Kinetics of syntrophic cultures: A theoretical treatise on butyrate fermentation

Numerous microbial conversions in methanogenic environments proceed at (Gib bs) free energy changes close to thermodynamic equilibrium. In this paper w e attempt to describe the consequences of this thermodynamic boundary condi tion on the kinetics of anaerobic conversions in methanogen ic environments . The anaerobic fermentation of butyrate is used as an example. Based on a simple metabolic network stoichiometry, the free energy change based balanc es in the cell, and the flux of substrates and products in the catabolic an d anabolic reactions are coupled. In butyrate oxidation, a mechanism of ATP -dependent reversed electron transfer has been proposed to drive the unfavo rable oxidation of butyryl-CoA to crotonyl-CoA. A major assumption in our m odel is that ATP-consumption and electron translocation across the cytoplas mic membrane do not proceed according to a fixed stoichiometry, but depend on the cellular concentration ratio of ATP and ADP. The energetic and kinet ic impact of product inhibition by acetate and hydrogen are described. A ma jor consequence of the derived model is that Monod-based kinetic descriptio n of this type of conversions is not feasible, because substrate conversion and biomass growth are proposed to be uncoupled. It furthermore suggests t hat 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 stoichiometr ies for the membrane associated processes, the required flexibility of anae robic bacteria to adapt to varying environmental conditions can be describe d. (C) 2000 John Wiley & Sons, Inc.