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

The origin of life on Earth is a central question in science. A pivotal step was the formation of cell membranes, which create internal microenvironments crucial for metabolic activity and ion gradients driving energy metabolism. Long-chain fatty acids (≥8 carbons) are prime candidates for these early membranes because they spontaneously self-assemble into vesicles, creating semi-permeable barriers. However, the source of these amphiphilic molecules remains a major unsolved problem. One promising environment for the origin of life is the mixing zone between bicarbonate- and CO2-rich seawater (or potentially freshwater) and upwelling hydrogen-rich fluids from alkaline hydrothermal vents (AHVs). AHVs exist in various settings today (deep ocean floor, shallow seas, freshwater-tidal zones, terrestrial environments), and were likely far more prevalent on the Hadean-Archaean Earth. The H2-rich fluids, produced by serpentinization of ultramafic rocks, create far-from-equilibrium conditions promoting inorganic carbon reduction and the formation of organic compounds, potentially including membrane components. Modern AHV fluids contain various organic compounds, including some possibly generated abiotically. The oxygen-free, CO2-rich atmosphere and abundance of catalytic Fe/Ni minerals on the early Earth could have made organic molecule generation even more efficient in ancient AHVs. Previous experiments have shown CO2 reduction to C1-C3 organic compounds, including some monocarboxylic acids, using various minerals and conditions. However, these experiments often lacked continuous flow, limiting the sustained disequilibrium necessary for efficient CO2 reduction and failing to generate the longer-chain fatty acids (>C5) required for vesicle formation. This study hypothesizes that a continuous-flow, pressurized hydrogen environment would facilitate the synthesis of long-chain fatty acids on iron minerals. Therefore, we designed a pressurized continuous-flow reactor to simulate the mixing of H2-rich AHV fluids with bicarbonate-rich water, utilizing magnetite (a mineral associated with AHVs) as a substrate. The goal was to demonstrate the abiotic generation of long-chain fatty acids (≥C8) under relatively low-temperature (90°C) AHV-like conditions.

The literature extensively discusses the role of membranes in the origin of life, highlighting the importance of amphiphilic molecules like fatty acids in forming protocell membranes. Numerous studies explore potential abiotic synthesis pathways, focusing on environments like alkaline hydrothermal vents. Previous research has demonstrated the formation of shorter-chain organic molecules under various conditions, particularly using electrochemical methods or batch experiments. However, these studies often employed simplified systems or conditions that did not fully represent the complexities of early Earth environments. The generation of long-chain fatty acids under conditions relevant to early Earth hydrothermal systems has remained a significant challenge, despite its importance in understanding the origin of cellular life. This research builds upon existing work, aiming to address the gap in our understanding of how long-chain fatty acids could have been formed abiotically in a geologically plausible scenario.

A pressurized continuous-flow reactor was constructed to mimic key aspects of H2-rich upwelling AHV fluids mixing with HCO3−-rich water. A NaHCO3 (or 13C-labeled NaHCO3) solution was mixed with dissolved H2 at concentrations comparable to modern and ancient AHV, maintained at 90 °C and 16 bar, and passed over a magnetite bed. Control experiments were conducted using quartz instead of magnetite and/or omitting HCO3−. A range of analytical techniques were used to characterize the generated organic molecules and changes in inorganic chemistry. These techniques included: * **X-ray diffraction (XRD):** To confirm the mineral composition before and after the reactions. * **1H nuclear magnetic resonance (1H NMR):** To detect low molecular weight organic acids in the aqueous phase. * **Thermal desorption/volatilization-gas chromatography-mass spectrometry (TD-GC/MS):** To identify and quantify organic molecules generated on the magnetite surface. * **Attenuated Total Reflectance-Fourier Transform Infra-red Spectroscopy (ATR-FTIR):** To confirm functional group assignments made by TD-GC/MS and investigate the surface chemistry of the magnetite. * **X-ray Photoelectron Spectroscopy (XPS):** To analyze the surface chemistry of the magnetite before and after reaction, particularly focusing on carbon bonding environments. * **Thermogravimetric-differential scanning calorimetry coupled with quadrupole mass spectrometry (TG-DSC-QMS):** To analyze the nature and amount of carbonaceous material on the mineral surface after the reaction. * **Ion Chromatography (IC):** To quantify C1-C5 carboxylic acids in the effluent. * **Gas chromatography/mass spectrometry (GC/MS):** To analyze gases collected in the gas trap. * **Total organic carbon (TOC):** To quantify the total organic carbon added to the magnetite.

The experiments successfully generated a range of functionalised long-chain (C10-C20) aliphatic organic compounds on the magnetite surface under the simulated AHV conditions. Importantly, long-chain (up to C18) saturated and unsaturated monocarboxylic acids (fatty acids) were detected. These compounds were not detected in control experiments (without HCO3− or with quartz replacing magnetite), confirming that they were synthesized in situ from H2 and HCO3− in the presence of magnetite. Specific findings include: * The generation of methyl, hydroxyl, ketone, and ester functional groups alongside the fatty acids, suggesting complex chemical reactions. * The detection of both saturated and unsaturated fatty acids. * Variation in chain lengths and stereochemistry between replicate experiments, possibly influenced by magnetite surface topography. * Evidence from ATR-FTIR and XPS supporting the presence of the functional groups identified by TD-GC/MS. * The observation of aromatic functional groups in ATR-FTIR and XPS data, although not detected by TD-GC/MS due to the high desorption temperature. * TG-DSC-QMS analysis indicated an average of 1.6 mg C g−1 generated, supported by TOC analysis (1.5 mg C g−1). * High-resolution Fe2p XPS showed a slight increase (average 8.9%) in the FeOOH:Fe2O3 ratio on the magnetite surface after the reaction, suggesting a surface-mediated reaction where magnetite is involved in the transfer of oxygen rather than acting as a pure catalyst. * An experiment using 13C-labeled NaHCO3 confirmed that the organic molecules were derived from bicarbonate reduction.

The successful generation of long-chain fatty acids under mild (90°C) hydrothermal conditions with geologically realistic reactants (H2 and HCO3−) on a magnetite surface provides compelling evidence for a plausible abiotic pathway for the formation of protocellular membranes. The continuous flow and elevated H2 pressure likely contributed to continuous long-chain molecule synthesis, promoting either sequential carbon-carbon addition or a Fischer-Tropsch type (FTT) reaction. The presence of diverse functional groups suggests non-sequential reactions may have also occurred. The observed variation in chain lengths and stereochemistry suggests that the process may not be highly specific, potentially producing a diverse mix of amphiphilic molecules capable of forming relatively stable vesicles. The oxidation of the magnetite surface implies that the mineral played a significant role in transferring oxygen during the reaction, rather than simply acting as a catalyst. This finding is consistent with previous work suggesting that mineral surfaces can influence reaction pathways. The high concentrations of organic molecules that would have resulted from the rapid release of surface-bound compounds into the effluent (potentially through pH changes) are consistent with the concentrations required for vesicle formation. Such vesicles would have been more similar to the membranes of extant biological cells than previously proposed inorganic membrane alternatives.

This study demonstrates the facile abiotic synthesis of long-chain fatty acids and associated amphiphilic molecules under simulated alkaline hydrothermal vent conditions. The results support the hypothesis that mixing of H2-rich fluids with HCO3−-rich water over magnetite on the early Earth could have provided a significant source of the amphiphilic membranes of the first protocells. The findings suggest that a simple, non-enzymatic process could have generated the building blocks of early life. Future research could focus on investigating the specific reaction mechanisms, the influence of other minerals and environmental factors, and the potential for this process to occur in other environments, such as the subsurface oceans of icy moons.

The reactor setup, while aiming for realism, still represents a simplification of natural AHV systems. The specific mineral composition and surface characteristics of magnetite used may influence the results. While various analytical techniques were employed, it is possible that some minor components or transient intermediates were not fully characterized. The extrapolated concentration of organic molecules in a natural setting remains a subject for further investigation. The study focuses on the generation of fatty acids and other organic molecules; future work is needed to examine the subsequent formation of vesicles and their stability under early Earth conditions.