BIOLOGICAL FORMATION OF METHANE 17 



suggested the so-called "carbon dioxide reduction theory." 20 

 This theory postulates that the organic compounds fer- 

 mented by methane bacteria are oxidized completely to 

 carbon dioxide, and that this oxidation is coupled with a 

 reduction of some or all of the carbon dioxide to methane. 

 The application of this idea to the fermentation of acetate 

 is shown in the following equations. 



Oxidation: CH3COOH + 2H 2 — > 2C0 2 + 8H (1) 

 Reduction : 8H + CO2 — ■> CK 4 + 2H 2 (2) 



Net: CH3COOH — > CH 4 + C0 2 (3) 



The oxidation of acetate, according to the theory, gives 

 2 moles of carbon dioxide and 8 equivalents of hydrogen. 

 One mole of carbon dioxide is reduced to methane and 

 water. The net result, the conversion of 1 mole of acetate 

 to 1 mole each of methane and carbon dioxide, is in accord- 

 ance with the experimental facts. 



The main evidence for the general theory at the time it 

 was developed was provided by the early observations of 

 Sohngen 22 on the fermentation of a mixture of hydrogen 

 and carbon dioxide. Sohngen found that enrichment cul- 

 tures could couple the oxidation of hydrogen with the 

 reduction of carbon dioxide to methane according to equa- 

 tion 4. Sohngen's experimental observations were later 



4H 2 + C0 2 — > CH 4 + 2H 2 (4) 



confirmed in several laboratories by the use of pure 

 cultures. 14,18 



Much of the subsequent work on the chemistry of the 

 fermentation has been concerned with determining whether 

 the carbon dioxide reduction theory also applies to fer- 



