Generation of long-chain fatty acids by hydrogen-driven bicarbonate reduction in ancient alkaline hydrothermal vents

The study addresses how inorganic geochemical processes could have produced amphiphilic molecules necessary for protocell membranes on the early Earth. The context is alkaline hydrothermal vents where hydrogen-rich, high-pH fluids from serpentinization mix with bicarbonate- and CO2-rich waters, creating redox and pH disequilibria favorable for CO2 fixation. The authors hypothesize that continuous flow conditions and pressurized dissolved H2 reacting with bicarbonate over Fe-bearing minerals (magnetite) at low temperatures (<100 °C) can drive the abiotic synthesis of long-chain fatty acids (≥C8) required for self-assembling vesicles, a key step toward cellular life.

- Modern alkaline hydrothermal vent systems (e.g., Lost City) contain a variety of organic compounds, including C8–C18 carboxylic acids and C9–C14 aliphatic hydrocarbons; some may form abiotically under high H2, CO2 conditions. - Prior laboratory work showed CO2 reduction to C1–C3 organics (including monocarboxylic acids) using Fe/Ni sulfides/oxides/metals under batch or electrochemical settings, typically at ≥100 °C, or room temperature electrochemistry. Continuous-flow and pH-gradient systems produced mainly formate. - Hydrocarbon generation up to C24 was achieved at ~300 °C and 30 MPa with cobalt oxides and bicarbonate, but production of ≥C5 fatty acids under AHV-like conditions below 100 °C had not been demonstrated. - Thus, an experimental gap existed for demonstrating long-chain fatty acid synthesis under mild, geochemically realistic AHV conditions with continuous disequilibrium.

- Reactor design: A pressurized continuous-flow alkaline hydrothermal vent (AHV) simulator mixed an aqueous bicarbonate solution with dissolved H2 and passed it over a magnetite (Fe3O4) or quartz (control) bed at 90 °C under back-pressure, mimicking vent–ocean mixing. - Fluids: 130 mM NaHCO3 (or 13C-NaHCO3) adjusted to pH 7 with HCl, yielding ~110 mM HCO3− and ~20 mM NaCl after CO2 degassing. Solution purged with N2 ≥1 h. During runs, pH rose to 8.5–9.3 within ~3 h. - Flow/pressure/temperature: System brought to 16 bar using 2 mL min−1, then operated at 0.1 mL min−1 (~500 mm h−1 linear velocity). Fluids heated to 90 °C via column heat exchanger. - Hydrogen delivery: H2 supplied at 7 bar through a tube-in-tube gas contactor, giving ~5 mM dissolved H2 (results section cites ~5.5 mM) in the flowing solution. - Substrates: Packed bed of magnetite (test) or quartz (control); additional control without bicarbonate. Reaction duration: 16 h per experiment; triplicate runs. - Analytics: - Gas: Headspace GC/MS for methane monitoring (none above background). - Aqueous: Ion chromatography for C1–C5 organic acids (C2–C4 detected at µM in test effluents). - Surface/bulk organics: Thermal desorption GC/MS (TD-GC/MS, 350 °C single-shot) of freeze-dried mineral to identify volatile/semi-volatile organics; ATR-FTIR for functional groups; XPS (C1s, Fe2p) for surface chemistry and oxidation state; TG-DSC-QMS for evolved gases (m/z 44 CO2, m/z 18 H2O) and decomposition profiles; TOC combustion for bulk carbon added. - Isotopic tracer: Repeat experiment with 13C-NaHCO3 to confirm inorganic carbon incorporation via evolved 13CO2 in TG-DSC-QMS. - Mineral integrity: XRD before/after to confirm magnetite/quartz phases unchanged by reaction. - Controls: Magnetite + H2 without HCO3−, and quartz + H2 with/without HCO3−, processed identically.

- Production of long-chain organics: Under 90 °C, 16 bar, pH 8.5–9.3, with ~5–5.5 mM dissolved H2 and 110 mM HCO3−, magnetite surfaces accumulated a diverse suite of functionalized aliphatic compounds including straight and branched chains C10–C20, with saturated and unsaturated fatty acids up to C18, ketones, alcohols, esters, and methylated species (TD-GC/MS). - Controls: These compounds were absent in controls lacking bicarbonate or with quartz replacing magnetite, indicating formation from H2 + HCO3− on magnetite. - Aqueous products: Low-molecular-weight organic acids (C2–C4) detected at µM in effluents from test runs but not in controls. - Functional group confirmation: ATR-FTIR and XPS corroborated aliphatic, carboxyl, hydroxyl, carbonyl, and aromatic functionalities on reacted magnetite. Aromatics were indicated by ATR-FTIR and XPS but likely under-represented in TD-GC/MS due to 350 °C desorption limit. - Quantification of carbon: TG-DSC-QMS and TOC indicated average organic carbon loading of ~1.6 mg C g−1 (TG-DSC-QMS) and 1.5 mg C g−1 (TOC) on magnetite (n=3). Quartz and magnetite controls showed no fresh carbonaceous material. - Isotopic confirmation: 13C-NaHCO3 experiments produced 13C-evolved CO2 signals consistent with 13C-labeled hydrocarbon combustion, confirming inorganic carbon as the source of synthesized organics. - Iron surface oxidation: High-resolution Fe2p XPS showed an average 8.9% increase in FeOOH:Fe2O3 ratio on magnetite in test runs (not in controls), suggesting net oxidation of the surface associated with HCO3− fixation and indicating the Fe surface acted as an electron donor (reactant) rather than a strict catalyst. - Methane: No methane above background detected in gas traps. - Thermal decomposition profiles: TG-DSC-QMS indicated a range of decomposition temperatures consistent with a distribution of molecular weights, including components consistent with aromatized material.

The experiments demonstrate that continuous-flow mixing of H2-rich alkaline fluids with bicarbonate-rich waters over magnetite at mild hydrothermal conditions can abiotically generate amphiphilic molecules, including long-chain fatty acids (up to C18) capable of forming protocell membranes (≥C8). This directly addresses the question of plausible prebiotic sources of membrane-forming lipids under early Earth AHV conditions. Mechanistically, the sustained disequilibrium and elevated dissolved H2 likely maintained electron flux for CO2/HCO3− reduction, enabling carbon–carbon coupling via sequential C1 additions and/or Fischer–Tropsch-type pathways on Fe surfaces. The observed surface oxidation (increase in FeOOH relative to Fe2O3) implies oxygen transfer from organics to the Fe mineral during synthesis, with magnetite acting as a sacrificial reactant promoting reduction chemistry rather than a purely catalytic surface. The diversity of chain lengths, branching, unsaturation, and functional groups suggests non-linear bond formation influenced by mineral surface topography/defects. Such molecular diversity could yield mixed amphiphile membranes that are more robust, aligning with evidence that mixed amphiphiles enhance protocell stability. If surface-bound amphiphiles are periodically released (e.g., via pH shifts causing electrostatic desorption), local concentrations could exceed 100 mM in the reactor effluent, far above the µM–mM thresholds needed for self-assembly, supporting scenarios for protocell formation at vent–ocean interfaces. The findings also have implications for abiotic fatty acid formation in planetesimals and potential low-temperature amphiphile synthesis in icy moon subsurfaces.

This study provides the first demonstration that long-chain fatty acids and associated amphiphiles can be generated abiotically under mild (<100 °C) alkaline hydrothermal conditions by reacting dissolved H2 and bicarbonate over magnetite in continuous flow. The results support AHV mixing zones as plausible sites for prebiotic membrane assembly and early bioenergetic compartmentalization. The inferred mechanisms include C1 addition and Fischer–Tropsch-type processes on Fe surfaces, with magnetite acting as a reactant. Future work should elucidate detailed reaction pathways, kinetics, and the roles of different Fe/Ni minerals, quantify yields to free solution, test vesicle formation directly from effluents, explore environmental variables (pH, temperature, pressure, flow), and extend to natural vent mineral assemblages and extraterrestrial analogs.

- Analytical constraints: TD-GC/MS desorption at 350 °C likely under-represented less volatile/aromatic compounds; aromatics were inferred mainly from ATR-FTIR/XPS. Surface-sensitive methods capture mineral-bound organics; concentrations in the effluent were not comprehensively quantified beyond low MW acids. - Variability: Replicate experiments produced differing chain length distributions and stereochemistry, likely due to heterogeneous mineral surface properties. - Mechanistic inference: Proposed pathways (C1 addition, FTT-like) are inferred from product distributions and surface chemistry; intermediate species and stepwise mechanisms were not directly resolved. - Model system: Laboratory flow reactor with pure magnetite simplifies complex natural vent systems (mixed minerals, variable flow/pH/temperature, natural inhibitors or promoters). Magnetite appears to act as a reactant (surface oxidation), raising questions about long-term sustainability and replenishment in natural settings. - Gas products: No methane above background was observed; broader volatile product spectra were not extensively reported.