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C. acetobutylicum is an anaerobic bacterium that produces acetic and butyric acids, hydrogen gas, and carbon dioxide during the exponential phase of growth. When the culture pH is allowed to remain near 4.5, the metabolism switches to the production of the neutral compounds (solvents) - acetone, n-butanol, and ethanol. The two metabolic phases are known as the acidogenic and solventogenic phases. The enzyme hydrogenase plays an important role in this bacterium because it converts excess reducing power into hydrogen gas to maintain a balance in the oxidation-reduction state in the cell. During solventogenesis, additional reducing power is used in the production of n-butanol and ethanol, which leaves excess reducing power to be vented as hydrogen gas. There are conflicting reports about the level of hydrogenase in acidogenic and solvento...

(ABSTRACT) The onset of solvent production by the clostridia involves regulation at the transcriptional level. The signal triggering the onset has not been identified, but redox and energetic states have been suggested as possible factors. Because several solvent-producing clostridia, including Clostridium acetobutylicum and Clostridium beijerinckii, are nitrogen-fixing organisms and both nitrogen-fixation and alcohol production (n-butanol, isopropanol and ethanol) are reductant-dependent processes, the effect of nitrogen fixation on the onset and progression of solvent production in C. beijerinckii NRRL B593 and vice versa was investigated. For this purpose, a defined growth medium containing three amino acids was developed for C. beijerinckii NRRL B593, and this medium was used for growing solvent-producing and nitrogen-fixing cultur...