissolution and precipitation rates for clay-rocks minerals. Applied Geochemistry, 55, 108‑118. https://doi.org/10.1016/j.apgeochem.2014.10.012 McCollom, T. M., Seewald, J. S. (2001). A reassessment of the potential for reduction of dissolved CO 2 to hydrocarbons during serpentinization of olivine. Geochimica et Cosmochimica Acta, 65(21), 3769‑3778. https://doi.org/10.1016/s0016-7037(01)00655-x McCollom, T. M., Seewald, J. S. (2006). Carbon isotope composition of organic compounds produced by abiotic synthesis under hydrothermal conditions. Earth And Planetary Science Letters, 243(1‑2), 74‑84. https://doi.org/10.1016/j.epsl.2006.01.027 Moody, J. B. (1976). Serpentinization : a review. Lithos, 9(2), 125‑138. https://doi.org/10.1016/0024-4937(76)90030-x Muñoz-Santiburcio, D., & Marx, D. (2017). Chemistry in nanoconfined water. Chemical Science, 8(5), 3444‑3452. https://doi.org/10.1039/c6sc04989c Neubeck, A., Duc, N. T., Hellevang, H., Oze, C., Bastviken, D., Bacsik, Z., & Holm, N. G. (2014). Olivine alteration and H2 production in carbonate-rich, low temperature aqueous environments. Planetary And Space Science, 96, 51‑61. https://doi.org/10.1016/j.pss.2014.02.014 Nitschke, W., & Russell, M. J. (2013). Beating the acetyl coenzyme A-pathway to the origin of life. Philosophical Transactions Of The Royal Society B Biological Sciences, 368(1622), 20120258. https://doi.org/10.1098/rstb.2012.0258 Nitschke, W., Schoepp‐Cothenet, B., Duval, S., Zuchan, K., Farr, O., Baymann, F., Panico, F., Minguzzi, A., Branscomb, E., & Russell, M. J. (2022). Aqueous electrochemistry : The toolbox for life’s emergence from redox disequilibria. Electrochemical Science Advances, 3(2). https://doi.org/10.1002/elsa.202100192 Nitschke, W., Farr, O., Gaudu, N., Truong, C., Guyot, F., Russell, M. J., & Duval, S. (2024). The Winding Road from Origin to Emergence (of Life). Life, 14(5), 607. https://doi.org/10.3390/life14050607 Okumura, T., Ohara, Y., Stern, R. J., Yamanaka, T., Onishi, Y., Watanabe, H., Chen, C., Bloomer, S. H., Pujana, I., Sakai, S., Ishii, T., & Takai, K. (2016). Brucite chimney formation and carbonate alteration at the Shinkai Seep Field, a serpentinite‐hosted vent system in the southern Mariana forearc. Geochemistry Geophysics Geosystems, 17(9), 3775‑3796. https://doi.org/10.1002/2016gc006449 O’Neill, H. S., Rubie, D. C., Canil, D., Geiger, C. A., Ross, C. R., Seifert, F., & Woodland, A. B. (2011). Ferric Iron in the Upper Mantle and In Transition Zone Assemblages : Implications for Relative Oxygen Fugacities in the Mantle. Dans Geophysical monograph (p. 73‑88). https://doi.org/10.1029/gm074p0073 Pouyan, S., Bassett, W. A., & Lin-Gun, L. (1983). Experimental determination of the effects of pressure and temperature on the stoichiometry and phase relations of wüstite. Geochimica et Cosmochimica Acta, 47(4), 773‑778. https://doi.org/10.1016/0016-7037(83)90110-2 Proskurowski, G., Lilley, M. D., Seewald, J. S., Früh-Green, G. L., Olson, E. J., Lupton, J. E., Sylva, S. P., & Kelley, D. S. (2008). Abiogenic Hydrocarbon Production at Lost City Hydrothermal Field. Science, 319(5863), 604‑607. https://doi.org/10.1126/science.1151194 Refait, P., Bon, C., Simon, L., Bourrié, G., Trolard, F., Bessière, J., & Gènin, J. R. (1999). Chemical composition and Gibbs standard free energy of formation of Fe(II)-Fe(III) hydroxysulphate green rust and Fe(II) hydroxide. Clay Minerals, 34(3), 499‑510. https://doi.org/10.1180/000985599546280 Refait, P., Géhin, A., Abdelmoula, M., & Génin, J. (2002). Coprecipitation thermodynamics of iron(II–III) hydroxysulphate green rust from Fe(II) and Fe(III) salts. Corrosion Science, 45(4), 659‑676. https://doi.org/10.1016/s0010-938x(02)00138-5 Revie, R. W., & Uhlig, H. H. (2008). Corrosion and Corrosion Control. https://doi.org/10.1002/9780470277270 Ruby, C., Aïssa, R., Géhin, A., Cortot, J., Abdelmoula, M., & Génin, J. (2006). Green rusts synthesis by coprecipitation of FeII–FeIII ions and mass-balance diagram. Comptes Rendus Géoscience, 338(6‑7), 420‑432. https://doi.org/10.1016/j.crte.2006.04.008 Russell, M. J. (2003). The Importance of Being Alkaline. Science, 302(5645), 580‑581. https://doi.org/10.1126/science.1091765 Russell, M. J., & Martin, W. (2004). The rocky roots of the acetyl-CoA pathway. Trends In Biochemical Sciences, 29(7), 358‑363. https://doi.org/10.1016/j.tibs.2004.05.007 Russell, M. J. (2006). First Life : Billions of years ago, deep under the ocean, the pores and pockets in minerals that surrounded warm, alkaline springs catalyzed the beginning of life on JSTOR. (s. d.). https://www.jstor.org/stable/27858705 Russell, M. J., Hall, A. J., & Martin, W. (2010). Serpentinization as a source of energy at the origin of life. Geobiology, 8(5), 355‑371. https://doi.org/10.1111/j.1472-4669.2010.00249.x Russell, M. J., Barge, L. M., Bhartia, R., Bocanegra, D., Bracher, P. J., Branscomb, E., Kidd, R., McGlynn, S., Meier, D. H., Nitschke, W., Shibuya, T., Vance, S., White, L., & Kanik, I. (2014). The Drive to Life on Wet and Icy Worlds. Astrobiology, 14(4), 308‑343. https://doi.org/10.1089/ast.2013.1110 Russell, M. J. (2018). Green Rust : The Simple Organizing ‘Seed’ of All Life ? Life, 8(3), 35. https://doi.org/10.3390/life8030035 Schiffbauer, J. D., & Xiao, S. (2009). Novel application of focused ion beam electron microscopy (FIB-EM) in preparation and analysis of microfossil ultrastructures : A new view of complexity in early Eukaryotic organisms. Palaios, 24(9), 616‑626. https://doi.org/10.2110/palo.2009.p09-003r Schikorr, G. (1929). Über die Reaktionen zwischen Eisen, Seinen Hydroxyden und Wasser. Zeitschrift Für Elektrochemie Und Angewandte Physikalische Chemie, 35(2), 65‑70. https://doi.org/10.1002/bbpc.19290350204 Seewald, J. S., Zolotov, M. Y., & McCollom, T. (2006). Experimental investigation of single carbon compounds under hydrothermal conditions. Geochimica et Cosmochimica Acta, 70(2), 446‑460. https://doi.org/10.1016/j.gca.2005.09.002 Sugimoto, T., & Matijević, E. (1980). Formation of uniform spherical magnetite particles by crystallization from ferrous hydroxide gels. Journal Of Colloid And Interface Science, 74(1), 227‑243. https://doi.org/10.1016/0021-9797(80)90187-3 Van der Lee J. and Windt L. D. (2002), CHESS Tutorial and Cookbook, Version 3.0. Technical Report 1±2. Ecole des Mines de Paris, Fontainebleau, France. Van Genuchten, C., Behrends, T., Kraal, P., Stipp, S., & Dideriksen, K. (2018). Controls on the formation of Fe(II,III) (hydr)oxides by Fe(0) electrolysis. Electrochimica Acta, 286, 324‑338. https://doi.org/10.1016/j.electacta.2018.08.031 Yakovlev, O. I., Dikov, Y. P., & Gerasimov, M. V. (2009). Effect of the disproportionation reaction of ferrous iron in impact-evaporation processes. Geochemistry International, 47(2), 134‑142. https://doi.org/10.1134/s0016702909020025 Zhu, M., Frandsen, C., Wallace, A. F., Legg, B., Khalid, S., Zhang, H., Mørup, S., Banfield, J. F., & Waychunas, G. A. (2015). Precipitation pathways for ferrihydrite formation in acidic solutions. Geochimica et Cosmochimica Acta, 172, 247‑264. https://doi.org/10.1016/j.gca.2015.09.015 Additional Declarations The authors declare no competing interests. Supplementary Files Supplementary.docx Supplementary Informations Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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\u003cstrong\u003eB)\u003c/strong\u003eExperimental setup to reproduce a simplified analogue of an AHV chimney wall formed in a pH gradient, at the interface of a basic solution (in orange, pH 14) and of an iron-rich acidic solution (in blue, pH 2).\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7036221/v1/dafa8d99a743bd29532f392f.png"},{"id":86028502,"identity":"3308df25-1661-410d-a496-47364b5b4702","added_by":"auto","created_at":"2025-07-04 13:40:41","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":165163,"visible":true,"origin":"","legend":"\u003cp\u003eX-ray diffraction under anoxic conditions using Co Kα radiation (λ = 1.79026 Å) of the alkaline vent analogues formed in a pH gradient after 1h, 5h, 24h, 48h, 72h and 10 days. For each phase, peaks are labeled with carbonated green rust (PDF4 ID: 00-052-0163 / noted GR in green), chloride green rust (PDF4 ID: 00-040-0127 / noted Cl - GR in green), magnetite (PDF4 ID: 01-076-1849/ noted M in blue), amakinite (COD ID: 00-900-9104 / noted A in orange), halite (PDF4 ID: 00-005-0628 / noted H in black) and the most intense peak (1 1 0) of metallic iron a-Fe (PDF4 ID: 01-087-0722 / noted Fe in red). The shift of the green rust plane (0 0 3) is indicated by a black arrow. See the complete indexing Table S1 in Supplementary Information.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7036221/v1/0e0f48c4f100f0b850afeb2f.png"},{"id":86028504,"identity":"8b4acc7f-62cc-42d0-ba18-bd54617ec087","added_by":"auto","created_at":"2025-07-04 13:40:41","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":617348,"visible":true,"origin":"","legend":"\u003cp\u003eSEM observations of the AHV analogues formed after 1h, 5h and 24h. \u003cstrong\u003eA)\u003c/strong\u003eAHV analogue formed after 1h. The blue area in \u003cstrong\u003eB) \u003c/strong\u003eprecipitates on contact with the dialysis membrane and\u003cstrong\u003e \u003c/strong\u003efeatures laminations. \u003cstrong\u003eC)\u003c/strong\u003e AHV analogue formed after 5h. \u003cstrong\u003eD)\u003c/strong\u003e The blue area near the dialysis membrane also features laminations while in \u003cstrong\u003eE)\u003c/strong\u003e the cyan area from \u003cstrong\u003eC)\u003c/strong\u003eshows a heterogeneous and disorganized texture. \u003cstrong\u003eF)\u003c/strong\u003e AHV analogue formed after 24h. \u003cstrong\u003eG)\u003c/strong\u003e The yellow area from \u003cstrong\u003eF)\u003c/strong\u003efeatures laminations characterized by the regular alternation of two different minerals, as shown in \u003cstrong\u003eH)-J).\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7036221/v1/406e946c00c0b8b97c3113ed.png"},{"id":86029423,"identity":"57ed2842-05d0-49b3-a13d-3c1813d8ddb9","added_by":"auto","created_at":"2025-07-04 13:56:41","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":678763,"visible":true,"origin":"","legend":"\u003cp\u003eSEM observations of the AHV analogues formed after 48h, 72h and 10 days. \u003cstrong\u003eA)\u003c/strong\u003e AHV analogue formed after 48h. The yellow area from \u003cstrong\u003eA)\u003c/strong\u003e in \u003cstrong\u003eB) \u003c/strong\u003epresents laminations formed by the alternation of two minerals, as in the 24h analogue (see figure 3). \u003cstrong\u003eC)\u003c/strong\u003e The pink area from \u003cstrong\u003eA)\u003c/strong\u003e absent from shorter time samples is characterized by a dense, homogeneous texture which induces fractures in the material. \u003cstrong\u003eD)\u003c/strong\u003e Formation of metallic iron nails starting 48h. \u003cstrong\u003eE)\u003c/strong\u003e AHV analogue formed after 72h, showing the same yellow and pink areas as in \u003cstrong\u003eA)\u003c/strong\u003e. Red arrows indicate metallic iron formed in the pink area, as shown at higher magnification in \u003cstrong\u003eF)\u003c/strong\u003e. \u003cstrong\u003eG)\u003c/strong\u003e AHV analogue formed after 10 days, harboring the yellow and pink areas as in \u003cstrong\u003eA)\u003c/strong\u003eand \u003cstrong\u003eE)\u003c/strong\u003e with metallic iron nails indicated by red arrow \u003cstrong\u003eH)\u003c/strong\u003e Magnetite nanoparticles surrounding metallic iron.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7036221/v1/cb88275001a2213389b2aed9.png"},{"id":86028507,"identity":"f6b08902-408a-4f35-843b-95c2d7bb829b","added_by":"auto","created_at":"2025-07-04 13:40:41","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":301505,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eA)\u003c/strong\u003e Schematic representation of the AHV analogue formation in a pH gradient. Refer to the color code on Figure 3 and Figure 4. The blue area made of nanocrystalline magnetite is formed after 1h. The cyan area is formed after 5h, according to magnetite growth. The yellow area formed after 24h consists of alternating layers of green rust and magnetite, resulting in a laminated structure. Finally, the pink area is dominated by amakinite and metallic iron nails, surrounded by magnetite. For each area, values of Eh and pH are estimated based on a representative ternary system retrieved from iron Pourbaix diagrams (Refait et al., 2002; Génin et al., 2006) \u003cstrong\u003eB)\u003c/strong\u003e pH monitoring of the alkaline solution (in orange) and of the acidic iron-rich solution (in blue) during the formation of the AHV analogues for 72h.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7036221/v1/01d1cc7b036b3072576654e6.png"},{"id":86029705,"identity":"1d5cbd05-5deb-409e-a601-408222407abf","added_by":"auto","created_at":"2025-07-04 14:04:42","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":727264,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eA)\u003c/strong\u003e Localization of the FIB ultrathin section performed on a metallic iron nail (72h sample). \u003cstrong\u003eB)\u003c/strong\u003e TEM observation of the FIB section. \u003cstrong\u003eC)\u003c/strong\u003e Mapping of the oxygen percentage conducted on the FIB section. Metallic iron, in blue (\u0026gt; 2% O), is surrounded by iron oxides, in red (\u0026gt; 30% O). \u003cstrong\u003eD)\u003c/strong\u003e Corresponding EDXS spectra of the metallic iron (in blue) and of the iron oxides (in red). \u003cstrong\u003eE)\u003c/strong\u003e TEM observation of the metallic iron (in blue) and iron oxide (in red) interface, as attested by the measured interatomic distance (in red) and the electron diffraction pattern (in blue).\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-7036221/v1/1460d0cfd09f7271f5d5f8aa.png"},{"id":86030623,"identity":"cdf19a00-2e18-4630-9d77-af5d46e6c8e6","added_by":"auto","created_at":"2025-07-04 14:12:48","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3167805,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7036221/v1/012e7d07-4419-4141-bff2-a1f8361b1b1d.pdf"},{"id":86029424,"identity":"84124957-483b-4d54-a0a3-0cc0a83bc616","added_by":"auto","created_at":"2025-07-04 13:56:42","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":4615977,"visible":true,"origin":"","legend":"\u003cp\u003eSupplementary Informations\u003c/p\u003e","description":"","filename":"Supplementary.docx","url":"https://assets-eu.researchsquare.com/files/rs-7036221/v1/db1018fe8aa01eeb924f5122.docx"}],"financialInterests":"The authors declare no competing interests.","formattedTitle":"\u003cp\u003e\u003cstrong\u003eFe\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e2+\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003e disproportionation within iron-rich alkaline vent analogues reveals proto-bioenergetic systems\u003c/strong\u003e\u003c/p\u003e","fulltext":[{"header":"INTRODUCTION","content":"\u003cp\u003eSince their discovery in the early 2000\u0026rsquo;s, hydrothermal vents resulting from serpentinization processes have added new perspectives concerning the question of the transitions from geochemical to biochemical entities (Kelley et al., 2005; Russel, 2003; Russel et al., 2010). These systems arise from the circulation of high temperature/high pressure water through deep ultramafic rocks, yielding fluids enriched in dissolved H\u003csub\u003e2\u003c/sub\u003e and CH\u003csub\u003e4\u003c/sub\u003e (McCollom and Seewald, 2001). This results in highly reducing alkaline (pH 9\u0026ndash;11), moderate temperature (30\u0026ndash;90\u0026deg;C) effluents which, upon contact with seawater, precipitate chimneys with high content in carbonate minerals (Ludwig et al., 2006). In addition to the mentioned reductants H\u003csub\u003e2\u003c/sub\u003e and CH\u003csub\u003e4\u003c/sub\u003e, AHV-effluents contain various organic compounds, derived principally from CO\u003csub\u003e2\u003c/sub\u003e reduction at high temperature and high pressure in the subsurface (Horita and Berndt, 1999; McCollom and Seewald, 2006; Seewald et al., 2006; Proskurowski et al., 2008). Concomitantly, serpentinization processes occurring in the subsurface of the vents are linked to the formation of metal alloys, commonly awaruite (FeNi\u003csub\u003e3\u003c/sub\u003e), and even pure native iron (Fe\u003csup\u003e0\u003c/sup\u003e) (Chamberlain et al., 1965; Frost and Beard, 2007) which, in reaction with aqueous CO\u003csub\u003e2\u003c/sub\u003e, can also induce the formation of formate and acetate (He et al., 2010; Hudson et al., 2020; De Graaf et al., 2023). Such a strong potential for abiotic organic synthesis crystallized hypotheses on the emergence of life in hydrothermal serpentinization systems (Russell et al., 1997; Russell et al., 2006; Camprubi et al., 2017).\u003c/p\u003e \u003cp\u003eIn modern oceans, AHV are predominantly made up of brucite Mg(OH)\u003csub\u003e2\u003c/sub\u003e and calcium carbonates CaCO\u003csub\u003e3\u003c/sub\u003e, mainly aragonite and calcite (Ludwig et al., 2006; Okumura et al., 2016). A type of chimney dominated by brucite (up to 99 wt %) harboring a porous structure (~\u0026thinsp;50%) has been discovered at the Shinkai Seep Field (Okumura et al., 2016). The brucite crystals are oriented by the circulation of the effluent, leading to a stratified structure which results in the formation of micropores and nanopores in the mineral walls (Lee et al., 2024). The micro- and nanoconfined structures of such AHVs provide natural high-surface-area chemical reactors with more efficient reaction rates, selectivity and chemical complexity compared to bulk systems (Mu\u0026ntilde;oz-Santiburcio and Marx, 2017; Lee et al., 2024).\u003c/p\u003e \u003cp\u003eDue to a greater abundance of Mg-Fe rich ultramafic rocks, \u003cem\u003ee.g.\u003c/em\u003e komatiites, in the early Earth than at present, Archean serpentinization processes were likely to produce higher amount of H\u003csub\u003e2\u003c/sub\u003e than currently, although the qualitative composition of the fluid was expected to be similar to nowadays (Leong et al., 2021). Modern and Archaean alkaline chimneys also feature distinct mineralogical assemblages due to main differences in the vent-ocean interface. The primitive ocean was likely more acidic by 0.5 to 3 pH units compared to present one and, most importantly, devoid of dissolved O\u003csub\u003e2\u003c/sub\u003e (\u003cem\u003ee.g.\u003c/em\u003e Halevy et al., 2017). Fe(II) was also present in high concentrations in ancient anoxic oceans, making an abundant presence of iron (oxy-hydr)oxides (together with carbonates and silicates) in early Earth AHVs plausible (Russel et al., 2006; Kotopoulou et al., 2020; Johnson et al., 2024). Such reactive minerals are of prime interest for chemical reactions at the vent-ocean interface. For example, they can mediate the oxidation of methane to methanol and yield the formation of various organic compounds (Barge et al., 2019; Farr et al., 2023). Iron (oxy-hydr)oxides, in particular layered double hydroxides (LDH) such as green rust, are frequently discussed as ingredients in putative proto-bioenergetic systems. Since they tend to deprotonate in response to oxidation reactions while conserving their crystalline structure, LDH are capable of conformational changes during oxidation-reduction and protonation-deprotonation oscillations (G\u0026eacute;nin al., 2006; 2008). This conformational flexibility makes them potential non-enzymatic catalysts for primitive anabolic reactions (Russell, 2018), especially since these minerals share structural similarities with metal-cluster-containing enzymes involved in current bioenergetic processes (Duval et al., 2019).\u003c/p\u003e \u003cp\u003eIn order to better understand the formation of iron-rich early Earth mineral structures at the vent-ocean interface, we produced experimental analogues of such alkaline chimneys in the laboratory, focusing on the precipitation of the reactive minerals iron (oxy-hydr)oxides. The analogues were formed at the interface between an alkaline solution and an acidic solution with high iron concentration (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). These early Earth AHV analogues were allowed to precipitate under strictly anaerobic conditions for different periods of time (5h, 24h, 48h, 72h and 10 days) and were then analyzed combining multi-scale approaches. Mineral phases were identified in bulk using X-ray diffraction (XRD), then analogues with preserved integrity were studied using scanning electron microscopy (SEM) coupled to energy dispersive X-ray spectroscopy (EDXS) to obtain information concerning the structural assembly of minerals. Ultrathin sections were prepared from selected areas for nanoscale analysis using transmission electron microscopy (TEM).\u003c/p\u003e"},{"header":"RESULTS AND DISCUSSION","content":"\u003cp\u003eGrowth of the alkaline vent analogues over time\u003c/p\u003e\u003cp\u003eWithin the first hour, Fe\u003csup\u003e3+\u003c/sup\u003e-rich phases such as magnetite (Fe\u003csup\u003eII\u003c/sup\u003eFe\u003csup\u003eIII\u003c/sup\u003e\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e) are the prime minerals to form (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Fe\u003csup\u003e3+\u003c/sup\u003e precipitation is favored in Fe\u003csup\u003e2+\u003c/sup\u003e/Fe\u003csup\u003e3+\u003c/sup\u003e solution at the direct interface between the alkaline fluid and the acidic iron-rich solution. Hydroxylation of Fe\u003csup\u003e2+\u003c/sup\u003e occurs around pH 7 to 9 while it ranges from pH 1 to 5 for Fe\u003csup\u003e3+\u003c/sup\u003e (Jolivet et al., 2006). As ferric aquo complexes are more acidic than ferrous ones (Jolivet et al., 2004), the formation of Fe\u003csup\u003e3+\u003c/sup\u003e-rich phases suggests that the crystallization front at the direct interface of the acidic and basic solutions is exposed to acidic rather than basic conditions. Ferric complexes occur very rapidly to first form oligomers, then nuclei and eventually particles (Jolivet et al., 2004; 2006; Zhu et al., 2016), likely within the first minutes of the mineral barrier formation or possibly even right after adding the two solutions to the respective sides of the dialysis membrane. Magnetite thus results from the adsorption of Fe\u003csup\u003e2+\u003c/sup\u003e onto an amorphous or poorly crystalline Fe\u003csup\u003e3+\u003c/sup\u003e-rich precursor such as nanoparticulate ferrihydrite or an Fe\u003csup\u003e2+\u003c/sup\u003e/Fe\u003csup\u003e3+\u003c/sup\u003e gel (Sugimoto and Matijevic, 1979; Combes et al., 1988; Jolivet et al., 2006). As observed in SEM, the alternation of nanocrystalline magnetite and a more homogeneous material deposited in laminae also indicates the presence of two distinct mineral phases. Together, these mineral phases deposited directly onto the dialysis membrane form a thin laminar mineral structure about ten micrometers thick (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eA and B). The broad XRD peaks observed at 1h suggest nanocrystalline or poorly crystallized magnetite, evolving toward better ordered or bigger crystals after 5h (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). After 5 hours, the contact area with the dialysis membrane has a similar laminar texture as in the 1h sample (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eD, in blue), suggesting that no transformation of minerals occurs over this time interval. In the parts furthest from the dialysis membrane, \u003cem\u003ei.e.\u003c/em\u003e furthest from the basic solution, other regions feature larger magnetite crystals assembled in a disorderly pattern (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eE, in cyan). Over time, at 5h (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eC), the crystallinity and the abundance of magnetite thus increases, but magnetite remains the only mineral phase detected by XRD (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eStarting at about 24h, the layered double hydroxide (LDH) green rust ([Fe\u003csup\u003eII\u003c/sup\u003e\u003csub\u003e(6\u0026minus;x)\u003c/sub\u003eFe\u003csup\u003eIII\u003c/sup\u003e\u003csub\u003ex\u003c/sub\u003e(OH\u003csub\u003e12\u003c/sub\u003e]\u003csub\u003ex\u003c/sub\u003e + [\u003cem\u003ex/n\u003c/em\u003eA\u003csup\u003en\u0026minus;\u003c/sup\u003e \u0026middot; \u003cem\u003ey\u003c/em\u003eH\u003csub\u003e2\u003c/sub\u003eO] \u003csup\u003ex\u0026minus;\u003c/sup\u003e with A\u0026thinsp;=\u0026thinsp;Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e or CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e) precipitates, containing chloride ions in the interlayers (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Green rust can be formed abiotically by several processes, for example through the partial air oxidation of amakinite Fe(OH)\u003csub\u003e2\u003c/sub\u003e (\u003cem\u003ee.g.\u003c/em\u003e Refait et al., 1999), the partial reduction of ferrihydrite (Hansen, 1989) or by co-precipitation of Fe\u003csup\u003e2+\u003c/sup\u003e and Fe\u003csup\u003e3+\u003c/sup\u003e in solution under anoxic conditions (e.g. Ruby et al., 2006). Here, green rust is found to be formed prior to Fe(OH)\u003csub\u003e2\u003c/sub\u003e, ruling out the Fe(OH)\u003csub\u003e2\u003c/sub\u003e oxidation pathway. The co-precipitation pathway represents the most straightforward rationalization of the observations of the present study. Concomitant with the formation of green rust, we observe the formation of a new laminar region alternating between two mineral phases (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eF and G, in yellow), magnetite with a rounded morphology and green rust platelets (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eH, I and J).\u003c/p\u003e \u003cp\u003eThe iron oxyhydroxide formation is determined by both the pH and the E\u003csub\u003eh\u003c/sub\u003e, as described in the Pourbaix diagram of iron (\u003cem\u003ee.g.\u003c/em\u003e Jolivet et al, 2004; G\u0026eacute;nin et al., 2006). The alternation of magnetite and green rust thus indicates local variation of pH and/or E\u003csub\u003eh\u003c/sub\u003e at the crystallization front. While magnetite and green rust can form in the same pH range, \u003cem\u003ei.e.\u003c/em\u003e 8.5 to 12 (Jolivet et al., 2004), their E\u003csub\u003eh\u003c/sub\u003e range is more constrained: green rust being formed in more reductive conditions, \u003cem\u003ei.e.\u003c/em\u003e at lower redox potential than magnetite, due to a greater amount of Fe\u003csup\u003e2+\u003c/sup\u003e in the crystal lattice (G\u0026eacute;nin et al., 2006; Ruby et al., 2006). Formation of green rust may thus be explained by the local reservoir of iron (enrichment of Fe\u003csup\u003e2+\u003c/sup\u003e initially added in solution) at the crystallization front. We propose that magnetite and possibly ferrihydrite massive precipitation in the first hours of the experiment result in a depletion of Fe\u003csup\u003e3+\u003c/sup\u003e at the solidification front, thus driving an enrichment of Fe\u003csup\u003e2+\u003c/sup\u003e which allows green rust to form preferentially over magnetite. In turn, this preferential formation of green rust leads to a relative depletion of Fe\u003csup\u003e2+\u003c/sup\u003e at the crystallization front, driving an enrichment of Fe\u003csup\u003e3+\u003c/sup\u003e explaining alternances with magnetite.\u003c/p\u003e \u003cp\u003eStarting at about 48h and until 10 days (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), all the assemblages described above (blue, cyan and yellow, see Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e) harbor the same characteristics (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eA, B, E and G, Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, Fig. S2, Fig. S3), corroborating the hypothesis that the mineral phases, in particular the alternating layers of magnetite and green rust, are formed directly at the crystallization front and undergo little or no subsequent transformation. After 48h, hydroxychloride green rust evolves into the more stable hydroxycarbonate green rust, as evidenced by a shift of the (0 0 3) XRD diffraction peak from 7.82 \u0026Aring; to 7.59 \u0026Aring; (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, Table S1). Iron hydroxide amakinite Fe\u003csup\u003eII\u003c/sup\u003e(OH)\u003csub\u003e2\u003c/sub\u003e precipitates (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), corresponding to the formation of a new region with a homogeneous texture marked by fractures (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eA, C, E and G, Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eA). The precipitation of Fe(OH)\u003csub\u003e2\u003c/sub\u003e indicates a strong increase of the pH and a decrease of the E\u003csub\u003eh\u003c/sub\u003e at the crystallization front (G\u0026eacute;nin et al., 2006) (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eA). Concomitantly, the precipitation of Fe(OH)\u003csub\u003e2\u003c/sub\u003e is related to the formation of metallic iron \u0026ldquo;nails\u0026rdquo;, observed from 48h onwards (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eD, F and H). De facto, the pH and redox conditions allowing the formation of Fe(OH)\u003csub\u003e2\u003c/sub\u003e and Fe\u003csup\u003e0\u003c/sup\u003e are very close (Van Genuchten et al., 2018) (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eA). From 48h to 10 days, the system does not significantly evolve, except from further growth of the metallic iron nails and from increasing abundance of magnetite surrounding Fe\u003csup\u003e0\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eD, F and H).\u003c/p\u003e \u003cp\u003eIn the iron-rich acidic solution remains, pH remains at a value of 2 (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eB) whereas the ternary system Fe\u003csup\u003e0\u003c/sup\u003e/Fe(OH)\u003csub\u003e2\u003c/sub\u003e/GR has an alkaline equilibrium pH value around 8 (Refait et al. 2002; G\u0026eacute;nin et al., 2006) (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eA). This discontinuity can also be applied to the Eh values. The Fe\u003csup\u003e2+\u003c/sup\u003e/Fe\u003csup\u003e3+\u003c/sup\u003e solution has a midpoint potential around +\u0026thinsp;0.68 V while the ternary system Fe\u003csup\u003e0\u003c/sup\u003e/Fe(OH)\u003csub\u003e2\u003c/sub\u003e/GR has an equilibrium potential around \u0026minus;\u0026thinsp;0.54 V (Refait et al., 2002; G\u0026eacute;nin et al., 2006) (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eA). Overall, the pH gradient induces the speciation of Fe\u003csup\u003e3+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e through their mobilization by mineral phases, from the most oxidized to the most reduced iron mineral phases. In other words, the initial pH gradient mediates a redox gradient and leads to a strong reductive effect on the (oxy-hydr)oxides mineral formation. The preferential precipitation of Fe(III) (oxy-hydr)oxides due to the pH gradient leads to the formation of Fe(OH)\u003csub\u003e2\u003c/sub\u003e (amakinite or ferroan brucite) at a well-defined stage and therefore at a well-defined spatial location in the AHV analogue. Thus, the formation of the mineral phases is in opposition to the imposed pH and redox constraint, which is likely due to the OH\u003csup\u003e\u0026minus;\u003c/sup\u003e having a diffusion rate higher than the iron complexation. This can be seen through the increase of the pH at the crystallization front, enabling the precipitation of stable mineral phases at very alkaline pH. Otherwise, only ferric iron complexes, stable at lower pH values, would be observed.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFormation of Fe\u003csup\u003e0\u003c/sup\u003e, a thermodynamic conundrum\u003c/p\u003e \u003cp\u003eWhile the alternating sequence of iron oxide/hydroxide minerals from magnetite through green rust and on to amakinite is straightforwardly rationalized by considering local pH-values and precipitation-induced variations of soluble Fe\u003csup\u003e2+\u003c/sup\u003e/Fe\u003csup\u003e3+\u003c/sup\u003e ion concentrations as described above, the formation of Fe\u003csup\u003e0\u003c/sup\u003e is thermodynamically puzzling. Only Fe ions in the 2\u0026thinsp;+\u0026thinsp;and 3\u0026thinsp;+\u0026thinsp;oxidation states were present in the starting solution and both halves of the reaction vessel were hermetically sealed. The bulk ambient redox potential therefore is exclusively determined by the initial Fe\u003csup\u003e2+\u003c/sup\u003e/Fe\u003csup\u003e3+\u003c/sup\u003e ratio, and while local potentials can be lower than that of the bulk when predominantly Fe\u003csup\u003e2+\u003c/sup\u003e is present (together with appropriate pH-values resulting in the formation of amakinite), there is no simple exergonic reaction scheme that would account for the reduction of Fe\u003csup\u003e2+\u003c/sup\u003e (and, even less, Fe\u003csup\u003e3+\u003c/sup\u003e) to metallic iron. Fe\u003csup\u003e0\u003c/sup\u003e is out-of-thermodynamic-equilibrium with the redox potential of the solution and its generation therefore corresponds to an endergonic reaction.\u003c/p\u003e \u003cp\u003eA possible lead to solve this conundrum may be provided by the observation that the regions made up from metallic iron have formed within the (strongly reduced) amakinite-rich precipitation front but are coated by a layer of (less reduced) iron-oxides (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eD, F and H, Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eA, Fig. S4). Ultrathin FIB sections prepared from a metallic iron nail show that these iron oxides are closely associated to the entire surface of the nails (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eB and E, Fig. S4). Estimation of oxygen percentage performed via EDXS confirms that the surrounding minerals are rich in oxygen (\u0026gt;\u0026thinsp;30%) (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eC and D), identified as magnetite given the 4.8 A\u0026deg; interplanar distance of the (1 1 1) lattice plane (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eE and F). Such a pattern is characteristic of an iron disproportionation reaction (O\u0026rsquo;Neill et al., 1993; Frost et al., 2004), that is, the production of compounds part of which is in a more oxidized state and another part in a more reduced one as compared with the starting material (Yakovlev et al., 2009). Here, Fe\u003csup\u003e2+\u003c/sup\u003e would thus be disproportionated into Fe\u003csup\u003e3+\u003c/sup\u003e (more oxidized state) and Fe\u003csup\u003e0\u003c/sup\u003e (more reduced state). Such a type of reaction is reminiscent of the dismutation of w\u0026uuml;stite (FeO) minerals (Eq.\u0026nbsp;\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) (Yakovlev et al., 2009) which is thermodynamically possible.\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:4\\:FeO\\to\\:\\:{Fe}^{0}+\\:{Fe}_{3}{O}_{4}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eThis mechanism has been experimentally observed at high temperatures and high pressures, \u003cem\u003ei.e.\u003c/em\u003e under the conditions of the mantle (Frost et al., 2004), as well as at lower temperatures, yet still up to 300\u0026deg;C (Pouyan et al., 1983). In these cases, disproportionation can be explained by the stability domain of w\u0026uuml;stite, which decreases concomitantly with temperature and pressure, while the stability field of metallic iron and magnetite increases at lower temperature and pressure (Pouyan et al., 1983; Yakovlev et al., 2009). In our case, all three mineral phases involved, namely amakinite Fe(OH)\u003csub\u003e2\u003c/sub\u003e, magnetite Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and metallic iron Fe\u003csup\u003e0\u003c/sup\u003e are stable at low temperatures and in the same range of redox conditions (Moody, 1976; Revie and Uhlig, 2008). Still, Fe\u003csup\u003e2+\u003c/sup\u003e from amakinite has to be oxidized into Fe\u003csup\u003e3+\u003c/sup\u003e, and Fe\u003csup\u003e0\u003c/sup\u003e has to be obtained from Fe\u003csup\u003e2+\u003c/sup\u003e (or Fe\u003csup\u003e3+\u003c/sup\u003e, which is thermodynamically less favorable). Moreover, these two redox reactions likely are causally related, since the formation of metallic iron is seen to be structurally related to the formation of magnetite.\u003c/p\u003e \u003cp\u003eIn the temporal evolution of the analogue described here, increasingly reduced mineral phases bear witness of a progressive and local reduction of the fluid at the crystallization front (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), likely explained by an enrichment of Fe\u003csup\u003e2+\u003c/sup\u003e in the local reservoir of iron. In such acidic environments, free protons are abundant and able to accept electrons from Fe\u003csup\u003e2+\u003c/sup\u003e, thus representing the most likely oxidant for Fe(OH)\u003csub\u003e2\u003c/sub\u003e minerals, deprotonating as a function of the oxidation (Eq.\u0026nbsp;\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) (G\u0026eacute;nin et al., 2006) and forming Fe\u003csup\u003e3+\u003c/sup\u003e and H\u003csub\u003e2\u003c/sub\u003e (Eq.\u0026nbsp;\u003cspan refid=\"Equ3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, Fig. S4). Oxidation and deprotonation of amakinite is better known as the Schikorr reaction, where ferrous hydroxide Fe(OH)\u003csub\u003e2\u003c/sub\u003e is dismutated into magnetite and H\u003csub\u003e2\u003c/sub\u003e, even at low temperature (below 100\u0026deg;C) (Eq.\u0026nbsp;\u003cspan refid=\"Equ4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) (Schikorr, 1929; Neubeck et al., 2014).\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:3\\:{Fe\\left(OH\\right)}_{2}\\:\\rightleftarrows\\:\\:{Fe}_{3}{O}_{4}+2\\:{H}_{2}O\\:+\\:2\\:{H}^{+}\\:+\\:2\\:{e}^{-}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:3\\:{Fe\\left(OH\\right)}_{2}\\:\\rightleftarrows\\:\\:{Fe}_{3}{O}_{4}+\\:{H}_{2}\\:+\\:2\\:{H}_{2}O\\:$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAn endergonic reaction performed by a proto-bioenergetic system\u003c/p\u003e \u003cp\u003eBy thermodynamically modeling the Schikorr reaction with production of H\u003csub\u003e2\u003c/sub\u003e, knowing Eq.\u0026nbsp;\u003cspan refid=\"Equ4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, it appears that this reaction has a ΔG of +\u0026thinsp;10 kJ/mol, resulting in an equilibrium H\u003csub\u003e2\u003c/sub\u003e pressure of 0.13 bar. However, in the experiments of Carlin et al., (2024) at 100\u0026deg;C, the H\u003csub\u003e2\u003c/sub\u003e pressures controlled by kinetics were measured at a pressure of 0.003 bar. By taking this value of 0.003 which is likely an overestimation in our conditions, we obtained a ΔG of -5 kJ/mol, meaning the ΔG of the Schikorr reaction remains largely negative in experiments at ambient temperature. Thus, the Schikorr reaction in our early Earth AHV analogue is a highly exergonic reaction.\u003c/p\u003e \u003cp\u003eOn the other hand, the amakinite dismutation (or Fe\u003csup\u003e2+\u003c/sup\u003e disproportionation) into magnetite and metallic iron described in Eq.\u0026nbsp;\u003cspan refid=\"Equ4\" class=\"InternalRef\"\u003e4\u003c/span\u003e leads to a highly positive ΔG of +\u0026thinsp;35 kJ/mol. Another possibility would be to consider H\u003csub\u003e2\u003c/sub\u003e as a sufficiently powerful reductant to reduce Fe(OH)\u003csub\u003e2\u003c/sub\u003e to Fe\u003csup\u003e0\u003c/sup\u003e, oxidizing the H\u003csub\u003e2\u003c/sub\u003e to H\u003csub\u003e2\u003c/sub\u003eO (Eq.\u0026nbsp;\u003cspan refid=\"Equ5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). However, for the reaction (Eq.\u0026nbsp;\u003cspan refid=\"Equ5\" class=\"InternalRef\"\u003e5\u003c/span\u003e) to occur, the H-H bond dissociation requires the astonishing ΔG of +\u0026thinsp;435 kJ/mol at ambient temperature (Herzberg and Shoosmith, 1959), making the direct amakinite dismutation (Eq.\u0026nbsp;\u003cspan refid=\"Equ4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) more likely. In any case, the observation of metallic iron undoubtedly means that the system of AHV analogues presented here performs an endergonic reaction.\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$$\\:4\\:{Fe\\left(OH\\right)}_{2}\\:\\to\\:\\:{Fe}_{3}{O}_{4}+\\:Fe\\:+\\:4\\:{H}_{2}O\\:$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ5\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ5\" name=\"EquationSource\"\u003e\n$$\\:{Fe\\left(OH\\right)}_{2}\\:+\\:{H}_{2}\\:\\to\\:\\:Fe\\:+\\:2\\:{H}_{2}O$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e5\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWe speculate that during the Schikorr reaction (Eq.\u0026nbsp;\u003cspan refid=\"Equ3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), a portion of the electrons and protons respectively produced by the oxidation of Fe\u003csup\u003e2+\u003c/sup\u003e to Fe\u003csup\u003e3+\u003c/sup\u003e and the associated the dehydroxylation of OH groups associated with Fe\u003csup\u003e2+\u003c/sup\u003e (Eq.\u0026nbsp;\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), are decoupled in a mineral system dominated by the layer double hydroxide amakinite (or ferroan brucite). This allows the migration of proton and hydroxide ions in a pH gradient. As a result, the electrons remaining almost on site in Fe(OH)\u003csub\u003e2\u003c/sub\u003e have a very reducing electrochemical potential and are able to reduce Fe(II) to Fe(0). By allowing some electrons to carry out a positive ΔG reaction (the endergonic reduction of Fe\u003csup\u003e2+\u003c/sup\u003e into Fe\u003csup\u003e0\u003c/sup\u003e) relying on a negative ΔG redox reaction (the exergonic oxidation of Fe\u003csup\u003e2+\u003c/sup\u003e into Fe\u003csup\u003e3+\u003c/sup\u003e coupled to H\u003csub\u003e2\u003c/sub\u003e production), this early Earth AHV analogue therefore generates local ambient potentials that are thermodynamically out-of-equilibrium (within regions containing Fe\u003csup\u003e0\u003c/sup\u003e) while taking the entire system closer to equilibrium, the latter process making this dismutation process thermodynamics-compliant.\u003c/p\u003e \u003cp\u003eIt is precisely this type of process which is the hallmark of all living entities. Life converts environmental free energy into intracellular disequilibria (\u003cem\u003ei.e.\u003c/em\u003e low entropy) which subsequently drive the quasi-totality of all metabolic pathways (Branscomb and Russel, 2013; Branscomb, 2023; Nitschke et al., 2024). The mechanism performing this conversion of free energy in living organisms is called bioenergetics. The two fundamental intracellular disequilibria built up by bioenergetic processes are far-from-equilibrium ratios of phosphates to polyphosphates and reducing potentials exceeding those of the environment (Nitschke, 2022). The latter disequilibrium is achieved by several bioenergetic mechanisms, one of which is called electron bifurcation (Buckel and Thauer, 2018), a process which disproportionates a 2-electron redox compound into a more reducing redox center on one side and a less reducing one on the other side. Fe\u003csup\u003e0\u003c/sup\u003e has been previously mentioned as an indispensable precursor for the electronic bifurcation process (Brabender et al., 2024). Accordingly, the disproportionation mechanism itself of amakinite into magnetite and metallic iron represents a tantalizing analogue in a purely abiotic system of the electron bifurcation reaction, fundamental to bioenergetics and hence to life\u0026rsquo;s thermodynamic prerequisites.\u003c/p\u003e"},{"header":"CONCLUSION","content":"\u003cp\u003eThe formation of iron oxyhydroxides at the interface between an alkaline solution and an acidic solution with high iron concentrations results in a spatially highly structured barrier of iron oxyhydroxide minerals. The initial pH conditions imposed by the two solutions induce a speciation of Fe\u003csup\u003e3+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e mobilized by the mineral phases and lead to a strong reductive effect. The formation of the mineral phases is in complete opposition with the imposed pH and redox constraints: the precipitation of minerals favored in acidic conditions, such as Fe(III) complexes, occurred at the direct interface with the basic solution (directly onto the dialysis membrane), while the precipitation of minerals requiring more basic conditions, such as amakinite, occurred at the direct interface with the acidic solution. We interpret this phenomenon as the result of the OH\u003csup\u003e−\u003c/sup\u003e diffusion rate on iron complexes.\u003c/p\u003e \u003cp\u003eUltimately, this strong pH constraint into AHV analogues leads to the formation of metallic iron Fe\u003csup\u003e0\u003c/sup\u003e due to the disproportionation of Fe\u003csup\u003e2+\u003c/sup\u003e (from amakinite minerals) into Fe\u003csup\u003e3+\u003c/sup\u003e (magnetite) and Fe\u003csup\u003e0\u003c/sup\u003e (metallic iron). The dismutation of amakinite at ambient temperature implies the coupling of the exergonic Schikorr reaction with the endergonic reduction of Fe\u003csup\u003e2+\u003c/sup\u003e into Fe\u003csup\u003e0\u003c/sup\u003e, a process reminiscent of the bioenergetic mechanism of electron bifurcation. The system we describe here thus generates a local decrease in entropy in the form of thermodynamic out-of-equilibrium ambient redox potentials mediated by the disproportionation of Fe\u003csup\u003e2+\u003c/sup\u003e ions. Such local lowering entropy driven by bulk free energy is the thermodynamic prerequisite for allowing the emergence of life in the framework of thermodynamics.\u003c/p\u003e "},{"header":"METHODS","content":"\u003cp\u003ePrecipitation protocol for the iron-rich early Earth alkaline vents analogue\u003c/p\u003e\u003cp\u003eThe experimental setup for the AHV analogue precipitation is based on Barge et al., (2014). The experiments were conducted in strict anoxia under N\u003csub\u003e2\u003c/sub\u003e atmosphere in a Jacomex™ glove box (\u0026lt; 10 ppm O\u003csub\u003e2\u003c/sub\u003e). We used a piece of dialysis membrane (Spectra-Por® Membrane MWCO: 3,500) placed at the interface of two glass vials (DEK Research™) hermetically sealed at the center. Two solutions were prepared: a basic solution (pH 14) composed of 0.27 M NaOH and 0.033 M Na\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e 35mL of degassed water; and an acidic solution (pH 2) composed of 0.06 M FeCl\u003csub\u003e3\u003c/sub\u003e•6H2O and 0.12 M FeCl\u003csub\u003e2\u003c/sub\u003e•4H\u003csub\u003e2\u003c/sub\u003eO into 35mL of degassed water. Both solutions were added simultaneously on either side of the dialysis membrane and let to incubate at room temperature for different durations (1h, 5h, 24h, 48h, 72h, 96h and 10d). The mineral membrane formed on the iron-rich acid solution side, directly onto the dialysis membrane (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), was then dried in the glovebox and prepared for the various analyses.\u003c/p\u003e\u003cp\u003eX-ray diffraction (XRD)\u003c/p\u003e\u003cp\u003eSample preparations and measurements were conducted under N\u003csub\u003e2\u003c/sub\u003e atmosphere in strictly anoxic conditions (\u0026lt; 10 ppm O\u003csub\u003e2\u003c/sub\u003e). AHV analogues were crushed in an agate mortar and resuspended into degassed pure ethanol before being deposited on a zero-background Si wafer. The wafer was inserted in a custom-built anoxic sample chamber equipped with a Kapton\u003csup\u003eR\u003c/sup\u003e window. The sealed chamber was then removed from the glove-box and XRD patterns were collected on a XPert Pro Panalytical™ diffractometer at the IMPMC diffractometry platform. Data were collected using Co Kα radiation in continuous scan mode with an equivalent 0.033° 2θ step counting 1 hour (2 scans of 30 min) per sample over the 5–80° 2θ range. Minerals were identified using XPert HighScore Plus software, PDF4 and COD databases: iron hydroxychloride green rust (PDF4 ID: 00-040-0127), iron hydroxycarbonate green rust (PDF4 ID: 00-052-0163), magnetite (PDF4 ID: 01-076-1849), amakinite (COD ID: 00-900-9104), halite (PDF4 ID: 00-005-0628) and metallic iron α-Fe (PDF4 ID: 01-087-0722).\u003c/p\u003e\u003cp\u003eScanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDXS)\u003c/p\u003e\u003cp\u003eThe whole mineral membranes were inserted into Epoxy Araldite 2020 resin directly in the glovebox to limit oxygen exposure. Once within the resin, the samples were taken out of the glovebox and placed in the oven at 35°C to accelerate the polymerization of the resin. The samples were then cut in half using a wire saw and polished, half on the top and half on the edge, with 6, 3 and 1µm diamond suspensions and an alcohol-based lubricant to limit oxygen exposure during the preparation. The samples were carbon-coated immediately after polishing and investigated using a Scanning Electron Microscope (SEM) coupled with Energy-Dispersive X-ray Spectroscopy (EDXS). SEM-EDXS data were collected at the CINaM microscopy platform, using a JEOL JSM-7900F equipped with a QUANTAX XFlash® Flat QUAD annular four-channel silicon drift (Bruker) for EDXS analysis. Images were collected using an acceleration voltage of 15 kV at a working distance of 11 mm, while EDXS analysis were conducted using a tension of 6 kV.\u003c/p\u003e\u003cp\u003eUltrathin sections by focused ion beam (FIB)\u003c/p\u003e\u003cp\u003eUltrathin FIB sections (20 µm * 5 µm * 100 nm) were extracted at Eurofins Biophy Research from one analogue of alkaline vents formed after 72h, using Ga milling with a SEM-FIB TESCAN CLARA. Two FIB sections were extracted from a selected region at the interface between the metallic iron and surrounding minerals, \u003cem\u003ei.e.\u003c/em\u003e magnetite. A layer of Pt was deposited on the sample surface to protect it from Ga sputtering. Milling was performed gradually at low Ga currents (500 pA, 250 pA and 50 pA) with a final milling step at 20 pA to minimize common artefacts including local Ga implantation, mixing of components, or redeposition of the sputtered material onto the sample surface (Bernard et al., 2009; Schiffbauer and Xiao, 2009).\u003c/p\u003e\u003cp\u003eTransmission electron microscopy (TEM)\u003c/p\u003e\u003cp\u003eNanoscale characterization of the minerals surrounding the metallic iron was performed on FIB sections at the CINaM microscopy platform. TEM data were collected using a JEOL JEM-2010 (LaB6) operating at 200 kV and equipped with a QUANTAX XFlash® Flat QUAD annular four-channel silicon drift SDD detector and a GATAN Ultrascan 1000XP camera. Mineral identification was achieved using selected-area electron diffraction (SAED) and energy-dispersive X-ray spectroscopy (EDXS).\u003c/p\u003e\u003cp\u003eCHESS thermodynamic modeling\u003c/p\u003e\u003cp\u003eThermodynamic modeling was performed using CHESS code 4.0.3. (Van der Lee and Windt, 2002) with Thermodem database (Blanc et al., 2012; Marty et al., 2014). A precipitation medium was modelled at 25°C containing for 1 litre of solution: 0.27 M NaOH, 0.06 M \u003csub\u003e(aq)\u003c/sub\u003eFe\u003csup\u003e3+\u003c/sup\u003e, 0.12 M \u003csub\u003e(aq)\u003c/sub\u003eFe\u003csup\u003e2+\u003c/sup\u003e. The partial pressure of H\u003csub\u003e2\u003c/sub\u003e was fixed at 0.003 bar based on Carlin et al., (2024) experiments.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eACKNOWLEDGEMENTS\u003c/h2\u003e \u003cp\u003eWe warmly thank Isabelle Pinet (BIP) for her administrative support, Damien Chaudansson and Alexandre Alti\u0026eacute; (CINaM) for their valuable help with SEM-EDXS, TEM and samples preparation for the microscopy, Amandine David (Eurofins Scientific) for the production of the FIB sections, Ludovic Delbes (IMPMC) and Vasile Heresanu (CINaM) for their help with XRD. This work was supported by the French Agence Nationale pour la Recherche (grant no. ANR-22-CE30-0035-01).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eBarge, L. M., Doloboff, I. J., Russell, M. J., VanderVelde, D., White, L. M., Stucky, G. D., Baum, M. M., Zeytounian, J., Kidd, R., \u0026amp; Kanik, I. (2013). Pyrophosphate synthesis in iron mineral films and membranes simulating prebiotic submarine hydrothermal precipitates. Geochimica et Cosmochimica Acta, 128, 1‑12. https://doi.org/10.1016/j.gca.2013.12.006\u003c/li\u003e\n\u003cli\u003eBarge, L. M., Flores, E., Baum, M. M., VanderVelde, D. G., \u0026amp; Russell, M. J. (2019). Redox and pH gradients drive amino acid synthesis in iron oxyhydroxide mineral systems. Proceedings Of The National Academy Of Sciences, 116(11), 4828‑4833. https://doi.org/10.1073/pnas.1812098116\u003c/li\u003e\n\u003cli\u003eBernard, S., Benzerara, K., Beyssac, O., Brown, G., Stamm, L. G., \u0026amp; Duringer, P. (2008). Ultrastructural and chemical study of modern and fossil sporoderms by Scanning Transmission X-ray Microscopy (STXM). Review Of Palaeobotany And Palynology, 156(1‑2), 248‑261. https://doi.org/10.1016/j.revpalbo.2008.09.002\u003c/li\u003e\n\u003cli\u003eBlanc, P., Lassin, A., Piantone, P., Azaroual, M., Jacquemet, N., Fabbri, A., \u0026amp; Gaucher, E. (2012). Thermoddem : A geochemical database focused on low temperature water/rock interactions and waste materials. Applied Geochemistry, 27(10), 2107‑2116. https://doi.org/10.1016/j.apgeochem.2012.06.002\u003c/li\u003e\n\u003cli\u003eBrabender, M., Pereira, D. P. H., Mrnjavac, N., Schlikker, M. L., Kimura, Z., Sucharitakul, J., Kleinermanns, K., T\u0026uuml;ys\u0026uuml;z, H., Buckel, W., Preiner, M., \u0026amp; Martin, W. F. (2024). Ferredoxin reduction by hydrogen with iron functions as an evolutionary precursor of flavin-based electron bifurcation. Proceedings Of The National Academy Of Sciences, 121(13). https://doi.org/10.1073/pnas.2318969121\u003c/li\u003e\n\u003cli\u003eBranscomb, E., \u0026amp; Russell, M. J. (2013). Turnstiles and bifurcators: The disequilibrium converting engines that put metabolism on the road. Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1827(2), 62‑78. https://doi.org/10.1016/j.bbabio.2012.10.003\u003c/li\u003e\n\u003cli\u003eBranscomb, E. (2023). Boltzmann\u0026rsquo;s casino and the unbridgeable chasm in emergence of life research. arXiv (Cornell University). https://doi.org/10.48550/arxiv.2312.00932\u003c/li\u003e\n\u003cli\u003eBuckel, W., \u0026amp; Thauer, R. K. (2018). Flavin-Based Electron Bifurcation, A New Mechanism of Biological Energy Coupling. Chemical Reviews, 118(7), 3862‑3886. https://doi.org/10.1021/acs.chemrev.7b00707\u003c/li\u003e\n\u003cli\u003eCamprubi, E., Jordan, S. F., Vasiliadou, R., \u0026amp; Lane, N. (2017c). Iron catalysis at the origin of life. IUBMB Life, 69(6), 373‑381. https://doi.org/10.1002/iub.1632\u003c/li\u003e\n\u003cli\u003eCarlin, W., Malvoisin, B., Brunet, F., Lanson, B., Findling, N., Lanson, M., Fargetton, T., Jeannin, L., \u0026amp; Lhote, O. (2024). Kinetics of low-temperature H2 production in ultramafic rocks by ferroan brucite oxidation. Geochemical Perspectives Letters, 29, 27‑32. https://doi.org/10.7185/geochemlet.2408\u003c/li\u003e\n\u003cli\u003eCombes, J. M., Manceau, A., Calas, G., \u0026amp; Bottero, J. Y. (1989). Formation of ferric oxides from aqueous solutions: A polyhedral approach by X-ray absorption spectroscopy: I. Hydrolysis and formation of ferric gels. Geochimica et Cosmochimica Acta, 53(3), 583-594. https://doi.org/10.1016/0016-7037(89)90001-X\u003c/li\u003e\n\u003cli\u003eDe Graaf, R., De Decker, Y., Sojo, V., \u0026amp; Hudson, R. (2023). Quantifying Catalysis at the Origin of Life**. Chemistry - A European Journal, 29(53). https://doi.org/10.1002/chem.202301447\u003c/li\u003e\n\u003cli\u003eDuval, S., Baymann, F., Schoepp-Cothenet, B., Trolard, F., Bourri\u0026eacute;, G., Grauby, O., Branscomb, E., Russell, M. J., \u0026amp; Nitschke, W. (2019). Fougerite: the not so simple progenitor of the first cells. Interface Focus, 9(6), 20190063. https://doi.org/10.1098/rsfs.2019.0063\u003c/li\u003e\n\u003cli\u003eFarr, O., Gaudu, N., Danger, G., Russell, M. J., Ferry, D., Nitschke, W., \u0026amp; Duval, S. (2023). Methanol on the rocks: green rust transformation promotes the oxidation of methane. Journal of the Royal Society Interface, 20(206), 20230386.\u003c/li\u003e\n\u003cli\u003eFrost, D. J., Liebske, C., Langenhorst, F., McCammon, C. A., Tr\u0026oslash;nnes, R. G., \u0026amp; Rubie, D. C. (2004). Experimental evidence for the existence of iron-rich metal in the Earth\u0026rsquo;s lower mantle. Nature, 428(6981), 409‑412. https://doi.org/10.1038/nature02413\u003c/li\u003e\n\u003cli\u003eFrost, B. R., \u0026amp; Beard, J. S. (2007). On Silica Activity and Serpentinization. Journal Of Petrology, 48(7), 1351‑1368. https://doi.org/10.1093/petrology/egm021\u003c/li\u003e\n\u003cli\u003eG\u0026eacute;nin, J. R., Ruby, C., G\u0026eacute;hin, A., \u0026amp; Refait, P. (2006). Synthesis of green rusts by oxidation of Fe(OH)2, their products of oxidation and reduction of ferric oxyhydroxides ; \u0026ndash;pH Pourbaix diagrams. Comptes Rendus G\u0026eacute;oscience, 338(6‑7), 433‑446. https://doi.org/10.1016/j.crte.2006.04.004\u003c/li\u003e\n\u003cli\u003eG\u0026eacute;nin, J. R., Renard, A., \u0026amp; Ruby, C. (2008). Fougerite FeII \u0026minus; III oxyhydroxycarbonate in environmental chemistry and nitrate reduction. Hyperfine Interactions, 186(1‑3), 31‑37. https://doi.org/10.1007/s10751-008-9837-z\u003c/li\u003e\n\u003cli\u003eHalevy, I., \u0026amp; Bachan, A. (2017). The geologic history of seawater pH. Science, 355(6329), 1069-1071.\u003c/li\u003e\n\u003cli\u003eHansen, H. C. B. (1989). Composition, stabilization, and light absorption of Fe(II)Fe(III) hydroxy-carbonate (\u0026lsquo;green rust\u0026rsquo;). Clay Minerals, 24(4), 663‑669. https://doi.org/10.1180/claymin.1989.024.4.08\u003c/li\u003e\n\u003cli\u003eHe, C., Tian, G., Liu, Z., \u0026amp; Feng, S. (2010). A Mild Hydrothermal Route to Fix Carbon Dioxide to Simple Carboxylic Acids. Organic Letters, 12(4), 649‑651. https://doi.org/10.1021/ol9025414\u003c/li\u003e\n\u003cli\u003eHerzberg, G., \u0026amp; Shoosmith, J. (1959). Spectrum and Structure of the Free Methylene Radical. Nature, 183(4678), 1801‑1802. https://doi.org/10.1038/1831801a0\u003c/li\u003e\n\u003cli\u003eHorita, J., Berndt, M. E. (1999). Abiogenic Methane Formation and Isotopic Fractionation Under Hydrothermal Conditions. Science, 285(5430), 1055‑1057. https://doi.org/10.1126/science.285.5430.1055\u003c/li\u003e\n\u003cli\u003eHudson, R., De Graaf, R., Rodin, M. S., Ohno, A., Lane, N., McGlynn, S. E., Yamada, Y. M. A., Nakamura, R., Barge, L. M., Braun, D., \u0026amp; Sojo, V. (2020c). CO 2 reduction driven by a pH gradient. Proceedings Of The National Academy Of Sciences, 117(37), 22873‑22879. https://doi.org/10.1073/pnas.2002659117\u003c/li\u003e\n\u003cli\u003eJohnson, J. E., Present, T. M., \u0026amp; Valentine, J. S. (2024). Iron : Life\u0026rsquo;s primeval transition metal. Proceedings Of The National Academy Of Sciences, 121(38). https://doi.org/10.1073/pnas.2318692121\u003c/li\u003e\n\u003cli\u003eJolivet, J., Chan\u0026eacute;ac, C., \u0026amp; Tronc, E. (2004). Iron oxide chemistry. From molecular clusters to extended solid networks. Chemical Communications, 5, 477‑483. https://doi.org/10.1039/b304532n\u003c/li\u003e\n\u003cli\u003eJolivet, J., Tronc, E., \u0026amp; Chan\u0026eacute;ac, C. (2006). Iron oxides : From molecular clusters to solid. A nice example of chemical versatility. Comptes Rendus G\u0026eacute;oscience, 338(6‑7), 488‑497. https://doi.org/10.1016/j.crte.2006.04.014\u003c/li\u003e\n\u003cli\u003eKelley, D. S., Karson, J. A., Früh-Green, G. L., Yoerger, D. R., Shank, T. M., Butterfield, D. A., Hayes, J. M., Schrenk, M. O., Olson, E. J., Proskurowski, G., Jakuba, M., Bradley, A., Larson, B., Ludwig, K., Glickson, D., Buckman, K., Bradley, A. S., Brazelton, W. J., Roe, K.,. . . Sylva, S. P. (2005). A Serpentinite-Hosted ecosystem : the Lost City hydrothermal Field. Science, 307(5714), 1428‑1434. https://doi.org/10.1126/science.1102556\u003c/li\u003e\n\u003cli\u003eKotopoulou, E., Lopez‐Haro, M., Gamez, J. J. C., \u0026amp; Garc\u0026iacute;a‐Ruiz, J. M. (2020). Nanoscale Anatomy of Iron‐Silica Self‐Organized Membranes : Implications for Prebiotic Chemistry. Angewandte Chemie International Edition, 60(3), 1396‑1402. https://doi.org/10.1002/anie.202012059\u003c/li\u003e\n\u003cli\u003eLee, H., Okumura, T., Ooka, H., Adachi, K., Hikima, T., Hirata, K., Kawano, Y., Matsuura, H., Yamamoto, M., Yamamoto, M., Yamaguchi, A., Lee, J., Takahashi, H., Nam, K. T., Ohara, Y., Hashizume, D., McGlynn, S. E., \u0026amp; Nakamura, R. (2024). Osmotic energy conversion in serpentinite-hosted deep-sea hydrothermal vents. Nature Communications, 15(1). https://doi.org/10.1038/s41467-024-52332-3\u003c/li\u003e\n\u003cli\u003eLeong, J. A. M., Ely, T., \u0026amp; Shock, E. L. (2021). Decreasing extents of Archean serpentinization contributed to the rise of an oxidized atmosphere. Nature Communications, 12(1). https://doi.org/10.1038/s41467-021-27589-7\u003c/li\u003e\n\u003cli\u003eLi, Y. (2022). Minerals as Prebiotic Catalysts for Chemical Evolution towards the Origin of Life. Dans IntechOpen eBooks. https://doi.org/10.5772/intechopen.102389\u003c/li\u003e\n\u003cli\u003eLudwig, K. A., Kelley, D. S., Butterfield, D. A., Nelson, B. K., \u0026amp; Fr\u0026uuml;h-Green, G. (2006). Formation and evolution of carbonate chimneys at the Lost City Hydrothermal Field. Geochimica et Cosmochimica Acta, 70(14), 3625‑3645. https://doi.org/10.1016/j.gca.2006.04.016\u003c/li\u003e\n\u003cli\u003eMarty, N. C., Claret, F., Lassin, A., Tremosa, J., Blanc, P., Mad\u0026eacute;, B., Giffaut, E., Cochepin, B., \u0026amp; Tournassat, C. (2014). A database of dissolution and precipitation rates for clay-rocks minerals. Applied Geochemistry, 55, 108‑118. https://doi.org/10.1016/j.apgeochem.2014.10.012\u003c/li\u003e\n\u003cli\u003eMcCollom, T. M., Seewald, J. S. (2001). A reassessment of the potential for reduction of dissolved CO 2 to hydrocarbons during serpentinization of olivine. Geochimica et Cosmochimica Acta, 65(21), 3769‑3778. https://doi.org/10.1016/s0016-7037(01)00655-x\u003c/li\u003e\n\u003cli\u003eMcCollom, T. M., Seewald, J. S. (2006). Carbon isotope composition of organic compounds produced by abiotic synthesis under hydrothermal conditions. Earth And Planetary Science Letters, 243(1‑2), 74‑84. https://doi.org/10.1016/j.epsl.2006.01.027\u003c/li\u003e\n\u003cli\u003eMoody, J. B. (1976). Serpentinization : a review. Lithos, 9(2), 125‑138. https://doi.org/10.1016/0024-4937(76)90030-x\u003c/li\u003e\n\u003cli\u003eMu\u0026ntilde;oz-Santiburcio, D., \u0026amp; Marx, D. (2017). Chemistry in nanoconfined water. Chemical Science, 8(5), 3444‑3452. https://doi.org/10.1039/c6sc04989c\u003c/li\u003e\n\u003cli\u003eNeubeck, A., Duc, N. T., Hellevang, H., Oze, C., Bastviken, D., Bacsik, Z., \u0026amp; Holm, N. G. (2014). Olivine alteration and H2 production in carbonate-rich, low temperature aqueous environments. Planetary And Space Science, 96, 51‑61. https://doi.org/10.1016/j.pss.2014.02.014\u003c/li\u003e\n\u003cli\u003eNitschke, W., \u0026amp; Russell, M. J. (2013). Beating the acetyl coenzyme A-pathway to the origin of life. Philosophical Transactions Of The Royal Society B Biological Sciences, 368(1622), 20120258. https://doi.org/10.1098/rstb.2012.0258\u003c/li\u003e\n\u003cli\u003eNitschke, W., Schoepp‐Cothenet, B., Duval, S., Zuchan, K., Farr, O., Baymann, F., Panico, F., Minguzzi, A., Branscomb, E., \u0026amp; Russell, M. J. (2022). Aqueous electrochemistry : The toolbox for life\u0026rsquo;s emergence from redox disequilibria. Electrochemical Science Advances, 3(2). https://doi.org/10.1002/elsa.202100192\u003c/li\u003e\n\u003cli\u003eNitschke, W., Farr, O., Gaudu, N., Truong, C., Guyot, F., Russell, M. J., \u0026amp; Duval, S. (2024). The Winding Road from Origin to Emergence (of Life). Life, 14(5), 607. https://doi.org/10.3390/life14050607\u003c/li\u003e\n\u003cli\u003eOkumura, T., Ohara, Y., Stern, R. J., Yamanaka, T., Onishi, Y., Watanabe, H., Chen, C., Bloomer, S. H., Pujana, I., Sakai, S., Ishii, T., \u0026amp; Takai, K. (2016). Brucite chimney formation and carbonate alteration at the Shinkai Seep Field, a serpentinite‐hosted vent system in the southern Mariana forearc. Geochemistry Geophysics Geosystems, 17(9), 3775‑3796. https://doi.org/10.1002/2016gc006449\u003c/li\u003e\n\u003cli\u003eO\u0026rsquo;Neill, H. S., Rubie, D. C., Canil, D., Geiger, C. A., Ross, C. R., Seifert, F., \u0026amp; Woodland, A. B. (2011). Ferric Iron in the Upper Mantle and In Transition Zone Assemblages : Implications for Relative Oxygen Fugacities in the Mantle. Dans Geophysical monograph (p. 73‑88). https://doi.org/10.1029/gm074p0073\u003c/li\u003e\n\u003cli\u003ePouyan, S., Bassett, W. A., \u0026amp; Lin-Gun, L. (1983). Experimental determination of the effects of pressure and temperature on the stoichiometry and phase relations of w\u0026uuml;stite. Geochimica et Cosmochimica Acta, 47(4), 773‑778. https://doi.org/10.1016/0016-7037(83)90110-2\u003c/li\u003e\n\u003cli\u003eProskurowski, G., Lilley, M. D., Seewald, J. S., Früh-Green, G. L., Olson, E. J., Lupton, J. E., Sylva, S. P., \u0026amp; Kelley, D. S. (2008). Abiogenic Hydrocarbon Production at Lost City Hydrothermal Field. Science, 319(5863), 604‑607. https://doi.org/10.1126/science.1151194\u003c/li\u003e\n\u003cli\u003eRefait, P., Bon, C., Simon, L., Bourri\u0026eacute;, G., Trolard, F., Bessi\u0026egrave;re, J., \u0026amp; G\u0026egrave;nin, J. R. (1999). Chemical composition and Gibbs standard free energy of formation of Fe(II)-Fe(III) hydroxysulphate green rust and Fe(II) hydroxide. Clay Minerals, 34(3), 499‑510. https://doi.org/10.1180/000985599546280\u003c/li\u003e\n\u003cli\u003eRefait, P., G\u0026eacute;hin, A., Abdelmoula, M., \u0026amp; G\u0026eacute;nin, J. (2002). Coprecipitation thermodynamics of iron(II\u0026ndash;III) hydroxysulphate green rust from Fe(II) and Fe(III) salts. Corrosion Science, 45(4), 659‑676. https://doi.org/10.1016/s0010-938x(02)00138-5\u003c/li\u003e\n\u003cli\u003eRevie, R. W., \u0026amp; Uhlig, H. H. (2008). Corrosion and Corrosion Control. https://doi.org/10.1002/9780470277270\u003c/li\u003e\n\u003cli\u003eRuby, C., A\u0026iuml;ssa, R., G\u0026eacute;hin, A., Cortot, J., Abdelmoula, M., \u0026amp; G\u0026eacute;nin, J. (2006). Green rusts synthesis by coprecipitation of FeII\u0026ndash;FeIII ions and mass-balance diagram. Comptes Rendus G\u0026eacute;oscience, 338(6‑7), 420‑432. https://doi.org/10.1016/j.crte.2006.04.008\u003c/li\u003e\n\u003cli\u003eRussell, M. J. (2003). The Importance of Being Alkaline. Science, 302(5645), 580‑581. https://doi.org/10.1126/science.1091765\u003c/li\u003e\n\u003cli\u003eRussell, M. J., \u0026amp; Martin, W. (2004). The rocky roots of the acetyl-CoA pathway. Trends In Biochemical Sciences, 29(7), 358‑363. https://doi.org/10.1016/j.tibs.2004.05.007\u003c/li\u003e\n\u003cli\u003eRussell, M. J. (2006). First Life : Billions of years ago, deep under the ocean, the pores and pockets in minerals that surrounded warm, alkaline springs catalyzed the beginning of life on JSTOR. (s. d.). https://www.jstor.org/stable/27858705\u003c/li\u003e\n\u003cli\u003eRussell, M. J., Hall, A. J., \u0026amp; Martin, W. (2010). Serpentinization as a source of energy at the origin of life. Geobiology, 8(5), 355‑371. https://doi.org/10.1111/j.1472-4669.2010.00249.x\u003c/li\u003e\n\u003cli\u003eRussell, M. J., Barge, L. M., Bhartia, R., Bocanegra, D., Bracher, P. J., Branscomb, E., Kidd, R., McGlynn, S., Meier, D. H., Nitschke, W., Shibuya, T., Vance, S., White, L., \u0026amp; Kanik, I. (2014). The Drive to Life on Wet and Icy Worlds. Astrobiology, 14(4), 308‑343. https://doi.org/10.1089/ast.2013.1110\u003c/li\u003e\n\u003cli\u003eRussell, M. J. (2018). Green Rust : The Simple Organizing \u0026lsquo;Seed\u0026rsquo; of All Life ? Life, 8(3), 35. https://doi.org/10.3390/life8030035\u003c/li\u003e\n\u003cli\u003eSchiffbauer, J. D., \u0026amp; Xiao, S. (2009). Novel application of focused ion beam electron microscopy (FIB-EM) in preparation and analysis of microfossil ultrastructures : A new view of complexity in early Eukaryotic organisms. Palaios, 24(9), 616‑626. https://doi.org/10.2110/palo.2009.p09-003r\u003c/li\u003e\n\u003cli\u003eSchikorr, G. (1929). \u0026Uuml;ber die Reaktionen zwischen Eisen, Seinen Hydroxyden und Wasser. Zeitschrift F\u0026uuml;r Elektrochemie Und Angewandte Physikalische Chemie, 35(2), 65‑70. https://doi.org/10.1002/bbpc.19290350204\u003c/li\u003e\n\u003cli\u003eSeewald, J. S., Zolotov, M. Y., \u0026amp; McCollom, T. (2006). Experimental investigation of single carbon compounds under hydrothermal conditions. Geochimica et Cosmochimica Acta, 70(2), 446‑460. https://doi.org/10.1016/j.gca.2005.09.002\u003c/li\u003e\n\u003cli\u003eSugimoto, T., \u0026amp; Matijević, E. (1980). Formation of uniform spherical magnetite particles by crystallization from ferrous hydroxide gels. Journal Of Colloid And Interface Science, 74(1), 227‑243. https://doi.org/10.1016/0021-9797(80)90187-3\u003c/li\u003e\n\u003cli\u003eVan der Lee J. and Windt L. D. (2002), CHESS Tutorial and Cookbook, Version 3.0. Technical Report 1\u0026plusmn;2. Ecole des Mines de Paris, Fontainebleau, France.\u003c/li\u003e\n\u003cli\u003eVan Genuchten, C., Behrends, T., Kraal, P., Stipp, S., \u0026amp; Dideriksen, K. (2018). Controls on the formation of Fe(II,III) (hydr)oxides by Fe(0) electrolysis. Electrochimica Acta, 286, 324‑338. https://doi.org/10.1016/j.electacta.2018.08.031\u003c/li\u003e\n\u003cli\u003eYakovlev, O. I., Dikov, Y. P., \u0026amp; Gerasimov, M. V. (2009). Effect of the disproportionation reaction of ferrous iron in impact-evaporation processes. Geochemistry International, 47(2), 134‑142. https://doi.org/10.1134/s0016702909020025\u003c/li\u003e\n\u003cli\u003eZhu, M., Frandsen, C., Wallace, A. F., Legg, B., Khalid, S., Zhang, H., M\u0026oslash;rup, S., Banfield, J. F., \u0026amp; Waychunas, G. A. (2015). Precipitation pathways for ferrihydrite formation in acidic solutions. Geochimica et Cosmochimica Acta, 172, 247‑264. https://doi.org/10.1016/j.gca.2015.09.015\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"Agence Nationale de la Recherche","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"bioenergetics, iron, minerals, redox, alkaline vents, early Earth, endergonic, thermodynamic, origin of life. ","lastPublishedDoi":"10.21203/rs.3.rs-7036221/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7036221/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eAlkaline hydrothermal vents are plausible environments for emergence of life on Earth. By means of a simplified analogical reconstruction of the vent-ocean interface of these systems reproducing early Earth conditions, we show that iron (oxy-hydr)oxide minerals may have carried out proto-bioenergetic processes driven by pH and redox gradients. The initial pH gradient precipitates the iron (oxy-hydr)oxide mineral barriers (magnetite, green rust and amakinite) and yields reducing conditions, enabling the production of metallic iron at room temperature via the disproportionation of Fe\u003csup\u003e2+\u003c/sup\u003e to Fe\u003csup\u003e3+\u003c/sup\u003e and Fe\u003csup\u003e0\u003c/sup\u003e. The association of Fe\u003csup\u003e0\u003c/sup\u003e with magnetite suggests the coupling of Fe\u003csup\u003e3+\u003c/sup\u003e / H\u003csub\u003e2\u003c/sub\u003e co-production by amakinite oxidation with the thermodynamically unfavorable reduction of Fe\u003csup\u003e2+\u003c/sup\u003e to Fe\u003csup\u003e0\u003c/sup\u003e. This abiotic disproportionation process coupling exergonic and endergonic reactions resembles a proto-bioenergetic mechanism increasing the non-equilibrium reduction state of the system and offers an interesting analogue of the electronic bifurcation reaction, fundamental to the thermodynamic requirements of life.\u003c/p\u003e","manuscriptTitle":"Fe2+ disproportionation within iron-rich alkaline vent analogues reveals proto-bioenergetic systems","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-04 13:40:37","doi":"10.21203/rs.3.rs-7036221/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"5097ecfa-efec-4f13-bb62-dc9cc2136851","owner":[],"postedDate":"July 4th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":50987590,"name":"Geology"},{"id":50987591,"name":"Thermodynamics and statistical mechanics"},{"id":50987592,"name":"Biophysics"}],"tags":[],"updatedAt":"2025-07-04T13:40:37+00:00","versionOfRecord":[],"versionCreatedAt":"2025-07-04 13:40:37","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7036221","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7036221","identity":"rs-7036221","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}