Hydrogen Partial Pressures in a Thermophilic Acetate-Oxidizing Methanogenic Coculture

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Abstract
Hydrogen partial pressures were measured in a thermophilic coculture comprised of a eubacterial rod which oxidized acetate to H2 and CO2 and a hydrogenotrophic methanogen, Methanobacterium sp. strain THF. Zinder and Koch (S. H. Zinder and M. Koch, Arch. Microbiol. 138:263-272, 1984) originally predicted, on the basis of calculations of Gibbs free energies of reactions, that the H2 partial pressure near the midpoint of growth of the coculture should be near 4 Pa (ca. 4 × 10−5 atm; ca. 0.024 μM dissolved H2) for both organisms to be able to conserve energy for growth. H2 partial pressures in the coculture were measured to be between 20 and 50 Pa (0.12 to 0.30 μM) during acetate utilization, approximately one order of magnitude higher than originally predicted. However, when ΔGf (free energy of formation) values were corrected for 60°C by using the relationship ΔGf = ΔHf − TΔS (ΔHf is the enthalpy or heat of formation, ΔS is the entropy value, and T is the temperature in kelvins), the predicted value was near 15 Pa, in closer agreement with the experimentally determined values. The coculture also oxidized ethanol to acetate, a more thermodynamically favorable reaction than oxidation of acetate to CO2. During ethanol oxidation, the H2 partial pressure reached values as high as 200 Pa. Acetate was not used until after the ethanol was consumed and the H2 partial pressure decreased to 40 to 50 Pa. After acetate utilization, H2 partial pressures fell to approximately 10 Pa and remained there, indicating a threshold for H2 utilization by the methanogen. Axenic cultures of the acetate-oxidizing organism were combined with pure cultures of either Methanobacterium sp. strain THF or Methanobacterium thermoautotrophicum ΔH to form reconstituted acetate-oxidizing cocultures. The H2 partial pressures measured in both of these reconstituted cocultures were similar to those measured in the original acetate-oxidizing rod coculture. Since M. thermoautotrophicum ΔH did not use formate as a substrate, formate is not necessarily involved in interspecies electron transfer in this coculture.