Ethanol is one of the major desirable products of syngas fermentation. Ethanol is com­monly used as a direct additive to gasoline. It has an octane value of 129 and the energy con­tent is about 70% of that of gasoline. Most of the syngas-fermenting microbes use acetyl-CoA pathway to produce ethanol. During that process, CO and H2 are oxidized and produce electrons and H+ ions necessary for the reactions, while CO2 gets reduced to Co-CH3 by accepting the electrons and H+ ions. Toward the end of the pathway, Co-CH3 and Co-A react with CO and produce acetyl-CoA under the influence of CODH and acetyl-CoA synthase enzymes. Acetyl-CoA acts as a building block for the production of a variety of biofuels including ethanol (Figure 1).

C. Ijungdahlii is one of the most frequently used microorganisms in syngas fermentation to eth­anol. Younesi et al. (2005) achieved an ethanol concentration of 0.6 g/L maintaining a syngas pressure of 1.8 atm in their bioreactor. The authors further reported that the high syngas pressure did not have a significant impact on acetic acid production, though it enhanced the ethanol yield. Klasson et al. (1990) reported a higher ethanol yield (3.0 g/L) by adding 0.02% yeast extract followed by cellobiose. The study further showed improvement in molar ratio of ethanol to ace­tate (>1.1) with the addition of 30 mg/L benzyl-viologen. Klasson et al. (1993) reported the highest ethanol concentration ever recorded (^48 g/L) with C. Ijungdahlii at a pH of 4.0-4.5 in a completely stirred tank reactor under nutrient-limited condition during 560 h of fermentation.