Bioenergetic Conditions of Butyrate Metabolism by a Syntrophic, Anaerobic Bacterium in Coculture with Hydrogen-Oxidizing Methanogenic and Sulfidogenic Bacteria

The butyrate-oxidizing, proton-reducing, obligately anaerobic bacterium NSF-2 was grown in batch cocultures with either the hydrogen-oxidizing bacterium Methanospirillum hungatei PM-1 or Desulfovibrio sp. strain PS-1. Metabolism of butyrate occurred in two phases. The first phase exhibited exponential growth kinetics (phase a) and had a doubling time of 10 h. This value was independent of whether NSF-2 was cultured with a methanogen or a sulfate reducer and likely represents the maximum specific growth rate of NSF-2. This exponential growth phase was followed by a second phase with a nearly constant rate of degradation (phase b) which dominated the time course of butyrate degradation. The specific activity of H₂ uptake by the hydrogen-oxidizing bacterium controlled the bioenergetic conditions of metabolism in phase b. During this phase both the Gibbs free energy (ΔG′) and the butyrate degradation rate (v) were greater for NSF-2-Desulfovibrio sp. strain PS-1 (ΔG′ = −17.0 kJ/mol; v = 0.20 mM/h) than for NSF-2-M. hungatei PM-1 (ΔG′ = −3.8 kJ/mol, v = 0.12 mM/h). The ΔG′ value remained stable and characteristic of the two hydrogen oxidizers during phase b. The stable ΔG′ resulted from the close coupling of the rates of butyrate and H₂ oxidation. The addition of 2-bromoethanesulfonate to a NSF-2-methanogen coculture resulted in the total inhibition of butyrate degradation; the inhibition was relieved when Desulfovibrio sp. strain PS-1 was added as a new H₂ sink. When the specific activity of H₂ consumption was increased by adding higher densities of the Desulfovibrio sp. to 2-bromoethanesulfonate-inhibited NSF-2-methanogen cocultures, lower H₂ pool sizes and higher rates of butyrate degradation resulted. Thus, it is the kinetic parameters of H₂ consumption, not the type of H₂ consumer per se, that establishes the thermodynamic conditions which in turn control the rate of fatty acid degradation. The bioenergetic homeostasis we observed in phase b was a result of the kinetics of the coculture members and the feedback inhibition by hydrogen which prevents butyrate degradation rates from reaching their theoretical V_max.