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

Methanol and the O-methyl group of vanillate did not support the growt h of Clostridium formicoaceticum in defined medium under CO2-limited c onditions, however, they were growth supportive when fumarate was prov ided concomitantly. Fumarate alone was not growth supportive under the se conditions. Fumarate reduction (dissimilation) to succinate was the predominant electron-accepting, energy-conserving process for methano l-derived reductant under CO2-limited conditions. However, when both r eductant sinks, i.e., fumarate and CO2, were available, reductant was redirected towards CO2 in defined medium. In contrast, in undefined me dium with both reductant sinks available, C. formicoaceticum simultane ously engaged fumarate dismutation and the concomitant usage of CO2 an d fumarate as reductant sinks. With Clostridium aceticum, fumarate als o substituted for CO2, and H-2 became growth supportive under CO2-limi ted conditions. Fumarate dissimilation was the predominant electron-ac cepting process under CO2-limited conditions; however, when both reduc tant sinks were available, H-2-derived reductant was routed towards CO 2, indicating that acetogenesis was the preferred electron-accepting p rocess when reductant flow originated from H-2. Collectively, these fi ndings indicate that fumarate dissimilation, not dismutation, is selec tively used under certain conditions and that such usage of fumarate i s subject to complex regulation.