BIOLOGICAL FORMATION OF METHANE 19 



equation 6. In this fermentation also tracer experiments 

 have shown that at least 98% of the methane is derived 

 from carbon dioxide. 



2CH 3 CH 2 CH 2 COOH + 2H 2 + C0 2 — > 



4GH 3 COOH + CH 4 (6) 



In the preceding examples of methane fermentations 

 involving carbon dioxide reduction, essentially no carbon 

 dioxide is formed by the oxidation of the organic sub- 

 strate. The fermentation of propionate by Mbact. pro- 

 pionicum, shown in the following equations, is somewhat 

 more complicated because it involves both carbon dioxide 

 formation and utilization. 11 



Oxidation : 4CH 3 CH 2 COOH + 8H2O — > 



4GH 3 COOH + 4C0 2 + 24H (7) 



Reduction : 3C0 2 + 24H — > 3CH 4 + 6H 2 (8) 



Observed : 4CH 3 CH 2 COOH + 2H 2 — > 



4CH 3 COOH + C0 2 + 3CH 4 (9) 



The observed reaction, shown in equation 9, results in the 

 production of 1 mole of carbon dioxide from 4 moles of 

 propionate. However, although 1 mole of carbon dioxide 

 accumulates, it is reasonable to assume, on the basis of what 

 is known about the oxidation of propionate in other bio- 

 logical systems, that the oxidation of 4 moles of propionate 

 initially gives 4 moles of acetate, 4 moles of carbon dioxide, 

 and 24 "hydrogens" as is shown in equation 7. If these 

 24 "hydrogens" serve to reduce carbon dioxide, 3 moles 

 would be converted to methane as is shown in equation 8. 

 This accounts for the observed yields of methane and carbon 

 dioxide (equation 9) . 



