Geochemical constraints on the origin of organic compounds in hydrothermal systems

It is proposed that abiotic synthesis of organic compounds occurs in metastable states. These states are permitted by kinetic barriers which inhibit the approach to stable equilibrium in the C-H-O-N system. Evidence for metastable equilibrium among organic compounds in sedimentary basins is reviewed, and further evidence is elucidated from hydrous pyrolysis experiments reported in the literature. This analysis shows that at hydrothermal conditions, organic compounds are formed or destroyed primarily through oxidation/reduction reactions, and that the role of temperature is to lower the kinetic barriers to these reactions. These lines of evidence allow the development of a scenario in which abiotic synthesis can occur at hydrothermal conditions through the reduction of CO₂ and N₂. This scenario can be tested quantitatively with distribution of species calculations as functions of temperature, pressure, hydrogen fugacity (fH₂) and initial composition. One example of such a test is given for an early, sudden outgassing of the Earth, in which CO₂, H₂O, and N₂ are transported from the mantle to the atmosphere by hydrothermal solutions. Activities of metastable aqueous organic species which form as a consequence of this process are evaluated at conditions appropriate for seafloor hydrothermal systems, and are found to maximize at about 200 °C and between the oxidation states set by two mineral assemblages common in the oceanic crust.     

Evidence for organic synthesis in high temperature aqueous media — Facts and prognosis

Hydrothermal systems are common along the active tectonic areas of the earth. Potential sites being studied for organic matter alteration and possible organic synthesis are spreading ridges, off-axis systems, back-arc activity, hot spots, volcanism, and subduction. Organic matter alteration, primarily reductive and generally from immature organic detritus, occurs in these high temperature and rapid fluid flow hydrothermal regimes. Hot circulating water (temperature range — warm to >400 °C) is responsible for these molecular alterations, expuslion and migration. Compounds that are obviously synthesized are minor components because they are generally masked by the pyrolysis products formed from contemporary natural organic precursors. Heterocyclic sulfur compounds have been identified in high temperature zones and hydrothermal petroleums of the Guaymas Basin vent systems. They can be interpreted as being synthesized from formaldehyde and sulfur or HS _x ^– in the hydrothermal fluids.Other products from potential synthesis reactions have not yet been found in the natural systems but are expected based on known industrial processes and inferences from experimental simulation data. Various industrial processes have been reviewed and are of relevance to hydrothermal synthesis of organic compounds. The reactivity of organic compounds in hot water (200–350 °C) has been studied in autoclaves, and supercritical water as a medium for chemistry has also been evaluated. This high temperature aqueous organic chemistry and the strong reducing conditions of the natural systems suggest this as an important route to produce organic compounds on the primitive earth. Thus a better understanding of the potential syntheses of organic compounds in hydrothermal systems will require investigations of the chemistry of condensation, autocatalysis, catalysis and hydrolysis reactions in aqueous mineral buffered systems over a range of temperatures from warm to >400 °C.Presented in part at the International Society for the Study of the Origin of Life Meeting, Barcelona, Spain, July 1993.     

The Abiotic Formation of Hydrocarbons from Dissolved CO2 Under Hydrothermal Conditions with Cobalt-Bearing Magnetite

Conversion of CO₂ to organic compounds in hydrothermal systems is important in understanding prebiotic chemical evolution leading to the origin of life. However, organic compounds with carbon number of more than 3 have never been produced from dissolved CO₂ in simulated hydrothermal experiments. In this paper, we report that not only CH₄, C₂H₆ and C₃H₈, but also n-C₄H₁₀ and n-C₅H₁₂ could be produced from dissolved CO₂ and H₂ in the presence of cobalt-bearing magnetite at 300°C and 30 MPa. It is shown that unbranched alkanes in Anderson–Schulz–Flory distribution were the dominant hydrocarbon products produced from dissolved CO₂ catalyzed by cobalt-bearing magnetite under certain hydrothermal conditions. It is proposed that magnetite with other transition metals may act potentially as effective mineral catalysts for abiotic formation of organic compounds from dissolved CO₂ in hydrothermal systems.     

Abiotic Condensation Synthesis of Glyceride Lipids and Wax Esters Under Simulated Hydrothermal Conditions

Precursor compounds for abiotic proto cellular membranes are necessary for the origin of life. Amphipathic compounds such as fatty acids and acyl glycerols are important candidates for micelle/bilayer/vesicle formation. Two sets of experiments were conducted to study dehydration reactions of model lipid precursors in aqueous media to form acyl polyols and wax esters, and to evaluate the stability and reactions of the products at elevated temperatures. In the first set, mixtures of n-nonadecanoic acid and ethylene glycol in water, with and without oxalic acid, were heated at discrete temperatures from 150 ^∘C to 300 ^∘C for 72 h. The products were typically alkyl alkanoates, ethylene glycolyl alkanoates, ethylene glycolyl bis-alkanoates and alkanols. The condensation products had maximum yields between 150 ^∘C and 250 ^∘C, and were detectable and thus stable under hydrothermal conditions to temperatures < 300 ^∘C. In the second set of experiments, mixtures of n-heptanoic acid and glycerol were heated using the same experimental conditions, with and without oxalic acid, between 100 ^∘C and 250 ^∘C. The main condensation products were two isomers each of monoacylglycerols and diacylglycerols at all temperatures, as well as minor amounts of the fatty acid anhydride and methyl ester. The yield of glyceryl monoheptanoates generally increased with increasing temperature and glyceryl diheptanoates decreased noticeably with increasing temperature. The results indicate that condensation reactions and abiotic synthesis of organic lipid compounds under hydrothermal conditions occur easily, provided precursor concentrations are sufficiently high.     

Photochemical synthesis of biomolecules under anoxic conditions

We report the long-wavelength UV anoxic photosynthesis of uracil, various sugars (including deoxyribose and glycoaldehyde), amino acids, and other organic photoproducts. These reactions occur in mixtures of water, calcium carbonate, formaldehyde and hydrazine. Our data demonstrate that under several sets of conditions biomolecules can be formed in variety and abundance from reduced compounds (formaldehyde and hydrazine) derived from anoxic dinitrogen/carbon dioxide environments. The formaldehyde concentrations were varied from 10 mM to 0.005 mM, and the hydrazine concentrations were varied from 1 mM to 0.01 mM. The highest of these reactant concentrations were 500 and 6 times greater than those reported for earlier experiments upon the synthesis of these precursors from CO₂ or N₂, while the lowest of reactant concentrations employed here were 0.5 (formaldehyde) and 0.006 (hydrazine). Product yields were greatest when the hydrazine/formaldehyde ratio was 1, and when the reactant concentrations were low. These data suggest that organic products can be formed in variety from those amounts of formaldehyde and hydrazine precursors which are themselves formed under anoxic UV photochemical conditions. Hence these various reactions would seem to have prebiotic relevance. The UV 254 nm photon flux employed was 100 times higher than unattenuated solar flux. Durations of UV exposure were 24 hrs and 72 hrs. No experiments have been addressed to the possibility of UV flux dependency.     

Organic sulfur compounds resulting from the interaction of iron sulfide,hydrogen sulfide and carbon dioxide in an anaerobic aqueous environment

The reaction of iron sulfide (FeS) with H₂S in water, in presence of CO₂ under anaerobic conditions was found to yield H₂ and a variety of organic sulfur compounds, mainly thiols and small amounts of CS₂ and dimethyldisulfide. The same compounds were produced when H₂S was replaced by HCl, in the H₂S-generating system FeS/HCl/CO₂. The identification of the products was confirmed by GC-MS analyses and the incorporation of H₂ in the organic sulfur compounds was demonstrated by experiments in which all hydrogen compounds were replaced by deuterium compounds. Generation of H₂ and the synthesis of thiols were both dependent upon the relative abundance of FeS and HCl or H₂S, i.e. the FeS/HCl- or FeS/H₂S-proportions. Whether thiols or CS₂ were formed as the main products depended also on the FeS/HCl-ratio: All conditions which create a H₂ deficiency were found to initiate a proportional increase in the amount of CS₂. The quantities of H₂ and thiols generated depended on temperature: the production of H₂ was significantly accelerated from 50°C onward and thiol synthesis above 75°C. The yield of thiols increased with the amount of FeS and HCl (H₂S), given a certain FeS/HCl-ratio and a surplus of CO₂. A deficiency of CO₂ results in lower thiol systhesis. The end product, pyrite (FeS₂), was found to appear as a silvery granular layer floating on the aqueous surface. The identity of the thiols was confirmed by mass spectrometry, and the reduction of CO₂ demonstrated by the determination of deuterium incorporation with DCl and D₂O. The described reactions can principally proceed under the conditions comparable to those obtaining around submarine hydrothermal vents, or the global situation about 4 billion years ago, before the dawn of life, and could replace the need for a reducing atmosphere on the primitive earth.     

The parageneses thermodynamic analysis of chemoautotrophic CO2 fixation archaic cycle components, their stability and self-organization in hydrothermal systems

The parageneses physico-chemical analysis based on a method of thermodynamic potentials has been used to study the system of C-H-O organic compounds, which are, in particular, components of biomimetically built primordial cycles of carbon dioxide chemoautotrophic fixation. Thermodynamic data for aqueous organic compounds allowed one to construct the chemical potential diagrams and establish the areas of thermodynamic stability (facies) of components of CO₂ fixation pathways in hydrothermal systems, in particular, a reductive citric cycle (RCC), 3-hydroxypropionate cycle (3-HPC) and acetyl-CoA pathway. An alternative deep source of carbon (hydrocarbons) proved by the data on endogenous emission of hydrocarbons in hydrothermal fields of oceanic ridges was suggested. The system was determined, which combines hydrocarbons, CO₂ and components of RCC, 3-HPC and acetyl-CoA pathway with characteristic parageneses of methane and ethylene with acetate in two-component CH₄-CO₂ and C₂H₄-O₂ subsystems, respectively. The thermodynamic analysis of a redox mode at various pressures and temperatures allowed one to uniquely determine hydrocarbon-organic system able to independently generate acetate and succinate at oxidation of deep hydrothermal hydrocarbon fluids emerging on sea surface. The limits for thermodynamic stability of CO₂ archaic fixation (CAF) components responsible for generation and self-organization in hydrothermal environment was identified. The tentative integrated system of CAF was developed as a combined acetyl-CoA pathway, 3-HPC and RCC containing a succinate-fumarate core, capable of switching electron flow in forward or reverse direction depending on redox potential of geochemical environment that is governed by the (CH)₂(COOH)₂+H₂(CH₂)₂(COOH)₂ reaction. This core is a “redox switch”, which is sensitive to certain conditions of hydrothermal environment and defines electron flow direction. The redox geochemical mode caused by temperature, pressure, composition of a hydrothermal fluid and a mineralogical setting defines stability of CAF cycle components in paragenesis with hydrocarbons and possibility of cycle self-organization.     

Organic sulfur metabolisms in hydrothermal environments

Sulfur is central to the metabolisms of many organisms that inhabit extreme environments. While biotic and abiotic cycling of organic sulfur compounds has been well documented in low‐temperature anaerobic environments, cycling of organic sulfur in hydrothermal environments has received less attention. Recently published thermodynamic data have been used to estimate aqueous alkyl thiol and sulfide activities in deep‐sea hydrothermal systems. Here we use geochemical mixing models to predict fluid compositions that result from mixing end‐member hydrothermal fluid from the East Pacific Rise with bottom seawater. These fluid compositions are combined with estimates of methanethiol and dimethylsulfide activities to evaluate energy yields for potential organic sulfur‐based metabolisms under hydrothermal conditions. Aerobic respiration has the highest energy yields (over ?240 kJ/mol e^?) at lower temperature; however, oxygen is unlikely to persist at high temperatures, restricting aerobic respiration to mesophilic communities. Nitrite reduction to N₂ has the highest energy yields at higher temperatures (greater than ～40 °C). Nitrate and nitrite reduction to ammonium also yield significant energy (up to ?70 kJ/mol e^?). Much lower, but still feasible energy yields are calculated for sulfate reduction, disproportionation, and reduction with H₂. Organic compound family and the activity of methanethiol and dimethylsulfide were less important than metabolic strategy in determining overall energy yields. All metabolic strategies considered were exergonic within some portion of the mixing regime suggesting that organic sulfur‐based metabolisms may be prevalent within deep‐sea hydrothermal vent microbial communities.     

Biomass fast pyrolysis in a fluidized bed reactor under N2, CO2, CO, CH4 and H2 atmospheres

Biomass fast pyrolysis is one of the most promising technologies for biomass utilization. In order to increase its economic potential, pyrolysis gas is usually recycled to serve as carrier gas. In this study, biomass fast pyrolysis was carried out in a fluidized bed reactor using various main pyrolysis gas components, namely N₂, CO₂, CO, CH₄ and H₂, as carrier gases. The atmosphere effects on product yields and oil fraction compositions were investigated. Results show that CO atmosphere gave the lowest liquid yield (49.6%) compared to highest 58.7% obtained with CH₄. CO and H₂ atmospheres converted more oxygen into CO₂ and H₂O, respectively. GC/MS analysis of the liquid products shows that CO and CO₂ atmospheres produced less methoxy-containing compounds and more monofunctional phenols. The higher heating value of the obtained bio-oil under N₂ atmosphere is only 17.8 MJ/kg, while that under CO and H₂ atmospheres increased to 23.7 and 24.4 MJ/kg, respectively.     

The effect of forty years of continuous corn cropping on soil organic matter characteristics

Changes in humus and some of the major extractable components of soil organic matter were examined, following forty years of conventionally tilled continuous corn cropping compared with those of an adjacent untilled native grassland soil. Results indicate that long-term continuous cropping caused a significant reduction in the content of alkali extractable and water soluble carbon, as well as in the phenolic and chloroform extractable compounds, while no differences in volatile acids and n-hexane extractable substances were found. On the basis of organic C, corn cropping led to a relative enrichement of phenolic compounds, volatile acids and substances extractable by Na₄P₂O₇, but it did not affect the substances extractable by water. Except for the humification ratio (HR), the humification parameters, such as humification degree (HD), the extracted humic and fulvic acids, the C_HA/C_FA ratio and the humification index (HI) revealed a higher degree of humification of the organic matter in native grassland than in the continuous corn cropping system. IR spectra of n-hexane, chloroform, alkaline and water extracts failed to show significant differences between sites under native grassland and sites under continuous corn.     

Influence of manure types and pyrolysis conditions on the oxidation behavior of manure char

Livestock manure can be quickly converted into valuable products (H₂, syn-gas and char) by low temperature gasification. Manure char combustion offers energy for the gasification reactions. In the paper, the influence of manure type and pyrolysis conditions on manure char reactivity is addressed. The results show that the oxidation behaviors of manure char are dependent strongly on manure type and pyrolysis conditions employed. The large difference between the oxidation behaviors of pig and hen manure chars can be attributed to the difference in the organic materials and minerals of the samples. High final temperature, flash pyrolysis and water steam atmosphere used for char preparation promote the resultant char reactivity.     

Prebiotic Synthesis of Methionine and Other Sulfur-Containing Organic Compounds on the Primitive Earth: A Contemporary Reassessment Based on an Unpublished 1958 Stanley Miller Experiment

Original extracts from an unpublished 1958 experiment conducted by the late Stanley L. Miller were recently found and analyzed using modern state-of-the-art analytical methods. The extracts were produced by the action of an electric discharge on a mixture of methane (CH₄), hydrogen sulfide (H₂S), ammonia (NH₃), and carbon dioxide (CO₂). Racemic methionine was formed in significant yields, together with other sulfur-bearing organic compounds. The formation of methionine and other compounds from a model prebiotic atmosphere that contained H₂S suggests that this type of synthesis is robust under reducing conditions, which may have existed either in the global primitive atmosphere or in localized volcanic environments on the early Earth. The presence of a wide array of sulfur-containing organic compounds produced by the decomposition of methionine and cysteine indicates that in addition to abiotic synthetic processes, degradation of organic compounds on the primordial Earth could have been important in diversifying the inventory of molecules of biochemical significance not readily formed from other abiotic reactions, or derived from extraterrestrial delivery.     

An evaluation of pyrolytic techniques with regard to the Apollo 11, 12 and 14 lunar samples analyses

Two different pyrolysis techniques have been used in the analysis of lunar fines. The first technique involved pyrolysis at 700°C under an inert atmosphere in a flowing He system at normal pressure. The products were collected at liquid N₂ temperature and then allowed to pass instantaneously into a combined capillary gas chromatograph-mass spectrometer. The second technique consisted of a vacuum pyrolysis where the sample was first degassed at 150°C and then pyrolyzed at 500°C and 1000°C consecutively. The products were again collected at liquid N₂ temperature and then they were directly introduced to the ion source of the mass spectrometer through a modified gas inlet system.An evaluation of the two techniques based on control experiments has shown that the probability of secondary reactions is greater in the inert atmosphere pyrolysis method. Pyrolysis of benzene in He under atmospheric pressure at 600°C showed the presence of small quantities of biphenyl and trace amounts of naphthalene. Biphenyl pyrolyzed under vacuum at 600, 700, 800 and 900°C by passing through a hot zone containing a quartz wool plug showed the presence of a wide range of synthesis and breakdown products as the temperature increased.These experiments have shown the importance of taking into account the factors that influence pyrolytic degradation and/or the synthesis of products. These can be diffusion effects, involving sample size, sample form, pyrolysis pressure conditions; temperature, catalytic effects from the pyrolysis vessel, contamination, perhaps other factors. Pyrolysis is an effective method of analysis if used under carefully controlled conditions. Pyrolysis of Apollo 14 lunar fines and scrapings from an astronaut's glove gave different products by mass spectrometry and showed different looking flaky materials upon scanning electron microscopy.     

Hydrothermal Reactions of Pyruvic Acid: Synthesis,Selection, and Self-Assembly of Amphiphilic Molecules

Selection and self-assembly of organic compounds in aqueous phases must have been a primary process leading to emergent molecular complexity and ultimately to the origin of life. Facile reactions of pyruvic acid under hydrothermal conditions produce a complex mixture of larger organic molecules, some of which are amphiphiles that readily self-assemble into cell-sized vesicular structures. Chemical characterization of major components of this mixture reveals similarities to the suite of organic compounds present in the Murchison carbonaceous chondrite, some of whose molecules also self-assemble into membranous vesicles. Physical properties of the products are thus relevant to understanding the prebiotic emergence of molecular complexity. These results suggest that a robust family of prebiotic reaction pathways produces similar products over a range of geochemical and astrochemical environments.     

Role of Grafting in Resistance to Water Stress in Tomato Plants: Ammonia Production and Assimilation

In general, drought depresses nutrient uptake by the root and transport to the shoot due to a restricted transpiration rate, which may contribute to growth limitation under water deprivation. Moreover, water stress may also restrict the ability of plants to reduce and assimilate nitrogen through the inhibition of enzymes implicated in nitrogen metabolism. The assimilation of nitrogen has marked effects on plant productivity, biomass, and crop yield, and nitrogen deficiency leads to a decrease in structural components. Plants produce significant quantities of NH₄ ⁺ through the reduction of NO₃ ^? and photorespiration, which must be rapidly assimilated into nontoxic organic nitrogen compounds. The aim of the present work was to determine the response of reciprocal grafts made between one tomato tolerant cultivar (Lycopersicon esculentum), Zarina, and a more sensitive cultivar, Josefina, to nitrogen reduction and ammonium assimilation under water stress conditions. Our results show that when cv. Zarina (tolerant cultivar) was used as rootstock grafted with cv. Josefina (ZarxJos), these plants showed an improved N uptake and NO₃ ^? assimilation, triggering a favorable physiological and growth response to water stress. On the other hand, when Zarina was used as the scion (JosxZar), these grafted plants showed an increase in the photorespiration cycle, which may generate amino acids and proteins and could explain their better growth under stress conditions. In conclusion, grafting improves N uptake or photorespiration, and increases leaf NO₃ ^? photoassimilation in water stress experiments in tomato plants.     

Iron: The Forgotten Driver of Nitrous Oxide Production in Agricultural Soil

In response to rising interest over the years, many experiments and several models have been devised to understand emission of nitrous oxide (N₂O) from agricultural soils. Notably absent from almost all of this discussion is iron, even though its role in both chemical and biochemical reactions that generate N₂O was recognized well before research on N₂O emission began to accelerate. We revisited iron by exploring its importance alongside other soil properties commonly believed to control N₂O production in agricultural systems. A set of soils from California''s main agricultural regions was used to observe N₂O emission under conditions representative of typical field scenarios. Results of multivariate analysis showed that in five of the twelve different conditions studied, iron ranked higher than any other intrinsic soil property in explaining observed emissions across soils. Upcoming studies stand to gain valuable information by considering iron among the drivers of N₂O emission, expanding the current framework to include coupling between biotic and abiotic reactions.     

The Use of Ascorbate as an Oxidation Inhibitor in Prebiotic Amino Acid Synthesis: A Cautionary Note

It is generally thought that the terrestrial atmosphere at the time of the origin of life was CO₂-rich and that organic compounds such as amino acids would not have been efficiently formed abiotically under such conditions. It has been pointed out, however, that the previously reported low yields of amino acids may have been partially due to oxidation by nitrite/nitrate during acid hydrolysis. Specifically, the yield of amino acids was found to have increased significantly (by a factor of several hundred) after acid hydrolysis with ascorbic acid as an oxidation inhibitor. However, it has not been shown that CO₂ was the carbon source for the formation of the amino acids detected after acid hydrolysis with ascorbic acid. We therefore reinvestigated the prebiotic synthesis of amino acids in a CO₂-rich atmosphere using an isotope labeling experiment. Herein, we report that ascorbic acid does not behave as an appropriate oxidation inhibitor, because it contributes amino acid contaminants as a consequence of its reactions with the nitrogen containing species and formic acid produced during the spark discharge experiment. Thus, amino acids are not efficiently formed from a CO₂-rich atmosphere under the conditions studied.     

Biodegradation of endocrine-disrupting phenolic compounds using laccase followed by activated sludge treatment

Endocrine-disrupting phenolic compounds in the water were degraded by laccase fromTrametes sp. followed by activated sludge treatment. The effect of temperature on the degradation of phenolic compounds and the production of organic compounds were investigated using endocrine-disrupting chemicals such as bisphenol A, 2,4-dichlorophenol, and diethyl phthalate. Bisphenol A and 2,4-dichlorophenol disappeared completely after the laccase treatment, but no disappearance of diethyl phthalate was observed. The Michaelis-Menten type equation was proposed to represent the degradation rate of bisphenol A by the lacasse under various temperatures. After the laccase treatment of endocrine-disrupting chemicals, the activated sludge treatment was attempted and it could convert about 85 and 75% of organic compounds produced from bisphenol A and 2,4-dichlorophenol into H₂O and CO₂, respectively.     

Comments on the Role of H2S in the Chemistry of Earth's Early Atmosphere and in Prebiotic Synthesis

Free energy calculations and experimental measurements have been used to show that H₂S/CO₂ mixtures outgassing from a prebiotic Earth's crust would have produced a reducing gas mixture containing CO, H₂, H₂O, and S _x as principal components. Due to rapid recombination of H₂, CO, and S _x to H₂S and CO₂ on cooling from a high temperature to ambient conditions, reducing components would have been retained only if efficient quenching of the reduced gas mixture had been possible. Consequently, subsea vents or vents with efficient infusion of water would have been ideal sites for retention of reduced species and for prebiotic organic synthesis. It is suggested that C/H/O/S ratios are important factors in controlling the degree of prebiotic organic synthesis and, hence, the emergence of life, since if oxygen is abundant, CO₂ and SO₂ would have been dominant species. Received: 5 March 1997 / Accepted: 15 December 1997     

Nanoheterogeneous catalytic hydrogenation of N-, O- or S-heteroaromatic compounds by re-usable aqueous colloidal suspensions of rhodium(0)

The hydrogenation of various nitrogen-, oxygen- or sulfur-heterocyclic aromatic compounds by various surfactant-stabilized aqueous rhodium(0) colloidal suspensions was investigated. The nanocatalysts in the size range of 2.1-2.4 nm have been synthesized by reducing RhCl₃ · 3H₂O with sodium borohydride and were stabilized by highly water soluble N,N-dimethyl-N-cetyl-N-(2-hydroxyethyl)ammonium bromide or chloride salts. The catalytic reactions were performed under mild reaction conditions, namely room temperature and under atmospheric hydrogen pressure. The influence of the bromide or chloride nature of the surfactant counter-ion on the recycling of the aqueous phase containing the Rh(0) particles was studied.