Effects of trace element concentrations on culturing thermophiles.

The majority of microorganisms in natural environments resist laboratory cultivation. Sometimes referred to as 'unculturable', many phylogenetic groups are known only by fragments of recovered DNA. As a result, the ecological significance of whole branches of the 'tree of life' remains a mystery; this is particularly true when regarding genetic material retrieved from extreme environments. Geochemically relevant media have been used to improve the success of culturing Archaea and Bacteria, but these efforts have focused primarily on optimizing pH, alkalinity, major ions, carbon sources, and electron acceptor-donor pairs. Here, we cultured thermophilic microorganisms from 'Sylvan Spring' (Yellowstone National Park, USA) on media employing different trace element solutions, including one that mimicked the source fluid of the inocula. The growth medium that best simulated trace elements found in 'Sylvan Spring' produced a more diverse and faster growing mixed culture than media containing highly elevated trace element concentrations. The elevated trace element medium produced fewer phylotypes and inhibited growth. Trace element concentrations appear to influence growth conditions in extreme environments. Incorporating geochemical data into cultivation attempts may improve culturing success.

Energetics of amino acid synthesis in hydrothermal ecosystems.

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.

Energetics of overall metabolic reactions of thermophilic and hyperthermophilic Archaea and bacteria.

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.

Organic acids in hydrothermal solutions: standard molal thermodynamic properties of carboxylic acids and estimates of dissociation constants at high temperatures and pressures.

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.

High-temperature life without photosynthesis as a model for Mars.

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.

The biological potential of Mars, the early Earth, and Europa.

The potential biomass that could have existed on Mars is constrained by the total amount of energy available to construct it. From an inventory of the available geochemical sources of energy, we estimate that from the time of the onset of the visible geologic record 4 b.y. ago to the present, as much as 20 g cm-2 of biota could have been created. This is the same amount that could have been constructed on the early Earth in only 100 million years. This indicates that there likely was sufficient energy available to support an origin of life on Mars but not sufficient energy to create a ubiquitous and lush biosphere. Similar calculations for Europa suggest that even less would have been available there.

Hydrothermal hydration of Martian crust: illustration via geochemical model calculations.

If hydrothermal Systems existed on Mars, hydration of crustal rocks may have had the potential to affect the water budget of the planet. We have conducted geochemical model calculations to investigate the relative roles of host rock composition, temperature, water-to-rock ratio, and initial fluid oxygen fugacity on the mineralogy of hydrothermal alteration assemblages, as well as the effectiveness of alteration to store water in the crust as hydrous minerals. In order to place calculations for Mars in perspective, models of hydrothermal alteration of three genetically related Icelandic volcanics (a basalt, andesite, and rhyolite) are presented, together with results for compositions based on SNC meteorite samples (Shergotty and Chassigny). Temperatures from 150 degrees C to 250 degrees C, water-to-rock ratios from 0.1 to 1000, and two initial fluid oxygen fugacities are considered in the models. Model results for water-to-rock ratios less than 10 are emphasized because they are likely to be more applicable to Mars. In accord with studies of low-grade alteration of terrestrial rocks, we find that the major controls on hydrous mineral production are host rock composition and temperature. Over the range of conditions considered, the alteration of Shergotty shows the greatest potential for storing water as hydrous minerals, and the alteration of Icelandic rhyolite has the lowest potential.

A thermodynamic assessment of the potential synthesis of condensed hydrocarbons during cooling and dilution of volcanic gases.

The possibility for abiotic synthesis of condensed hydrocarbons in cooling/diluting terrestrial volcanic gases has been evaluated on the basis of the consideration of metastable chemical equilibria involving gaseous CO, CO2, H2 and H2O. The stabilities of n-alkanes and polycyclic aromatic hydrocarbons (PAHS) have been evaluated for several typical volcanic gas compositions under various conditions for cooling/diluting of quenched volcanic gas. The modeling shows that n-alkanes and PAHs have a thermodynamic potential to form metastably from H2 and CO below approximately 250 degrees C within the stability field of graphite. Despite the predominance of CO2 in volcanic gases, synthesis of hydrocarbons from CO2 and H2 is less favored energetically than from CO and H2. Both low temperature and a high H/C atomic ratio in volcanic gas generally favor stability of hydrocarbons with higher H/C ratios. PAHs are thermodynamically stable at temperatures approximately 10 degrees -50 degrees C higher than large n-alkanes; however, at lower temperatures, PAHs and n-alkanes have similar stabilities and are likely to form metastable mixtures. Both the energetic drive to form hydrocarbons and possible temperatures of formation increase as the oxidation state (fO2) of the volcanic gases decreases and as the cooling/dilution ratios of volcanic gases increase. Synthesis of hydrocarbons is energetically more likely in cooling trapped gases than in ashcloud eruptive columns. Mechanisms for hydrocarbon formation may include Fischer-Tropsch-type synthesis catalyzed by magnetite from solid volcanic products. On the early Earth, Mars, and Jupiter's satellite Europa, several factors would have provided more favorable conditions for hydrocarbon synthesis in volcanic gases than under current terrestrial conditions and might have contributed to the production of organic compounds required for the emergence of life.

An abiotic origin for hydrocarbons in the Allan Hills 84001 martian meteorite through cooling of magmatic and impact-generated gases.

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.

Hydrothermal systems and the emergence of life.

NASA: The author reviews current thought about life originating in hyperthermophilic microorganisms. Hyperthermophiles obtain food from chemosynthesis of sulfur and have an RNA nucleotide sequence different from bacteria and eucarya. It is postulated that a hyperthermophile may be the common ancestor of all life. Current research efforts focus on the synthesis of organic compounds in hydrothermal systems.^ieng

Spatial and temporal variability of biomarkers and microbial diversity reveal metabolic and community flexibility in Streamer Biofilm Communities in the Lower Geyser Basin, Yellowstone National Park.

Detailed analysis of 16S rRNA and intact polar lipids (IPLs) from streamer biofilm communities (SBCs), collected from geochemically similar hot springs in the Lower Geyser Basin, Yellowstone National Park, shows good agreement and affirm that IPLs can be used as reliable markers for the microbial constituents of SBCs. Uncultured Crenarchaea are prominent in SBS, and their IPLs contain both glycosidic and mixed glyco-phospho head groups with tetraether cores, having 0-4 rings. Archaeal IPL contributions increase with increasing temperature and comprise up to one-fourth of the total IPL inventory at >84 °C. At elevated temperatures, bacterial IPLs contain abundant glycosidic glycerol diether lipids. Diether and diacylglycerol (DAG) lipids with aminopentanetetrol and phosphatidylinositol head groups were identified as lipids diagnostic of Aquificales, while DAG glycolipids and glyco-phospholipids containing N-acetylgycosamine as head group were assigned to members of the Thermales. With decreasing temperature and concomitant changes in water chemistry, IPLs typical of phototrophic bacteria, such as mono-, diglycosyl, and sulfoquinovosyl DAG, which are specific for cyanobacteria, increase in abundance, consistent with genomic data from the same samples. Compound-specific stable carbon isotope analysis of IPL breakdown products reveals a large isotopic diversity among SBCs in different hot springs. At two of the hot springs, 'Bison Pool' and Flat Cone, lipids derived from Aquificales are enriched in (13) C relative to biomass and approach values close to dissolved inorganic carbon (DIC) (approximately 0‰), consistent with fractionation during autotrophic carbon fixation via the reversed tricarboxylic acid pathway. At a third site, Octopus Spring, the same Aquificales-diagnostic lipids are 10‰ depleted relative to biomass and resemble stable carbon isotope values of dissolved organic carbon (DOC), indicative of heterotrophy. Other bacterial and archaeal lipids show a similar variance, with values resembling the DIC or DOC pool or a mixture thereof. This variance cannot be explained by hot spring chemistry or temperature alone, but instead, we argue that intermittent input of exogenous organic carbon can result in metabolic shifts of the chemotrophic communities from autotrophy to heterotrophy and vice versa.

Science potential from a Europa lander.

The prospect of a future soft landing on the surface of Europa is enticing, as it would create science opportunities that could not be achieved through flyby or orbital remote sensing, with direct relevance to Europa's potential habitability. Here, we summarize the science of a Europa lander concept, as developed by our NASA-commissioned Science Definition Team. The science concept concentrates on observations that can best be achieved by in situ examination of Europa from its surface. We discuss the suggested science objectives and investigations for a Europa lander mission, along with a model planning payload of instruments that could address these objectives. The highest priority is active sampling of Europa's non-ice material from at least two different depths (0.5-2 cm and 5-10 cm) to understand its detailed composition and chemistry and the specific nature of salts, any organic materials, and other contaminants. A secondary focus is geophysical prospecting of Europa, through seismology and magnetometry, to probe the satellite's ice shell and ocean. Finally, the surface geology can be characterized in situ at a human scale. A Europa lander could take advantage of the complex radiation environment of the satellite, landing where modeling suggests that radiation is about an order of magnitude less intense than in other regions. However, to choose a landing site that is safe and would yield the maximum science return, thorough reconnaissance of Europa would be required prior to selecting a scientifically optimized landing site.

Microbiology and geochemistry of Little Hot Creek, a hot spring environment in the Long Valley Caldera.

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.

Methane: an open or shut case?

NASA: The author examines conflicting models, especially evidence from Price and Schoell in this issue (see comment), for the creation of ore deposits and the movement of petroleum. The Bakken formation in North Dakota is used as an example of a petroleum reservoir that has not migrated from its original site. Analysis of natural gases at the site indicates a difference in gas distribution between the Bakken site and migratory sites. Controversies are examined in the context of advances in fluid-flow calculations and high-temperature methanogenic reactions.^ieng

Hydrothermal systems as environments for the emergence of life.

Analysis of the chemical disequilibrium provided by the mixing of hydrothermal fluids and seawater in present-day systems indicates that organic synthesis from CO2 or carbonic acid is thermodynamically favoured in the conditions in which hyperthermophilic microorganisms are known to live. These organisms lower the Gibbs free energy of the chemical mixture by synthesizing many of the components of their cells. Primary productivity is enormous in hydrothermal systems because it depends only on catalysis of thermodynamically favourable, exergonic reactions. It follows that hydrothermal systems may be the most favourable environments for life on Earth. This fact makes hydrothermal systems logical candidates for the location of the emergence of life, a speculation that is supported by genetic evidence that modern hyperthermophilic organisms are closer to a common ancestor than any other forms of life. The presence of hydrothermal systems on the early Earth would correspond to the presence of liquid water. Evidence that hydrothermal systems existed early in the history of Mars raises the possibility that life may have emerged on Mars as well. Redox reactions between water and rock establish the potential for organic synthesis in and around hydrothermal systems. Therefore, the single most important parameter for modelling the geochemical emergence of life on the early Earth or Mars is the composition of the rock which hosts the hydrothermal system.

Amino-acid synthesis in carbonaceous meteorites by aqueous alteration of polycyclic aromatic hydrocarbons

It has been suggested that amino acids and other organic compounds found in carbonaceous meteorites formed by aqueous alteration in the meteorite parent bodies. Observations of carbonaceous material in interstellar grains and interplanetary dust particles indicate that condensed organic compounds may have been present in meteorite parent bodies at the time of aqueous alteration. One group of compounds thought to be representative of this carbonaceous material is the polycyclic aromatic hydrocarbons (PAHs). Recently it was proposed that PAHs condense on SiC grains in the molecular envelopes of carbon-rich red-giant stars, which would allow for their subsequent incorporation into meteorite parent bodies during accretion. This incorporation mechanism is supported by the identification of SiC grains in carbonaceous chondrites. The possibility therefore exists that PAHs, and/or other condensed organic compounds, represent the starting material for aqueous alteration which leads to the formation of amino acids and other water-soluble organic compounds. Here we present calculations of the distribution of aqueous organic compounds in metastable equilibrium with representative PAHs as functions of the fugacities of O2, CO2 and NH3. The results reported here for pyrene and fluoranthene, two PAHs with different structures but the same stoichiometry, differ greatly but indicate that the formation of amino and carboxylic acids is energetically favourable at probable parent-body alteration conditions. The actual reaction mechanisms involved could be revealed by consideration of isotope data for PAHs, amino acids, other organic compounds and carbonates in carbonaceous chondrites.

Summary and implications of reported amino acid concentrations in the Murchison meteorite

A study of literature reports of the concentrations of amino acids in extracts from the Murchison meteorite shows that many of the concentration ratios are constant. There are two possible interpretations of these ratios. One is that they are controlled by the pathways through which the amino acids formed, from which it follows that the amino acids are distributed in the same proportions throughout the meteorite. The other interpretation is that the ratios result from the analytical procedures used to extract the amino acids from the meteorite. These methods rely heavily on high-temperature (100 degrees C) aqueous extraction and subsequent high-temperature acid hydrolysis. A correlation was observed in the present study between the relative concentrations of several amino acids in the meteorite extracts and their relative aqueous solubilities at 100 degrees C (alanine, valine, leucine, isoleucine, norleucine, aspartic acid, glutamic acid and glycine). The extract solutions are dilute, and far from the saturation limits, but these correlations suggest that the sampling procedure affects directly the reported concentrations for these amino acids. Ratios of the concentration of serine to those of glycine are also constant but cannot be accounted for solely by relative solubilities, and, as suggested elsewhere, serine as well as phenylalanine and methionine may be terrestrial contaminants. Data for beta-alanine, alpha-aminobutyric acid, proline, sarcosine, alloisoleucine, beta-aminoisobutyric acid, beta-aminobutyric acid, and threonine also show constant abundances relative to glycine, but lack of solubility data at extraction conditions prohibits evaluating the extent of possible sampling bias for these amino acids. If the extraction process does not bias the results, and all extractable amino acids are removed from meteorite samples, then the properties of amino acids which control both their solubilities and their concentrations in the meteorite need to be established. The possibility of sampling bias needs to be tested experimentally before concluding that extraction is complete, and that the constant relative abundances indicate that the relative concentrations of amino acids are homogeneous in the meteorite.

Aldehydes in hydrothermal solution: standard partial molal thermodynamic properties and relative stabilities at high temperatures and pressures.

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.

Hydrothermal processing of cometary volatiles--applications to Triton.

Subsequent to its capture by Neptune, Triton could have experienced an episode of tidal heating sufficient to melt its icy mantle and possibly its rocky core as well. This heating would have driven hydrothermal circulation at the core-rock/mantle-ocean boundary. We consider the chemical consequences of this hydrothermal reprocessing on Triton's volatile budget by assuming an initial cometary composition for the icy mantle and evaluating the effects of changes in temperature and oxidation state. We assume that the latter would have been controlled by mineral assemblages in the rock. Such reprocessing could explain the lack of carbon monoxide in the atmosphere of Triton and its depletion relative to N2 and (apparently) CO2 in the satellite's surface ices. Our calculations also show that whatever the original source of nitrogen in Triton, N2 and/or NH3 are likely abundant products of hydrothermal reprocessing. Depending on the temperature and prevailing oxidation state, acetic acid, ethanol, urea, methanol, and ethanamine are possible important components, in addition to ammonia, of the resulting mantle material. Triton may thus preserve the organic chemistry that might have led to the origin of life in early terrestrial hydrothermal systems.

A geochemical model for the formation of hydrothermal carbonates on Mars.

It is often argued that substantially more carbon dioxide and water were degassed from the martian interior than can be found at present in the atmosphere, polar caps and regolith. Calculations have shown that atmospheric escape cannot account for all of the missing volatiles. Suggestions that carbon dioxide is stored as marine or lacustrine deposits, are challenged by Earth-based and spacecraft remote-sensing data. Moreover, recent modelling of the martian atmosphere suggests that rainfall or open bodies of water are in any case unlikely to have persisted for extended periods of time. Hydrothermal carbonates therefore provide a possible solution to this dilemma. Using an accessible terrestrial system (Iceland) as a guide to the underlying processes, and a host rock composition inferred from the least-altered martian meteorite, we present a geochemical model for the formation of carbonates in possible martian hydrothermal systems. Our results suggest that an extensive reservoir of carbonate minerals--equivalent to an atmospheric pressure of carbon dioxide of at least one bar--could have been sequestered beneath the surface by widespread hydrothermal activity in the martian past.