Solventogenesis in Clostridium acetobutylicum fermentations related to carboxylic acid and proton concentrations

Abstract

The mechanism primarily implicated in the solventogenesis process in batch fermentations of Clostridium acetobutylicum is examined in considerable detail. A variety of fermentations with or without pH control in the pH range of 3.7–6 have been carried out in order to examine which of a host of suspect parameters correlate with the initiation of solventogenesis. The parameters that did not correlate are the external (pH₀) and intracellular (pH_i) pH, and ΔpH, and the external or intracellular butyrate and acetate concentrations. Undissociated butyric acid (UBA) correlated well with the initiation of solventogenesis. A linear relationship between UBA and butanol concentrations was found at the onset of solventogenesis in all fermentations examined. The intercept of this linear relationship was 6–13mM UBA for the pH₀ range of 3.7–5 and approximately zero for pH₀ at or above 6. The required minimal UBA was interpreted as a dependency of the solventogenesis process on both H⁺ and butyrate concentrations. Undissociated acetic acid was found not to correlate with the initiation of solventogenesis. Addition of acetoacetate (AA) and propionate enhanced the effect of UBA on the solventogenesis process. The action of a nonmetabolizable (FCCP) and a metabolizable (AA) uncoupler on the ΔpH, pH₀, pH_i, and solventogenesis were also studied in order to gain further understanding of the solventogenesis mechanism.