2-14: Effect of Gas Sparging on Pyruvate Catabolism during Batch Fermentation of Clostridium thermocellum ATCC 27405

Growth, end-product synthesis, enzyme activities and associated transcription involved in pyruvate catabolism, H2 synthesis, and ethanol production were studied in the cellulolytic anaerobe, Clostridium thermocellum ATCC 27405, during batch fermentation of cellobiose to determine the effect of elevated N2 and H2 sparging on metabolism using a 14 liter fermenter. End product profiles revealed increases in acetate, CO2 and H2 production under high N2 sparging with respect to standard N2 gas flow rates. Alternatively, elevated H2 sparging shifted carbon and electron flow towards the production of formate and ethanol. Transcription levels of key genes involved in pyruvate catabolism as well as hydrogen and ethanol synthesis and associated enzyme activities (pyruvate:ferredoxin oxidoreductase, pyruvate:formate lyase, lactate dehydrogenase, NAD- and NADP- dependent alcohol dehydrogenase, methyl viologen- ferredoxin-, NAD- and NADP-dependent hydrogenase) revealed no significant differences under the conditions tested, with the exception of the hydrogenases. The results presented here suggest that gas sparging can be effectively used to shift carbon and electron flow and the observed shifts at the pyruvate branch-point are principally influenced by the availability of reduced electron carriers (NAD, NADP, ferredoxin) and thermodynamic considerations.