FUMARATE DISSIMILATION AND DIFFERENTIAL REDUCTANT FLOW BY CLOSTRIDIUM-FORMICOACETICUM AND CLOSTRIDIUM-ACETICUM

Methanol and the O-methyl group of vanillate did not support the growth of Clostridium formicoaceticum in defined medium under CO2-limited conditions, however, they were growth supportive when fumarate was provided concomitantly. Fumarate alone was not growth supportive under these conditions. Fumarate reduction (dissimilation) to succinate was the predominant electron-accepting, energy-conserving process for methanol-derived reductant under CO2-limited conditions. However, when both reductant sinks, i.e., fumarate and CO2, were available, reductant was redirected towards CO2 in defined medium. In contrast, in undefined medium with both reductant sinks available, C. formicoaceticum simultaneously engaged fumarate dismutation and the concomitant usage of CO2 and fumarate as reductant sinks. With Clostridium aceticum, fumarate also substituted for CO2, and H-2 became growth supportive under CO2-limited conditions. Fumarate dissimilation was the predominant electron-accepting process under CO2-limited conditions; however, when both reductant sinks were available, H-2-derived reductant was routed towards CO2, indicating that acetogenesis was the preferred electron-accepting process when reductant flow originated from H-2. Collectively, these findings indicate that fumarate dissimilation, not dismutation, is selectively used under certain conditions and that such usage of fumarate is subject to complex regulation.