The potential for abiotic organic synthesis and biosynthesis at seafloor hydrothermal systems

Abstract

Calculations are presented of the extent to which chemical disequilibria are generated when submarine hydrothermal fluids mix with sea water. These calculations involve quantifying the chemical affinity for individual reactions by comparing equilibrium compositions with the compositions of mixtures in which oxidation-reduction reactions are inhibited. The oxidation-reduction reactions that depart from equilibrium in these systems provide energy for chemotrophic microbial metabolism. Methanogenesis is an example of this phenomenon, in which the combination of carbon dioxide, hydrogen and methane induced by fluid mixing is far from equilibrium, which can be approached if more methane is generated. Similarly, the production of other organic compounds is also favorable under the same conditions that permit methanogenesis. Alkanes, alkenes, alcohols, aldehydes, carboxylic acids and amino acids are among the compounds that, if formed, would lower the energetic state of the chemical composition generated in mixed fluids. The resulting positive values of chemical affinity correspond to the thermodynamic drive required for abiotic organic synthesis. It is also possible that energy release accompanies biosynthesis by chemotrophic organisms. In this way, hydrothermal ecosystems differ radically from familiar ecosystems at Earth's surface. If captured, the energy released may be sufficient to drive biosynthesis of carbohydrates, purines, pyrimidines and other compounds which require energy inputs.

Organic, Gas, and Element Geochemistry of Hydrothermal Fluids of the Newly Discovered Extensive Hydrothermal Area in the Wallis and Futuna Region (SW Pacific)

“…Organic compounds in hydrothermal fluids may come from marine dissolved organic matter (DOM) recycling [12,13], subsurface biomass degradation [26], entrainment of organic detritus from local recharge zones, and subsequent degradation, or abiotic formation in the deep subsurface [27][28][29][30]. The latter is supported by many theoretical [31][32][33] and experimental work summarised in two reviews [34,35]. Conversely, some other studies reported the absence of organic compounds in hydrothermal fluids except at the Lost City alkaline vent field which is theoretically more favourable for abiotic synthesis [36].…”

“…The isotopic composition of CH 4 in the Fatu Kapa fluids falls into the abiotic gas category but differs from the typical isotopic signature of CH 4 at Mid-Atlantic Ridge's vent fields. and H 2 [31], nonane [78], or undecane [79]. As a difference the presence of C 16 and C 18 n-FAs in significant amount in the fluids from Fatu Kapa may represent a direct microbial contribution.…”

“…Carbon isotopes of CH 4 and CO 2 suggest that methane underwent oxidation, possibly by bacteria, at both sites and may explain the extremely low concentrations observed (Figure 8 in [73]). Consistently and according to thermodynamic calculations, methanogenesis should be limited under the , , and redox conditions present at the Futuna sites and CH 4 consumption might be prevalent [31].…”

“…Formate and acetate have been given more attention in both laboratory [91][92][93] and field hydrothermal studies [28,29] as these small molecules are likely to prevail according to thermodynamic studies (e.g., [31][32][33]). Where usually formate dominates, acetate was found to be more abundant in fluids from Fatu Kapa.…”

“…Several processes are likely to occur simultaneously and to use several C sources, resulting in a nondiagnostic bulk 13 C signature. Several experimental and theoretical studies indicate that a range of organic compounds including linear alkanes and FAs could form and persist in natural hydrothermal systems (e.g., [31][32][33][34][35]). However, according to the calculated H 2 at and of the study sites, the redox conditions are likely buffered by Hematite-Magnetite (HM) or an even more oxidizing mineral assemblage which appear less favourable for abiotic synthesis than Pyrite-Pyrrhotite-Magnetite, Fayalite-MagnetiteQuartz, or ultramafic rocks assemblages [27,32,33] (Table 4).…”