BIOLOGICAL FORMATION OF METHANE 21[ 



are converted to methane by chemical pathways not involv- 

 ing carbon dioxide. 



Schnellen, 9 working in Kluyver's laboratory, showed that 

 methyl alcohol is readily fermented by a species of Methano- 

 sarcina according to equation 10. At first this reaction was 



4CH 3 OH — > 3CH 4 + CO2 + 2H 2 (10) 



thought to be the result of a complete oxidation of the alco- 

 hol coupled with the reduction of three-fourths of the 

 resulting carbon dioxide. However, this possibility was 

 excluded by tracer experiments which showed that less 

 than 1% of the methane was derived from carbon dioxide. 25 

 Therefore the methane must be formed more or less directly 

 by reduction of methanol. 



With acetate, which is decomposed by several species of 

 methane bacteria, the situation is similar. The fermenta- 

 tion of acetate is equivalent to a decarboxylation. As we 

 have seen, according to van Niel's theory, acetate should be 

 completely oxidized to carbon dioxide, half of which is 

 simultaneously reduced to methane. This mechanism was 

 first critically tested by Buswell and Sollo 26 in 1948 by the 

 use of C 14 -labeled carbon dioxide. They demonstrated that 

 essentially none of the methane is derived from carbon 

 dioxide. Subsequently Thressa Stadtman and I 23 - 25 showed 

 by the tracer method that the methane is formed entirely 

 from the methyl carbon, and the carbon dioxide exclusively 

 from the carboxyl carbon, of acetate (equation 11) . These 



G*H 3 C°OOH — -> C*H 4 + C°0 2 (1 1) 



results are inconsistent with the carbon dioxide reduction 

 theory since the methyl carbon of acetate is not oxidized 

 to carbon dioxide and carbon dioxide is not a precursor 

 of methane. 



