Kinetics of syntrophic cultures: a theoretical treatise on butyrate fermentation

Numerous microbial conversions in methanogenic environments proceed at (Gibbs) free energy changes close to thermodynamic equilibrium. In this paper we attempt to describe the consequences of this thermodynamic boundary condition on the kinetics of anaerobic conversions in methanogenic environments. The anaerobic fermentation of butyrate is used as an example. Based on a simple metabolic network stoichiometry, the free energy change based balances in the cell, and the flux of substrates and products in the catabolic and anabolic reactions are coupled. In butyrate oxidation, a mechanism of ATP-dependent reversed electron transfer has been proposed to drive the unfavorable oxidation of butyryl-CoA to crotonyl-CoA. A major assumption in our model is that ATP-consumption and electron translocation across the cytoplasmic membrane do not proceed according to a fixed stoichiometry, but depend on the cellular concentration ratio of ATP and ADP. The energetic and kinetic impact of product inhibition by acetate and hydrogen are described. A major consequence of the derived model is that Monod-based kinetic description of this type of conversions is not feasible, because substrate conversion and biomass growth are proposed to be uncoupled. It furthermore suggests that the specific substrate conversion rate cannot be described as a single function of the driving force of the catabolic reaction but depends on the actual substrate and product concentrations. By using nonfixed stoichiometries for the membrane associated processes, the required flexibility of anaerobic bacteria to adapt to varying environmental conditions can be described.