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, Fe²⁺, 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, Fe³⁺, 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 (ΔG_r/⁰) as a function of temperature to 200°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 ΔG_r/⁰ for these and countless other reactions, the apparent standard molal Gibbs free energies of formation (ΔG⁰) at temperatures to 200°C are given for 307 solids, liquids, gases, and aqueous solutes. It is shown that values of ΔG_r/⁰ for many microbially mediated reactions are highly temperature dependent, and that adopting values determined at 25°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 (ΔG_r) 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.