Supercritical and liquid CO2 (sc-/liq-CO2) emitted from deep-sea hydrothermal vents create a unique dry environment distinct from seawater and hydrothermal fluids, whose physicochemical characteristics could play an important role in the ocean biogeochemical cycles of the present Earth and even in the prebiotic chemical evolution of the early Earth. While previous studies attempted to sample and analyze sc-/liq-CO2 in several hydrothermal fields, the sampling and analysis without seawater contamination have been unsuccessful. In this study, we developed the method and apparatus for sampling and analyzing CO2 in different phases in which in situ Raman measurements can be directly performed and applied them to the CO2 emissions in two hydrothermal vent fields in the Mariana Arc. The in situ Raman spectra taken at both fields indicate that the high purity of CO2 emissions without seawater contamination was successfully sampled and measured. In the North-West Eifuku seamount, the collected hydrothermal fluid was monitored from the seafloor (approximately 1600  m) to the surface. The phase transitions of CO2─hydrate, liquid, and gas─were successfully observed in the Raman spectra. At the Daikoku seamount, the in situ Raman spectra taken at the seafloor (approximately 400 m) identified that the CO2 emission consisted of the gas phase. The in situ Raman measurement also revealed that gas H2S was abundant in the emissions at both the fields. This study demonstrates the ability of the Raman spectroscopic technique to monitor the phase transition of hydrothermal CO2 emissions and the chemical composition in different phases of CO2 in the oceans in real time.

An increasing amount of evidence suggests that early ocean hydrothermal systems were sustained sources of ammonia, an essential nitrogen species for prebiotic synthesis of life’s building blocks. However, it remains a riddle how the abiotically generated ammonia was retained at the vent–ocean interface for the subsequent chemical evolution. Here, we demonstrate that, under simulated geoelectrochemical conditions in early ocean hydrothermal systems (≤−0.6 V versus the standard hydrogen electrode), mackinawite gradually reduces to zero-valent iron (Fe0), generating interlayer Fe0 sites. This reductive conversion leads to an up to 55-fold increase in the solid/liquid partition coefficient for ammonia, enabling over 90% adsorption of 1 mM ammonia in 1 M NaCl at neutral pH. A coordinative binding of ammonia on the interlayer Fe0 sites was computed to be the major mechanism of selective ammonia adsorption. Mackinawite is a ubiquitous sulfide precipitate in submarine hydrothermal systems. Given its reported catalytic function in amination, the extreme accumulation of ammonia on electroreduced mackinawite should have been a crucial initial step for prebiotic nitrogen assimilation, paving the way to the origin of life.

The life’s origin in submarine hydrothermal systems is a long-standing scenario supported by diverse scientific disciplines, while verification of its chemical plausibility remains a major bottleneck. Most scenarios of hydrothermal origin postulate peptides as key players in prebiotic chemistry; nevertheless, it remains unknown how peptides were formed from geochemically available amino acids that must be low in concentration. Here we show that a simple mixing with elemental sulfur (S0), hydrogen sulfide (HS–), and carbon monoxide (CO) enables an effective dimerization of glycine (Gly) at micro to several millimolar concentrations. Incubation of 1 mM Gly with the three inorganic compounds in a warm alkaline solution (pH 9.3 and 35 or 50 °C) led up to 18% conversion of Gly to glycylglycine (GlyGly). Dimerization of 0.01 mM Gly was also discerned in the yield far exceeding the thermodynamic equilibrium (0.4% yield). In this reaction, CO is oxidized to CO2 concomitant with the reduction of S0 to polysulfides, serving as the driving force for the Gly-to-GlyGly conversion. Although the formation of CO2 as a byproduct suppressed the formation of GlyGly, its adverse effect was mitigated by carbonate precipitation upon adding Mg2+ and Ca2+. To our knowledge, 0.01 mM is the lowest concentration of amino acid oligomerized experimentally in the prebiotic context. Given the ubiquity of S0, HS–, and CO in primordial ocean hydrothermal systems, and the abundance of Mg2+ and Ca2+ in seawater, our demonstrated favorable condition for amino acid dimerization is likely to have occurred on the Hadean seafloor. This possibility supports an abiotic appearance of peptides and thereby facilitates the hydrothermal origin of life.

Polysulfide is a candidate for the redox pairs for thermocell devices; however, its aqueous solution has never been investigated. Polysulfides are in complex equilibria in an aqueous system, and the open-circuit potential of the solution is made of their mixed potential; thus, the dominant redox equilibrium determines its thermoelectric property. In this paper, an aqueous polysulfide thermocell was fabricated, and the underlying mechanism of the polysulfide thermocell was investigated by electrochemical studies, UV-vis spectroscopy at various temperatures, and thermodynamic calculations.

A prevailing scenario of the origin of life postulates thioesters as key intermediates in protometabolism, but there is no experimental support for the prebiotic CO2 fixation routes to thioesters. Here we demonstrate that, under a simulated geoelectrochemical condition in primordial ocean hydrothermal systems (–0.6 to –1.0 V versus the standard hydrogen electrode), nickel sulfide (NiS) gradually reduces to Ni0, while accumulating surface-bound carbon monoxide (CO) due to CO2 electroreduction. The resultant partially reduced NiS realizes thioester (S-methyl thioacetate) formation from CO and methanethiol even at room temperature and neutral pH with the yield up to 35% based on CO. This thioester formation is not inhibited, or even improved, by 50:50 coprecipitation of NiS with FeS or CoS (the maximum yields; 27 or 56%, respectively). Such a simple thioester synthesis likely occurred in Hadean deep-sea vent environments, setting a stage for the autotrophic origin of life.

The presence of amino acids in diverse extraterrestrial materials has suggested that amino acids are widespread in our solar system, serving as a common class of components for the chemical evolution of life. However, there are a limited number of parameters available for modeling amino acid polymerization at mineral–water interfaces, although the interfacial conditions inevitably exist on astronomical bodies with surface liquid water. Here, we present a set of extended triple-layer model parameters for aspartate (Asp) and aspartyl-aspartate (AspAsp) adsorptions on two-line ferrihydrite, anatase, and γ-alumina determined based on the experimental adsorption data. By combining the parameters with the reported thermodynamic constants for amino acid polymerization in water, we computationally demonstrate how these minerals impact the AspAsp/Asp equilibrium over a wide range of environmental conditions. It was predicted, for example, that two-line ferrihydrite strongly promotes Asp dimerization, leading to the AspAsp/Asp ratio in the adsorbed state up to 41% even from a low Asp concentration (0.1 mM) at pH 4, which is approximately 5 × 107 times higher than that attainable without mineral (8.5 × 10−6%). Our exemplified approach enables us to screen wide environmental settings for abiotic peptide synthesis from a thermodynamic perspective, thereby narrowing down the geochemical situations to be explored for life’s origin on Earth and Earth-like habitable bodies.

Chemical evolution is an abiotic reaction process in which complex organic molecules arise from a combination of simple inorganic and organic chemical compounds. To assess the possible ongoing chemical evolution in the subsurface ocean of Saturn’s icy satellite Enceladus, we explored the water–rock aqueous reactions and the peptide formation capability under a hydrothermal environment similar to that on Enceladus. It has been suggested that the core of Enceladus has not experienced high temperatures from the time of satellite formation to the present. The major components of the core are assumed to be carbonaceous chondrites; thus, simple organic substances, including amino acids, are likely present in the alkaline seawater of Enceladus. In this study, we conducted a laboratory-based simulation experiment to describe the chemical alteration of six prebiotically abundant amino acids over 147 days under high pressure with thermal cycling (30 to 100 °C) to simulate the water–rock interface of the ocean on Enceladus. As a result, we detected 28 out of 36 possible dipeptide species during the entire reaction period. We propose that peptide-bond formation is coupled to rock surface chemisorption of amino acids under alkaline condition, which was further supported by the elemental analysis showing carbon and nitrogen signature on the rock surface only when amino acids are added. The above result suggests that ongoing chemical evolution on Enceladus is likely producing short abiotic peptides on the porous core surface.