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

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Abstract
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), and possibly mediated by biosynthetic enzymes. The formed longer‐chained acids dissipated when the P 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. When P 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.