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Integrating microbial metabolism into geochemical modeling allows assessments of energy and mass transfer between the geosphere and the microbial biosphere. Energy and power supplies and demands can be assessed from analytical geochemical data given thermodynamic data for compounds involved in catabolism and anabolism. Results are reported here from a critique of the available standard state thermodynamic data for organic acids and acid anions involved in the citric acid cycle (also known as the tricarboxylic acid cycle or the Krebs cycle). The development of methods for estimating standard state data unavailable from experiments is described, together with methods to predict corresponding values at elevated temperatures and pressures using the revised Helgeson–Kirkham–Flowers (HKF) equation of state for aqueous species. Internal consistency is maintained with standard state thermodynamic data for organic and inorganic aqueous species commonly used in geochemical modeling efforts. Standard state data and revised-HKF parameters are used to predict equilibrium dissociation constants for the organic acids in the citric acid cycle, and to assess standard Gibbs energies of reactions for each step in the cycle at elevated temperatures and pressures. The results presented here can be used with analytical data from natural and experimental systems to assess the energy and power demands of microorganisms throughout the habitable ranges of pressure and temperature, and to assess the consequences of abiotic organic compound alteration processes at conditions of subsurface aquifers, sedimentary basins, hydrothermal systems, meteorite parent bodies, and ocean worlds throughout the solar system.

A culture-independent community census was combined with chemical and thermodynamic analyses of three springs located within the Long Valley Caldera, Little Hot Creek (LHC) 1, 3, and 4. All three springs were approximately 80 degrees C, circumneutral, apparently anaerobic and had similar water chemistries. 16S rRNA gene libraries constructed from DNA isolated from spring sediment revealed moderately diverse but highly novel microbial communities. Over half of the phylotypes could not be grouped into known taxonomic classes. Bacterial libraries from LHC1 and LHC3 were predominantly species within the phyla Aquificae and Thermodesulfobacteria, while those from LHC4 were dominated by candidate phyla, including OP1 and OP9. Archaeal libraries from LHC3 contained large numbers of Archaeoglobales and Desulfurococcales, while LHC1 and LHC4 were dominated by Crenarchaeota unaffiliated with known orders. The heterogeneity in microbial populations could not easily be attributed to measurable differences in water chemistry, but may be determined by availability of trace amounts of oxygen to the spring sediments. Thermodynamic modeling predicted the most favorable reactions to be sulfur and nitrate respirations, yielding 40-70 kJ mol(-1) e(-) transferred; however, levels of oxygen at or below our detection limit could result in aerobic respirations yielding up to 100 kJ mol(-1) e(-) transferred. Important electron donors are predicted to be H(2), H(2)S, S(0), Fe(2+) and CH(4), all of which yield similar energies when coupled to a given electron acceptor. The results indicate that springs associated with the Long Valley Caldera contain microbial populations that show some similarities both to springs in Yellowstone and springs in the Great Basin.

The discovery of CO 2 , CH 4 , and N 2 in a plume at Enceladus provides useful clues about the chemistry and evolution of this moon of Saturn. Here, we use chemical equilibrium and kinetic calculations to estimate the oxidation state of hydrothermal systems on early Enceladus, with the assumption that the plume's composition was inherited from early hydrothermal fluids. Chemical equilibrium calculations are performed using the CO 2 /CH 4 ratio in the plume, and kinetic calculations are conducted using equations from fluid dynamics and chemical kinetics. Our results suggest that chemical equilibrium between CO 2 and CH 4 would have been reachable at temperatures above ∼200 °C in hydrothermal systems. The oxidation state of the hydrothermal systems would have been close to the pyrrhotite–pyrite–magnetite (PPM) or fayalite–magnetite–quartz (FMQ) redox buffer (i.e., terrestrial-like) if the plume's CO 2 and CH 4 equilibrated in hydrothermal systems long ago. As for minerals, we suggest that iron metal would have been oxidized to magnetite by the escape of H 2 from the early satellite. Our calculations also indicate that, assuming CO 2 and CH 4 reached chemical equilibrium, magnetite would not have been oxidized to hematite in hydrothermal systems, perhaps due to insufficient H 2 escape. It is shown that, if Enceladus accreted as much NH 3 as comets contain, the presence of N 2 and deficiency of NH 3 in the plume can be understood in the context of chemical equilibrium in the C–N–O–H system. We conclude by proposing an evolutionary hypothesis in which the fairly oxidized nature of the plume can be explained by a brief episode of oxidation caused by short-lived radioactivity. These suggestions can be rigorously tested by acquiring gravity and isotopic data in the future.

Analysis has been made of a large volume of data to obtain the partial molar volumes of aqueous solutes at infinite dilution for temperatures up to 700 K and pressures up to 50 MPa. Values of V 2 ∞ are expressed in terms of the infinite dilution water–solute direct correlation function integral (DCFI) at the conditions of interest reduced by the solute's DCFI at 298.15 K, 0.1 MPa. Empirical equations have been found for the temperature-density dependence of these reduced DCFIs for solute classes of: simple fluids, nonpolar compounds, aromatic compounds, monohydric alcohols, monocarboxylic acids, nitriles, amides, and other organic solutes. A reliable, but less accurate generalization, based on pure water DCFI, has also been developed for the reduced DCFI for solute classes of: simple fluids, nonpolar compounds, polar substances, and compounds containing chemically bonded water. At high densities, the reduced DCFI for all solutes is the same as that for water, which is consistent with the behavior of hard-sphere mixtures.

A compilation of experimental values of the infinite dilution partial molar Gibbs energy, enthalpy, and heat capacity of hydration together with partial molar volumes in water at 298.15 K and 0.1 MPa is presented for aliphatic nitriles and dinitriles. These data are treated in the framework of the first- and second-order group additivity methods. Thermodynamic properties are determined for the first-order CN group and for the following second-order groups: C-(CN)(C)(H)2, C-(CN)(C)2(H), and CN-(C). The relatively long-range dipole−dipole interactions of two terminal nitrile groups require additional corrections, {CN-(CH2)2-CN}corr and {CN-(CH2)3-CN}corr, for lower dinitriles. New experimental studies of aqueous branched mononitriles and lower dinitriles (particularly propanedinitrile) are required to expand the usefulness and accuracy of group contribution models for aqueous nitriles.

A compilation of experimental values of the infinite dilution partial molar Gibbs energy, enthalpy, and heat capacity of hydration, together with partial molar volumes in water at 298.15 K and 0.1 MPa is presented for aliphatic monoethers, diethers, and polyethers. These data are treated in the framework of the first- and second-order group additivity methods. However, third- and higher-order effects (i.e., interactions expressed beyond the nearest neighbors) are clearly present in aqueous ethers. The effects can be accounted for by the introduction of a number of corrections. For the second-order group contribution method, numerical values are determined for the following groups: C−(C)2(H)(O)ether, C−(C)3(O)ether, O−(C)2, C−(H)2(O)2, C−(C)(O)2(H), and corrections: a “ethoxyalkane” correction, {CH3−CH2−O−CH2}, and a “diether” correction, {O−(CH2)2−O}. For the first-order group contribution method, in addition to the “ether” O group, a large number of corrections appears to be necessary for accurate repr...

The Krichevskii parameter, AKr, for aqueous solutes can be determined from variations of the vapor–liquid distribution constants for a solute at temperatures that are 100–150 K below the critical temperature of water. Pragmatically this means that if one can reliably evaluate the vapor–liquid distribution constant for a solute in water at temperatures up to 500–550 K, then the value of AKr for this solute can be calculated. This is exactly the approach taken in this study. We use a virial equation of state truncated at the second virial coefficient to evaluate the fugacity coefficients of the solute in the vapor phase and a simple empirical method of thermodynamic integration to calculate Henry’s constant of the solute in water up to 550 K. These results allow prediction of the vapor–liquid distribution constants and the Krichevskii parameter for many volatile nonelectrolytes in water. It appears that the accuracy of predictions is limited mainly by the accuracy of the values of the thermodynamic functions of hydration of solutes at 298.15 K. The values of AKr for aqueous organic solutes follow group additivity systematics, and we derive a set of corresponding group contribution values for a large number of functional groups. © 2004 Elsevier B.V. All rights reserved.

The distribution of solutes between coexisting liquid and vapor phases of water can be expressed by the distribution constant, KD, defined as K D = lim x→0 y x , where y and x stand for the mole fraction concentrations of a solute in vapor and liquid phases, respectively. Research reported here is concerned with the prediction of this property, KD, for volatile nonelectrolytes, over the whole temperature range of existence of the vapor–liquid equilibrium for water, i.e. from 273 K to the critical temperature at 647.1 K. A simple empirical method is proposed to extrapolate the values of KD from 298 K to 500–550 K. Calculations at higher temperatures are based on the theoretical relation that establishes the proportionality between RTlnKD and the Krichevskii parameter, AKr, which is the single most important property of a solute at near-critical conditions, and can be evaluated using the method proposed here. The comparison of predicted and experimental values of KD and AKr for a few well-studied solutes reveals the satisfactory performance of the proposed method. It appears that the accuracy of predictions in the framework of this method is limited mainly by the accuracy of the values of the thermodynamic functions of hydration of solutes at 298 K, and that the best way to improve the quality of predictions of KD and AKr is to increase the inventory of accurate calorimetric enthalpy and heat capacity data for aqueous solutes at 298 K. We stress that the values of the Krichevskii parameter, such as those generated in this study, are of crucial importance for reliable predictions of the chemical potential and its derivatives (V2o, Cp2o) for aqueous solutes at near-critical and supercritical conditions. Values of KD and AKr are predicted for many inorganic volatile nonelectrolytes and some halogenated derivatives of methane and ethene. We show that both ln KD and AKr for aqueous organic solutes follow group additivity systematics, and we derive a set of corresponding group contribution values for several functional groups (material point, CH3, CH2, CH, C, C = C, HC = CH, C≡C, HCar, Car, Cfus, OH, O, S, SH, CO, COO, COH, COOH, CN, F, Cl, Br, NH2, NH, N, etc.).

A simple method is presented for estimating the second cross virial coefficients for interactions involving water. It is based on the Tsonopoulos corresponding-states correlation and consists of a semiempirical equation for evaluating the mixture-specific parameter k12 of this correlation. The general structure of this equation is guided by a theory-based relation, which is valid for interactions between spherical nonpolar molecules. An empirical extension of this relation to interactions involving water was done by replacing the “energy” term with a function of the Gibbs energy of hydration of a compound at 298 K and 0.1 MPa. This correlation is supported by experimental B12 values for interactions between water and numerous compounds that differ greatly in size and strength of water−solute interactions. Limitations of this empirical correlation are briefly discussed. In addition, available B12 data for interactions between water and normal alkanes, from ethane to octane, strongly suggest the applicabili...

Values of the second cross virial coefficients, B12, and isothermal Joule−Thomson coefficients, φ12, for interactions involving water are compiled and, as necessary, evaluated from existing experimental data including PVT properties of gas mixtures, measurements of the pressure dependence of the molar excess enthalpies of a gas mixture, and measurements of the solubility of the condensed phases of H2O (liquid water or ice) in compressed gases. In all, there are data for interactions between water and 32 other compounds, and among these, nine interactions involve compounds with appreciable dipole moments. These B12 values, typically available over a limited temperature range, are extrapolated to high temperatures by means of the Tsonopoulos corresponding-states correlation. Finally, the temperature dependence of the second cross virial coefficients between water and solutes at temperatures up to 1200 K is approximated by an empirical polynomial equation, the coefficients of which are tabulated for each int...

The main goal of this study is to revise the correlation algorithm for estimating parameters of the revised Helgeson-Kirkham-Flowers (HKF) model for aqueous nonelectrolyte solutes. The basis for the revision is the analysis of a large body of experimental values of the standard partial molar heat capacities and volumes of aqueous nonelectrolytes published mainly during the last decade. Unlike earlier estimation methods, we show that one of the most useful properties for developing correlations for uncharged species is the standard Gibbs energy of hydration of a solute at 298.15 K and 0.1 MPa, ΔhGo, which reflects the strength of water-solute interactions. Explicit correlations with ΔhGo seem to provide reliable means to estimate the “solvation” parameter ω and the “caloric” parameter c2 in the revised HKF model. “Volumetric” parameters a1 to a4 depend on both the size of a solute and its standard Gibbs energy of hydration. It is expected that the revised estimation strategy will improve the reliability of predictions of thermodynamic properties of aqueous nonelectrolytes in the framework of the revised HKF model. A related problem is the temperature and density ranges of the applicability of the model for uncharged species. A comparison of experimental properties and those fitted in the framework of the revised HKF model made in this study or in articles published elsewhere shows that the model can be used along the saturation vapor-liquid curve of pure water in the density region sufficiently remote from the critical point of water, say up to 630 K. At densities above 500 to 600 kg · m−3 the range of applicability of the revised HKF model for uncharged species may extend up to higher temperatures. At temperatures up to 500 K at pressures up to 50 MPa, the revised HKF model is capable of excellent description of the standard thermodynamic properties of aqueous nonelectrolytes, except in the narrow temperature range below 280–290 K.

Recent experimental studies of the standard-state heat capacities of aqueous nonelectrolytes at high temperatures make it possible to test predictions made with theoretical equations of state and correlation algorithms. Existing predictive methods are qualitatively correct and produce quantitative results that are in reasonable agreement with the new experimental data at temperature ≤ 350°C, but much better results are possible using new data. Regression of the new data for CO 2 (aq), H 2 S(aq), CH 4 (aq), NH 3 (aq), and SO 2 (aq) leads to revised parameters for the revised Helgeson-Kirkham-Flowers (HKF) equation of state. New correlations based on these parameters allow revised regressions of older data for Xe(aq), Ar(aq), and C 2 H 4 (aq) and lead to new predictions for several other aqueous nonelectrolytes. These predictions are tested against experimentally determined equilibrium constants for reactions involving several other aqueous nonelectrolytes, including He(aq), Ne(aq), Kr(aq), Rn(aq), H 2 (aq), N 2 (aq), O 2 (aq), and CO(aq). The correlations developed are used to make predictions for a variety of other aqueous nonelectrolytes, allowing incorporation of these species into models of geochemical processes.

The recently proposed model [Geochim. Cosmochim. Acta 64 (2000) 495; Geochim. Cosmochim. Acta 64 (2000) 2779] for correlating the infinite dilution partial molar properties of aqueous nonelectrolytes is briefly outlined. The approach is fundamentally based on the A 12 = V 0 2 /κRT parameter, related to the infinite dilute solute-water direct correlation function integral. The A 12 parameter is considered as a semiempirical function of temperature and density. At supercritical temperatures thermodynamic integrations of this function allow calculation of all thermodynamic functions of hydration (V 0 2 , Δ h G 0 , Δ h H 0 and Δ h C 0 p ) of a solute. An extension to subcritical conditions is done using an auxiliary Δ h C 0 p (T, P r )-function, which describes the temperature course of the heat capacity of hydration of a solute at P r = 28 MPa and subcritical temperatures. The variations of the Δ h C 0 p (T, P r )-function are constrained by known values of Δ h G 0 , Δ h H 0 and Δ h C 0 p at ambient and supercritical conditions. This model, which was used earlier to correlate properties of a few dissolved gases, is successfully employed here to describe V 0 2 , Δ h G 0 . Δ h H 0 and Δ h C 0 p experimental results for a number of aqueous nonelectrolytes, including ones of high polarity (alcohols, amines, acids and amides) and/or large size (hexane and benzene).

The Krichevskii parameter determines the sign and magnitude of the near-critical divergence of the partial molar thermodynamic properties of infinitely dilute solutions. Estimation of the values of the Krichevskii parameter, A Kr , would be helpful in improving the quality of predictions of the infinite dilution partial molar properties of aqueous nonelectrolytes at near-critical and supercritical conditions. In this paper, we discuss the methods to obtain values of A Kr from various types of experimental information on the thermodynamic properties of binary systems, and use these methods to evaluate the Krichevskii parameter for more than 30 aqueous nonelectrolytes. The possibility of calculating the values of the Krichevskii parameter for solutes at infinite dilution in water from the van der Waals and Peng–Robinson equations of state was also checked. It is shown that the classical cubic equations of state may be used to predict the values of A Kr for small nonpolar solutes in water, but grossly overestimate A Kr for larger compounds. As an alternative, we propose an empirical correlation between the Krichevskii parameter and the Gibbs energy of hydration at 298.15 K and 0.1 MPa for the corresponding compound. This correlation seems to hold for solutes with values of the Krichevskii parameter ranging from −200 to +200 MPa, and values of the Gibbs energy of hydration extending over 100 kJ mol −1 , and appears to be useful for estimating values of the Krichevskii parameter for many solutes of different sizes and polarities, from inert gases to aqueous silica. Predictions, related to the sign of the Krichevskii parameter for a number of aqueous inorganic and organic nonelectrolytes, are made, which can be verified experimentally.

Thermophilic and hyperthermophilic Archaea and Bacteria have been isolated from marine hydrothermal systems, heated sediments, continental solfataras, hot springs, water heaters, and industrial waste. They catalyze a tremendous array of widely varying metabolic processes. As determined in the laboratory, electron donors in thermophilic and hyperthermophilic microbial redox reactions include H2, Fe(2+), H2S, S, S2O3(2-), S4O6(2-), sulfide minerals, CH4, various mono-, di-, and hydroxy-carboxylic acids, alcohols, amino acids, and complex organic substrates; electron acceptors include O2, Fe(3+), CO2, CO, NO3(-), NO2(-), NO, N2O, SO4(2-), SO3(2-), S2O3(2-), and S. Although many assimilatory and dissimilatory metabolic reactions have been identified for these groups of microorganisms, little attention has been paid to the energetics of these reactions. In this review, standard molal Gibbs free energies (DeltaGr(0)) as a function of temperature to 200 degrees C are tabulated for 370 organic and inorganic redox, disproportionation, dissociation, hydrolysis, and solubility reactions directly or indirectly involved in microbial metabolism. To calculate values of DeltaGr(0) for these and countless other reactions, the apparent standard molal Gibbs free energies of formation (DeltaG(0)) at temperatures to 200 degrees C are given for 307 solids, liquids, gases, and aqueous solutes. It is shown that values of DeltaGr(0) for many microbially mediated reactions are highly temperature dependent, and that adopting values determined at 25 degrees C for systems at elevated temperatures introduces significant and unnecessary errors. The metabolic processes considered here involve compounds that belong to the following chemical systems: H-O, H-O-N, H-O-S, H-O-N-S, H-O-C(inorganic), H-O-C, H-O-N-C, H-O-S-C, H-O-N-S-C(amino acids), H-O-S-C-metals/minerals, and H-O-P. For four metabolic reactions of particular interest in thermophily and hyperthermophily (knallgas reaction, anaerobic sulfur and nitrate reduction, and autotrophic methanogenesis), values of the overall Gibbs free energy (DeltaGr) as a function of temperature are calculated for a wide range of chemical compositions likely to be present in near-surface and deep hydrothermal and geothermal systems.

A compilation of experimental values of the infinite dilution partial molar Gibbs energy, enthalpy, and heat capacity of hydration, together with molar volumes in water at 298.15 K and 0.1 MPa, is presented for aliphatic noncyclic ketones. These data, combined with the related results for aliphatic noncyclic hydrocarbons and monohydric alcohols, were treated in the framework of a simple first-order group additivity scheme. Numerical values of the contributions to each of the thermodynamic properties are obtained by the least-squares procedure for the following groups: CH3, CH2, CH, C, OH, and CO.

The volumetric equation proposed previously (Plyasunov et al., 2000) , for estimating the infinite dilution Gibbs energy of hydration of volatile nonelectrolytes at temperatures exceeding the critical temperature of pure water, T c , is extended to subcritical temperatures. The basis for the extension without inclusion of new fitting parameters besides the experimental values of the thermodynamic functions of hydration at 298.15 K, 0.1 MPa, is an auxiliary function, Δ h Cp 0 ( T , P r ), for the variation of the infinite dilution partial molar heat capacity of hydration of a solute in liquid-like water between temperatures T = 273.15 K and T = T s = 658 K along the isobar P r = 28 MPa. The analytical form of Δ h Cp 0 ( T , P r ) was found by globally fitting all available data for the seven best-studied solutes (CH 4 , CO 2 , H 2 S, NH 3 , Ar, Xe, and C 2 H 4 ). Four constraints were used to determine the values of four terms of the Δ h Cp 0 ( T , P r ) function: the numerical values of the temperature increments between T = 298.15 K and T = T s = 658 K for the Gibbs energy and the enthalpy of hydration, and numerical value of the heat capacity at T s and at 298.15 K, all at the selected isobar P r . This approach, in combination with the volumetric equation, may be used to describe and predict all the infinite dilution thermodynamic functions of hydration for nonelectrolytes over extremely wide ranges of temperature and pressure. The model allows calculation of the standard state partial molar properties, including the Gibbs energy of aqueous solutes in a single framework for conditions from high-temperature magmatic processes through hydrothermal phenomena to low-temperature conditions of hypergenesis.

Experimental results of phase equilibria studies at elevated temperatures for more than twenty hydrocarbon-water systems were uniformly correlated within the framework of the Peng-Robinson-Stryjek-Vera equation of state in combination with simple mixing rules. This treatment allows evaluation of the Gibbs energy of hydration for many alkanes, 1-alkenes, cycloalkanes (derivatives of cyclohexane) and alkylbenzenes up to 623 K at saturated water vapor pressure and up to 573 K at 50 MPa. Results for homologous series show regular changes with increasing carbon number, and confirm the applicability of the group contribution approach to the Gibbs energy of hydration of hydrocarbons at elevated temperatures. The temperature dependence of group contributions to the Gibbs energy of hydration were determined for CH3, CH2, and CH in aliphatic hydrocarbons; C=C and H for alkenes; c-CH2 and c-CH in cycloalkanes; and CHar and Car in alkylbenzenes (or aromatic hydrocarbons). Close agreement between calculated and experimental results suggests that this approach provides reasonable estimates of Gibbs energy of hydration for many alkanes, 1-alkenes, alkyl cyclohexanes and alkylbenzenes at temperatures up to 623 K and pressures up to 50 MPa.

Thermodynamic calculations of metastable equilibria were used to evaluate the potential for abiotic synthesis of aliphatic and polycyclic aromatic hydrocarbons (PAHs) in the martian meteorite Allan Hills (ALH) 84001. The calculations show that PAHs and normal alkanes could form metastably from CO, CO2, and H2 below approximately 250-300 degrees C during rapid cooling of trapped magmatic or impact-generated gases. Depending on temperature, bulk composition, and oxidation-reduction conditions, PAHs and normal alkanes can form simultaneously or separately. Moreover, PAHs can form at lower H/C ratios, higher CO/CO2 ratios, and higher temperatures than normal alkanes. Dry conditions with H/C ratios less than approximately 0.01-0.001 together with high CO/CO2 ratios also favor the formation of unalkylated PAHs. The observed abundance of PAHs, their low alkylation, and a variable but high aromatic to aliphatic ratio in ALH 84001 all correspond to low H/C and high CO/CO2 ratios in magmatic and impact gases and can be used to deduce spatial variations of these ratios. Some hydrocarbons could have been formed from trapped magmatic gases, especially if the cooling was fast enough to prevent reequilibration. We propose that subsequent impact heating(s) in ALH 84001 could have led to dissociation of ferrous carbonates to yield fine-grain magnetite, formation of a CO-rich local gas phase, reduction of water vapor to H2, reequilibration of the trapped magmatic gases, aromatization of hydrocarbons formed previously, and overprinting of the synthesis from magmatic gases, if any. Rapid cooling and high-temperature quenching of CO-, H2-rich impact gases could have led to magnetite-catalyzed hydrocarbon synthesis.

Mixing of hydrothermal fluids and seawater at the ocean floor, combined with slow reaction kinetics for oxidation/reduction reactions, provides a source of metabolic energy for chemolithotrophic microorganisms which are the primary biomass producers for an extensive submarine ecosystem that is essentially independent of photosynthesis. Thermodynamic models are used to explore geochemical constraints on the amount of metabolic energy potentially available from chemosynthetic reactions involving S, C, Fe, and Mn compounds during mixing of hydrothermal fluids with seawater. For the vent fluid used in the calculations (EPR 21 degrees N OBS), the model indicates that mixing environments are favorable for oxidation of H2S, CH4, Fe2+ and Mn2+ only below approximately 38 degrees C, with methanogenesis and reduction of sulfate or S degrees favored at higher temperatures, suggesting that environments dominated by mixing provide habitats for mesophilic (but not thermophilic) aerobes and thermophilic (but not mesophilic) anaerobes. A maximum of approximately 760 cal per kilogram vent fluid is available from sulfide oxidation while between 8 and 35 cal/kg vent fluid is available from methanotrophy, methanogenesis, oxidation of Fe or Mn, or sulfate reduction. The total potential for chemosynthetic primary production at deep-sea hydrothermal vents globally is estimated to be about 10(13) g biomass per year, which represents approximately 0.02% of the global primary production by photosynthesis in the oceans. Thermophilic methanogens and sulfate- and S degree-reducers are likely to be the predominant organisms in the walls of vent chimneys and in the diffuse mixing zones beneath warm vents, where biological processes may contribute to the high methane concentrations of vent fluids and heavy 34S/32S ratios of vent sulfide minerals. The metabolic processes taking place in these systems may be analogs of the first living systems to evolve on the Earth.

A large number of aqueous metal complexes contribute significantly to hydrothermal, metamorphic, and magmatic processes in the crust of the Earth. Nevertheless, relatively few thermodynamic data other than dissociation constants (K) for a few dozen of these complexes have been determined experimentally at elevated temperatures and pressures. The calculations summarized below are intended to supplement these experimental data by providing interim predictions of the thermodynamic properties of supercritical aqueous metal complexes using the revised HKF (Helgeson et al., 1981) equations of state for aqueous species (Tanger and Helgeson, 1988; Shock et al., 1992) and correlations among equations of state parameters and standard partial molal properties at 25 degrees C and 1 bar (Shock and Helgeson, 1988, 1990; Shock et al., 1989). These equations and correlations permit retrieval of the conventional standard partial molal entropies (mean S0), volumes (mean V0), and heat capacities (mean C0P) of aqueous metal complexes at 25 degrees C and 1 bar from published values of log K in the supercritical region and the limited number of experimental dissociation constants available in the literature over relatively short ranges of elevated temperature at PSAT (PSAT and SAT are used in the present communication to refer to pressures corresponding to liquid-vapor equilibrium for the system H2O except at temperatures

Correlations among experimentally determined standard partial molal thermodynamic properties of inorganic aqueous species at 25°C and 1 bar allow estimates of these properties for numerous monatomic cations and anions, polyatomic anions, oxyanions, acid oxyanions, neutral oxy-acid species, dissolved gases, and hydroxide complexes of metal cations. Combined with correlations among parameters in the revised Helgeson-Kirkham-Flowers (HKF) equation of state (Shock et al., 1992), these estimates permit predictions of standard partial molal volumes ( V ° ), heat capacities ( C ° P ), and entropies ( S ° ), as well as apparent standard partial molal enthalpies ( Δ H ° P,T ) and Gibbs free energies (ΔḠ° P,T ) of formation to 1000°C and 5 kb for hundreds of inorganic aqueous species of interest in geochemistry. Data and parameters for more than 300 inorganic aqueous species are presented. Close agreement between calculated and experimentally determined equilibrium constants for acid dissociation reactions and cation hydrolysis reactions supports the generality and validity of these predictive methods. These data facilitate the calculation of the speciation of major, minor, and trace elements in hydrothermal and metamorphic fluids throughout most of the crust of the Earth.

Many studies link the compositions of microbial communities to their environments, but the energetics of organism-specific biomass synthesis as a function of geochemical variables have rarely been assessed. We describe a thermodynamic model that integrates geochemical and metagenomic data for biofilms sampled at five sites along a thermal and chemical gradient in the outflow channel of the hot spring known as "Bison Pool" in Yellowstone National Park. The relative abundances of major phyla in individual communities sampled along the outflow channel are modeled by computing metastable equilibrium among model proteins with amino acid compositions derived from metagenomic sequences. Geochemical conditions are represented by temperature and activities of basis species, including pH and oxidation-reduction potential quantified as the activity of dissolved hydrogen. By adjusting the activity of hydrogen, the model can be tuned to closely approximate the relative abundances of the phyla observed in the community profiles generated from BLAST assignments. The findings reveal an inverse relationship between the energy demand to form the proteins at equal thermodynamic activities and the abundance of phyla in the community. The distance from metastable equilibrium of the communities, assessed using an equation derived from energetic considerations that is also consistent with the information-theoretic entropy change, decreases along the outflow channel. Specific divergences from metastable equilibrium, such as an underprediction of the relative abundances of phototrophic organisms at lower temperatures, can be explained by considering additional sources of energy and/or differences in growth efficiency. Although the metabolisms used by many members of these communities are driven by chemical disequilibria, the results support the possibility that higher-level patterns of chemotrophic microbial ecosystems are shaped by metastable equilibrium states that depend on both the composition of biomass and the environmental conditions.

Thermodynamic modelling of organic synthesis has largely been focused on deep-sea hydrothermal systems. When seawater mixes with hydrothermal fluids, redox gradients are established that serve as potential energy sources for the formation of organic compounds and biomolecules from inorganic starting materials. This energetic drive, which varies substantially depending on the type of host rock, is present and available both for abiotic (outside the cell) and biotic (inside the cell) processes. Here, we review and interpret a library of theoretical studies that target organic synthesis energetics. The biogeochemical scenarios evaluated include those in present-day hydrothermal systems and in putative early Earth environments. It is consistently and repeatedly shown in these studies that the formation of relatively simple organic compounds and biomolecules can be energy-yielding (exergonic) at conditions that occur in hydrothermal systems. Expanding on our ability to calculate biomass synthesis energetics, we also present here a new approach for estimating the energetics of polymerization reactions, specifically those associated with polypeptide formation from the requisite amino acids.

Accurate values of the apparent standard partial molal Gibbs free energies of formation (ΔG°) of aqueous species, minerals, and gases at high temperatures and pressures are a requisite for characterizing a variety of industrial and natural processes including corrosion of metals, solvent extraction, crystal growth, metamorphism, and the formation of hydrothermal ore deposits. Revision of the HKF equations of state for aqueous species other than H2O (Helgeson, Kirkham and Flowers, 1981) by Tanger and Helgeson (1988) and Shock et al. (1992) permits calculation of ΔG° for these species at temperatures to 1000 °C and pressures to 5000 bars. The revised equations of state were combined with parameters generated by Shock and Helgeson (1988, 1990), Shock and McKinnon (1993), Shock and Koretsky (1993), Schulte and Shock (1993), Pokrovskii and Helgeson (1995 a, b, and c), and Sverjensky et al. (1995) together with densities and electrostatic properties of H2O computed from equations summarized by Johnson and Norton (1991) to calculate values of ΔG° for aqueous species as a function of temperature and pressure. The results of these calculations are tabulated for 348 such species, including both inorganic and organic aqueous ions, neutral species, and metal ligand complexes. Similar calculations using equations, parameters, and thermodynamic data taken from Kelley (1960), Helgeson et al. (1982), Hill (1990), Shock (1993), and Pokrovskii and Helgeson (1995 a and b) were used to generate tables of ΔG° for H2O, 22 minerals, and 18 gases. The tabulated values of ΔG°, which were generated with the aid of SUPCRT92 (Johnson et al., 1992), facilitate considerably assessment of the thermodynamic behavior of chemical processes at both high and low temperatures and pressures.

Thermodynamic calculations provide the means to quantify the chemical disequilibrium inherent in the mixing of reduced hydrothermal fluids with seawater. The chemical energy available for metabolic processes in these environments can be evaluated by taking into account the pressure and temperature dependence of the apparent standard Gibbs free energies of reactions in the S-H2-H2O system together with geochemical constraints on pH, activities of aqueous sulfur species and fugacities of H2 and/or O2. Using present-day mixing of hydrothermal fluids and seawater as a starting point, it is shown that each mole of H2S entering seawater from hydrothermal fluids represents about 200,000 calories of chemical energy for metabolic systems able to catalyze H2S oxidation. Extrapolating to the early Earth, which was likely to have had an atmosphere more reduced than at present, shows that this chemical energy may have been a factor of two or so less. Nevertheless, mixing of hydrothermal fluids with seawater would have been an abundant source of chemical energy, and an inevitable consequence of the presence of an ocean on an initially hot Earth. The amount of energy available was more than enough for organic synthesis from CO2 or CO, and/or polymer formation, indicating that the vicinity of hydrothermal systems at the sea floor was an ideal location for the emergence of the first chemolithoautotrophic metabolic systems.

Experimental standard partial molal volumes, heat capacities, and entropies as well as apparent standard partial molal enthalpies and Gibbs free energies of mono- and dicarboxylic acids and their anions at low temperatures and pressures are used to generate correlations for predicting the same properties at high temperatures and pressures for 59 carboxylic and 18 hydroxyacid species with the revised Helgeson-Kirkham-Flowers (HKF) equation of state. Predicted equilibrium dissociation constants are compared with experimental values from the literature and tabulated as functions of pressure and temperature for 25 carboxylic acids and nine hydroxyacids. Close agreement between independent predictions and experimental data supports the generality of the computational techniques and the accuracy of predicted data. These results allow incorporation of a wide variety of organic acids into quantitative interpretations of geochemical processes.

Uncovering the chemical and physical links between natural environments and microbial communities is becoming increasingly amenable owing to geochemical observations and metagenomic sequencing. At the hot spring known as Bison Pool in Yellowstone National Park, the cooling of the water in the outflow channel is associated with an increase in oxidation potential estimated from multiple field-based measurements. Representative groups of proteins whose sequences were derived from metagenomic data also exhibit an increase in average oxidation state of carbon in the protein molecules with distance from the hot-spring source. The energetic requirements of reactions to form selected proteins used in the model were computed using amino-acid group additivity for the standard molal thermodynamic properties of the proteins, and the relative chemical stabilities of the proteins were investigated by varying temperature, pH and oxidation state, expressed as activity of dissolved hydrogen. The relative stabilities of the proteins were found to track the locations of the sampling sites when the calculations included a function for hydrogen activity that increases with temperature and is higher, or more reducing, than values consistent with measurements of dissolved oxygen, sulfide and oxidation-reduction potential in the field. These findings imply that spatial patterns in the amino acid compositions of proteins can be linked, through energetics of overall chemical reactions representing the formation of the proteins, to the environmental conditions at this hot spring, even if microbial cells maintain considerably different internal conditions. Further applications of the thermodynamic calculations are possible for other natural microbial ecosystems.

Aldehydes are common in a variety of geologic environments and are derived from a number of sources, both natural and anthropogenic. Experimental data for aqueous aldehydes were taken from the literature and used, along with parameters for the revised Helgeson-Kirkham-Flowers (HKF) equations of state, to estimate standard partial molal thermodynamic data for aqueous straight-chain alkyl aldehydes at high temperatures and pressures. Examples of calculations involving aldehydes in geological environments are given, and the stability of aldehydes relative to carboxylic acids is evaluated. These calculations indicate that aldehydes may be intermediates in the formation of carboxylic acids from hydrocarbons in sedimentary basin brines and hydrothermal systems like they are in the atmosphere. The data and parameters summarized here allow evaluation of the role of aldehydes in the formation of prebiotic precursors, such as amino acids and hydroxy acids on the early Earth and in carbonaceous chondrite parent bodies.

Correlations between standard partial molal entropies ( S j 0 ) of aqueous mono-, di-, and trivalent ions at 25°C and 1 bar and effective electrostatic radii (re,j) calculated from the corresponding crystallographic radii (rx,j) have been constructed for coordination numbers from 4 to 12. These correlations permit estimates of S j 0 to include information from spectroscopic studies about ionic coordination in aqueous solution. As a consequence, values of S j 0 can be estimated for many more ions than previously possible. Close agreement between predicted and experimental data for tetravalent cations supports the validity and generality of this approach. Combined with the experimentally determined values of S j 0 used to generate the correlations, estimates of standard partial molal entropies at 25°C and 1 bar for over 130 mon-atomic aqueous ions of charges ranging from −3 to +6 provide a set of S j 0 values for 188 monatomic aqueous ions for use in geochemical calculations.

Within the framework of the revised HKF (H. C. Helgeson, D. H. Kirkham and G. C. Flowers, Am. J. Sci., 1981, 281, 1249) equations of state (J. C. Tanger IV and H. C. Helgeson, Am. J. Sci., 1988, 288, 19), prediction of the standard partial molal thermodynamic properties of aqueous ions and electrolytes at high pressures and temperatures requires values of the effective electrostatic radii of the ions (re), as well as provision for the temperature and pressure dependence of the relative permittivity of the solvent, H2O. Values of the relative permittivity of H2O, together with the Born functions needed to compute the standard partial molal Gibbs free energy, enthalpy, entropy, heat capacity and volume of solvation were calculated as a function of temperature and density from a modified version of the Uematsu–Franck equation (M. Uematsu and E. U. Franck, J. Phys. Chem. Ref. Data, 1980, 9, 1291). The temperature/pressure dependence of re is described in terms of a solvent function designated by g, which was evaluated in the present study at temperatures and pressures to 1000 °C and 5 kbar by regressing experimental standard partial molal heat capacities and volumes of NaCl reported in the literature together with published dissociation constants for NaClo at supercritical temperatures and pressures using the revised HKF equations of state for aqueous species. The calculated values of re decrease substantially with increasing temperature at constant pressure ⩽2 kbar, and with decreasing pressure at constant temperature 400 °C. The equations and parameters summarized below permit calculation of the standard partial molal properties of aqueous species from the revised HKF equations of state over a much more extensive range of temperature than was previously possible.

A coordinated study of water chemistry, sediment mineralogy, and sediment microbial community was conducted on four73 degrees C springs in the northwestern Great Basin. Despite generally similar chemistry and mineralogy, springs with short residence time (approximately 5-20 min) were rich in reduced chemistry, whereas springs with long residence time (1 day) accumulated oxygen and oxidized nitrogen species. The presence of oxygen suggested that aerobic metabolisms prevail in the water and surface sediment. However, Gibbs free energy calculations using empirical chemistry data suggested that several inorganic electron donors were similarly favorable. Analysis of 298 bacterial 16S rDNAs identified 36 species-level phylotypes, 14 of which failed to affiliate with cultivated phyla. Highly represented phylotypes included Thermus, Thermotoga, a member of candidate phylum OP1, and two deeply branching Chloroflexi. The 276 archaeal 16S rDNAs represented 28 phylotypes, most of which were Crenarchaeota unrelated to the Thermoprotei. The most abundant archaeal phylotype was closely related to "Candidatus Nitrosocaldus yellowstonii", suggesting a role for ammonia oxidation in primary production; however, few other phylotypes could be linked with energy calculations because phylotypes were either related to chemoorganotrophs or were unrelated to known organisms.

Formate, a simple organic acid known to support chemotrophic hyperthermophiles, is found in hot springs of varying temperature and pH. However, it is not yet known how metabolic strategies that use formate could contribute to primary productivity in hydrothermal ecosystems. In an effort to provide a quantitative framework for assessing the role of formate metabolism, concentration data for dissolved formate and many other solutes in samples from Yellowstone hot springs were used, together with data for coexisting gas compositions, to evaluate the overall Gibbs energy for many reactions involving formate oxidation or reduction. The result is the first rigorous thermodynamic assessment of reactions involving formate oxidation to bicarbonate and reduction to methane coupled with various forms of iron, nitrogen, sulfur, hydrogen, and oxygen for hydrothermal ecosystems. We conclude that there are a limited number of reactions that can yield energy through formate reduction, in contrast to numerous formate oxidation reactions that can yield abundant energy for chemosynthetic microorganisms. Because the energy yields are so high, these results challenge the notion that hydrogen is the primary energy source of chemosynthetic microbes in hydrothermal ecosystems.

The common thread of energy release suggests that diverse microbial metabolic processes can be compared through thermodynamic analyses. The resulting energy and power requirements can provide quantitative constraints on habitability. Because previous thermodynamic analyses have focused on the minimum amount of energy needed for the growth of a microorganism or community, the focus of this study is to gain a fuller understanding of the microbial response to highly habitable conditions. This communication summarizes the results of a thermodynamic analysis of the energy and power consumed by microorganisms in experiments that were designed to optimize growth. Reports of microbial growth experiments taken from the literature were combined with speciation and standard state calculations to assess the overall Gibbs energy change during the experiments. Results show that similar numbers of cells (10(9) to 10(10) ) were produced in these experiments regardless of the duration of log phase growth (from2 to200 hours) or the total Gibbs energy change [from 1.3-29.6 kJ (mol electrons transferred)(1)]. As a result, optimal growth conditions appear to produce between 10(10) and 10(14) cells per watt of power consumed.

Densities of dilute aqueous solutions of isopropanol, 1,5-pentanediol, cyclohexanol, benzyl alcohol, diethyl ether, 1,2-dimethoxyethane, acetone, and 2,5-hexanedione were measured by means of a vibrating-tube flow densimeter at temperatures near T = (302, 373, 423, 473, and 521) K at a pressure of p = 28 MPa. At the lowest and highest temperatures, measurements were also made close to the saturation vapour pressure of water to investigate the effect of pressure on the volumes of solutes. Apparent molar volumes were calculated for each solute and extrapolated to give partial molar volumes at infinite dilution. The variation of the volume with temperature, pressure, and structure of solute is discussed qualitatively, and group contributions are determined at the temperatures of measurements and p = 28 MPa. Several equations proposed in the literature for correlating the partial molar volumes at infinite dilution as a function of state parameters are tested. Parameters of one selected equation are tabulated allowing calculation of the partial molar volumes at infinite dilution at temperatures and pressures up to T = 573 K and p = 40 MPa, respectively.

Discoveries in biology and developments in geochemistry over the past two decades have lead to a radical revision of concepts relating to the upper temperature at which life thrives, the genetic relationships among all life on Earth, links between organic and inorganic compounds in geologic processes, and the geochemical supply of metabolic energy. It is now apparent that given a source of geochemical energy, in the form of a mixture of compounds that is far from thermodynamic equilibrium, microorganisms can take advantage of the energy and thrive without the need for photosynthesis as a means of primary productivity. This means that life can exist in the subsurface of a planet such as Mars without necessarily exhibiting a surface expression. Theoretical calculations quantify the geochemically provided metabolic energy available to hyperthermophilic organisms in submarine hydrothermal systems on the Earth, and help to explain the enormous biological productivity of these systems. Efforts to place these models in the context of the early Earth reveal that substantial geochemical energy would have been available and that organic synthesis would have been thermodynamically favored as hydrothermal fluids mix with seawater.

Thermodynamic calculations showed that the autotrophic synthesis of all 20 protein-forming amino acids was energetically favored in hot (100 degrees C), moderately reduced, submarine hydrothermal solutions relative to the synthesis in cold (18 degrees C), oxidized, surface seawater. The net synthesis reactions of 11 amino acids were exergonic in the hydrothermal solution, but all were endergonic in surface seawater. The synthesis of the requisite amino acids of nine thermophilic and hyperthermophilic proteins in a 100 degreesC hydrothermal solution yielded between 600 and 8000 kilojoules per mole of protein, which is energy that is available to drive the intracellular synthesis of enzymes and other biopolymers in hyperthermophiles thriving in these ecosystems.

Submarine hydrothermal systems on the early Earth may have been the sites from which life emerged. The potential for Strecker synthesis to produce biomolecules (amino and hydroxy acids) from starting compounds (ketones, aldehydes, HCN and ammonia) in such environments is evaluated quantitatively using thermodynamic data and parameters for the revised Helgeson-Kirkham-Flowers (HKF) equation of state. Although there is an overwhelming thermodynamic drive to form biomolecules by the Strecker synthesis at hydrothermal conditions, the availability and concentration of starting compounds limit the efficiency and productivity of Strecker reactions. Mechanisms for concentrating reactant compounds could help overcome this problem, but other mechanisms for production of biomolecules may have been required to produce the required compounds on the early Earth. Geochemical constraints imposed by hydrothermal systems provide important clues for determining the potential of these and other systems as sites for the emergence of life.

Correlation algorithms permit prediction of species-dependent parameters in revised equations of state (tanger and Helgeson, 1988) for aqueous ions and electrolytes which can be used together with values at 25°C and 1 bar of the standard partial molal entropies (S0), volumes (V0), and heat capacities (C0p) of the ions to calculate their standard partial molal thermodynamic properties at pressures and temperatures to 5 kb and 1000°C. Values of V0 and C0p at 25°C and 1 bar have been calculated for 128 aqueous ions from analysis of published experimental data or from correlations of these properties with S0. Equation of state parameters have also been generated for these ions and examples are given of predicted standard partial molal thermodynamic properties of representative cations and anions at high pressures and temperatures. Close agreement between independently predicted and experimentally determined equilibrium constants for acid anion dissociation reactions at high temperatures supports the validity and generality of the predictive algorithms.

Consideration of interactions between neutral aqueous species and H2O dipoles in terms of effective Born coefficients permits extension of the revised HKF (Helgeson, Kirkham and Flowers, 1981) equations of state (Tanger and Helgeson, 1988) for the standard partial molal properties of ionic species at high pressures and temperatures to include inorganic gases, acids, and other neutral aqueous species. Correlation algorithms similar to those used to estimate equation of state parameters for ions and electrolytes (shock, and Helgeson, 1988) have also been developed for neutral aqueous species. Calculation of the standard partial molal thermodynamic properties of dissolved inorganic gases as well as other neutral aqueous species as a function of pressure and temperature indicates that the standard partial molal volume (V0), heat capacity (C0p), and entropy (S0), together with the apparent standard partial molal enthalpy of formation (ΔH0) of many of these species in the liquid phase minimize with increasing temperature at PSAT∗ and approach ∞ at the critical point of H2O. In the case of other neutral aqueous species such as SiO2(aq), V0, C0p, S0, and ΔH0 behave as functions of temperature and pressure like those of electrolytes in the liquid phase and maximize with increasing temperature at PSAT, approaching — ∞ at the critical point of H2O. Which of these types of behavior is exhibited by V0, C)p, S0, and ΔH0 for a given neutral aqueous species depends in part on the relative volatility of the aqueous species and the effect of the species on solvent dipole-dipole interaction. Close agreement between predicted and experimentally determined equilibrium constants for gas solubility and inorganic acid dissociation reactions at high temperatures and pressures supports the validity and generality of the equations of state and the predictive algorithms. High temperature/pressure equilibrium constants can be predicted for reactions involving a wide variety of neutral aqueous species for which few or no experimental data are available at temperatures > 25°C. Present capabilities permit such predictions to be made for hydrothermal and magmatic conditions at pressures and temperatures to 5 kb and 1000°C.

Potential influences on the distribution of alkylphenols in oilfield waters include simple partitioning between oil, water, and solid phases, generation of alkylphenols during kerogen maturation in source rocks, and reactions at the oil-water interface in the oil reservoir. Assessment of whether reactions at the oil-water interface influence phenol distributions in basinal aqueous waters requires thermodynamic properties of the aqueous alkylphenols. In this study we have calculated standard partial molal thermodynamic properties of aqueous phenol and cresols at 25°C and estimated standard partial molal thermodynamic properties of aqueous xylenols at 25°C. These thermodynamic properties (Gibbs free energy, enthalpy, entropy, heat capacity, and volume) together with estimated coefficients for the revised Helgeson-Kirkham-Flowers (HKF) equations of state allow prediction of thermodynamic properties of reactions involving these species at high temperatures and pressures. Computed logarithmic equilibrium constants (log K) for hydration reactions involving alkylphenol are in close agreement with log K values calculated from published experimental data (Dohnal and Fenclova, 1995). Resulting thermodynamic properties may be used together with analytical results, to assess whether metastable equilibrium is a major influence on the abundance and distribution of alkvlvhenols in vetroleum and associated water.