18 BACTERIAL FERMENTATIONS 



mentations of organic compounds. By now a considerable 

 amount of information has accumulated which shows that 

 the theory is correct for fermentations of many, but not 

 all, substrates. 



Examples of the reduction of carbon dioxide to methane 

 are provided by the fermentations of ethanol by Mbac. 

 omelianskii, of butyrate by Mbact. suboxydans, and of pro- 

 pionate by Mbact. propionicum. With all three of these 

 species, the interpretation of the experimental data is greatly 

 simplified by the fact that they oxidize their substrates to 

 acetate which is not further decomposed. 



Methanobacillus omelianskii oxidizes ethyl alcohol almost 

 quantitatively to acetate according to equation 5. 17 The 



2CH3CH2OH + C0 2 — ► 2CH3COOH + CH 4 (5) 



oxidation of alcohol is completely dependent upon the 

 supply of carbon dioxide; when the carbon dioxide is all 

 consumed the oxidation of alcohol stops. Other oxidants, 

 such as sulfate and nitrate, cannot replace carbon dioxide. 

 Now the above equation strongly indicates that the acetate 

 is derived from the alcohol, and that the methane is derived 

 from carbon dioxide. To confirm this interpretation by 

 an independent method, an experiment was done in which 

 unlabeled ethyl alcohol was incubated with C 14 -labeled 

 carbon dioxide. 23 At the end of the fermentation, the C 14 

 content per mole of the methane was found to be essen- 

 tially equal to that of the carbon dioxide, thus proving 

 that the methane produced in this fermentation is derived 

 entirely from carbon dioxide. 



The fermentation of butyrate by Mbact. suboxydans is 

 very similar in principle. 11 The oxidation of 2 moles of 

 butyrate to 4 moles of acetate is coupled with the reduc- 

 tion of 1 mole of carbon dioxide to methane according to 



