Reduced product formation following perturbation of ethanol- and propionate-fed methanogenic CSTRs.

Energetic analysis was applied to reduced product formation following perturbation of ethanol- and propionate-fed methanogenic continuous stirred tank reactors (CSTRs). Formation and dissipation of longer-chained n-carboxylic acids corresponded with the variation in Gibbs free energy change associated with beta-oxidation reactions. Formation appeared to occur from acetate and propionate by reductive back-reactions, made energetically favorable by elevated hydrogen partial pressure (P(H(2))), and possibly mediated by biosynthetic enzymes. The formed longer-chained acids dissipated when the P(H(2)) fell and equilibrium shifted to favor beta-oxidations. n-Propanol was found to be produced from propionate in a coupled ethanol oxidation/propionate reduction reaction, mediated by ethanol-oxidizing organisms during high rates of ethanol utilization and elevated P(H(2)). When P(H(2)) declined, n-propanol was oxidized back to its precursor propionate. Both reaction energetics and intracellular diffusion of the electron carrier may effect transient mediation of this coupled reaction.