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Diverse microbial communities and numerous energy-yielding activities occur in deeply buried sediments of the eastern Pacific Ocean. Distributions of metabolic activities often deviate from the standard model. Rates of activities, cell concentrations, and populations of cultured bacteria vary consistently from one subseafloor environment to another. Net rates of major activities principally rely on electron acceptors and electron donors from the photosynthetic surface world. At open-ocean sites, nitrate and oxygen are supplied to the deepest sedimentary communities through the underlying basaltic aquifer. In turn, these sedimentary communities may supply dissolved electron donors and nutrients to the underlying crustal biosphere.

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The first building blocks of life could be produced in ultramafic-hosted hydrothermal systems considering the large amounts of hydrogen and methane generated by serpentinisation and Fischer-Tropsch-Type synthesis, respectively, in those systems. The purpose of this study was to detect and characterise organic molecules in hydrothermal fluids from ultramafic-hosted hydrothermal systems in the Mid-Atlantic Ridge (MAR) region. During the EXOMAR cruise 2005, fluids from the Rainbow (36°14′N) and the Lost City (30°N) hydrothermal fields were collected and treated by Stir Bar Sorptive Extraction (SBSE) and Solid Phase Extraction (SPE). The extracts were analysed by Thermal Desorption-Gas Chromatography-Mass Spectrometry (TD-GC-MS) and GC-MS, respectively. Compared to nearby deep seawater, hydrothermal fluids were clearly enriched in organic compounds, with a more diverse spectrum of molecules. We observed a very similar range of organic compounds in fluids from both sites, with a dominance of aliphatic hydrocarbons (C9-C14), aromatic compounds (C6-C16) and carboxylic acids (C8-C18). The occurrence of these compounds is supported by other field studies on serpentinites and sulfide deposits. Literature on thermodynamic data and experimental work has suggested the possible abiogenic origin of hydrocarbons and organic acids. In addition, it has been shown elsewhere that catalytic reactions producing hydrocarbons likely occur at both Lost City and Rainbow hydrothermal fields as suggested by the evolution of δ 13 C with increasing C number for methane, ethane, propane and butane. In order to investigate the origin of the organic molecules in the fluids, compound-specific carbon isotope ratio measurements were performed on n-alkanes and carboxylic acids, for which the δ 13 C values were in the range of − 46 to − 20‰ (vs. V-PDB). These preliminary data did not allow conclusive support or rejection of an abiogenic origin of the compounds. Indeed, predicting δ 13 C signatures in hydrothermal systems is likely to be complicated, due to differences in source δ 13 C signatures (i.e., of the C building blocks), and a variety of, mostly unknown, fractionation steps which may occur along the synthesis pathways. In addition, even though a fraction of the compounds detected in the fluids is likely abiotically produced, a dominance of biogenic sources and/or processes might hide their characteristic signature

The adsorption of organic molecules onto the surfaces of inorganic solids has long been considered a process relevant to the origin of life. We have determined the equilibrium adsorption isotherms for the nucleic acid purine and pyrimidine bases dissolved in water on the surface of crystalline graphite. The markedly different adsorption behavior of the bases describes an elutropic series: guanine > adenine > hypoxanthine > thymine > cytosine > uracil. We propose that such differential properties were relevant to the prebiotic chemistry of the bases and may have influenced the composition of the primordial genetic architecture.T he purine and pyrimidine coding elements of nucleic acids are products of putative prebiotic chemistries that invoke cyanide (1, 2) and have been synthesized in reactions that also yield amino acids (3). The prebiotic availability of these compounds supports the RNA World Hypothesis (4) for the origin of life, which presupposes that the first living system was a polymer(s) of catalytic RNA capable of self-replication that subsequently evolved the ability to encode more versatile peptide catalysts. RNA can act as both information carrier and catalyst (5) and, in the laboratory, can be coerced into different catalytic functions through directed Darwinian evolution (6).Despite these properties, there are severe difficulties with the de novo appearance of RNA, and, even in the most optimistic scenario, information-bearing molecule(s) capable of selfreplication must have first formed fortuitously from an astronomical range of possibilities (7). Although RNA-mediated catalysis and the nonenzymic polymerization of nucleotides (8,9) are well demonstrated, nucleic acid structure incorporates carbohydrate moieties. Formaldehyde, a seemingly ubiquitous compound, is regarded as the most plausible precursor of carbohydrates; however, cyanohydrin (glyconitrile) is the major highly stable product of reactions between formaldehyde and cyanide, withdrawing the latter from being a putative source of bases and amino acids (10). The recovery of nonbiogenic amino acids and bases from extraterrestrial debris (11) suggests the spatial-temporal separation of formaldehyde and cyanide. Life may have been initiated in the absence of carbohydrates, and it has been proposed that modern biology was preceded by a non-nucleic acid informational architecture (12, 13).Aperiodicity is required to convey information (14), and it has been demonstrated that aperiodic structures can self-assemble from aqueous mixtures of purine and pyrimidine bases adsorbed onto the surface of an uncharged inorganic crystalline mineral (15). The spontaneous formation of such structures suggests the existence of an organic, nonpolymeric informational architecture that may have had relevance to the origin of life.The adsorption of organic molecules onto inorganic solids has long been considered a relevant prebiotic process (16). The purine and pyrimidine bases adsorb spontaneously from aqueous media onto inorganic solids and have been observed o...

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Concentrations and isotopic compositions of ethane and propane in cold, deeply buried sediments from the southeastern Pacific are best explained by microbial production of these gases in situ. Reduction of acetate to ethane provides one feasible mechanism. Propane is enriched in 13 C relative to ethane. The amount is consistent with derivation of the third C from inorganic carbon dissolved in sedimentary pore waters. At typical sedimentary conditions, the reactions yield free energy sufficient for growth. Relationships with competing processes are governed mainly by the abundance of H2. Production of C2 and C3 hydrocarbons in this way provides a sink for acetate and hydrogen but upsets the general belief that hydrocarbons larger than methane derive only from thermal degradation of fossil organic material.ethanogenesis ͉ hydrocarbon gases ͉ marine sediments ͉ propanogenesis ͉ stable carbon isotopes L eg 201 of the Ocean Drilling Program was dedicated to the study of microbial life in deeply buried marine sediments (1, 2). Cores were obtained from open-ocean sites in the Equatorial Pacific, where sediments deposited 40 million years ago are underlain by seafloor basalts through which oxygenated seawater is flowing, and from the Peruvian Margin, where drilling penetrated sediments up to 15 million years old (Fig. 1). Temperatures in sediments ranged from 2°C to 25°C. All sites are isolated from reservoirs of fossil hydrocarbons. At both openocean and ocean-margin sites, treatment of sediments with strong base released ethane and propane (Fig. 2). When the treatment was repeated with fresh sediment and isotopically labeled water (␦D ϭ ϩ4000‰), no excess deuterium appeared in the ethane or propane. Therefore, we conclude that the hydrocarbons were strongly sorbed, indigenous constituents of the sediment and did not derive from a chemical reaction between the strong base and an organic substrate.Earlier reports describe sediments offshore Peru (3) and Spitsbergen (4), from which similar mixtures of hydrocarbons could be released by treatment with hot solutions of phosphoric acid. In each case, the carbon-isotopic compositions and abundance ratios (C 1 ͞C 2ϩ ) led to reluctant suggestions that the gases must be of thermogenic origin and thus have migrated into the unconsolidated seafloor sediments: ''the [postulated] migration of C 2ϩ hydrocarbons. . . is somehow related to these fluids [brines that might have flowed from one basin to another]'' (3); and ''. . . elevated seepages [of thermogenic hydrocarbons] occurred irregularly but are not currently active. . . it remains speculative whether the detected hydrocarbon anomalies are related to reservoirs and͞or active source rocks'' (4). No mechanism for sorbing the putatively migrated hydrocarbons more strongly than indigenous microbial products has been offered.Ethane and propane with similar isotopic characteristics and abundance ratios have recently been reported in Cretaceous marine shales in the Western Canadian sedimentary basin (5). Previous work has also pointe...