20 BACTERIAL FERMENTATIONS 



The postulated intermediate role of carbon dioxide was 

 tested by experiments with C 14 -carbon dioxide or C 14 - 

 propionate. Without discussing the experimental data in 

 detail, it may be stated that the results could be interpreted 

 to indicate that approximately 1 mole of carbon dioxide is 

 formed per mole of propionate, in accordance with equa- 

 tion 7, and that carbon dioxide is a precursor of most, if 

 not all of the methane, as indicated in equation 8. 



Buswell and his associates 24 have also done tracer experi- 

 ments on the fermentation of propionate using enrichment 

 cultures, presumably containing a mixture of species, capa- 

 ble of converting propionate completely to carbon dioxide 

 and methane without the accumulation of acetate. These 

 cultures were shown to use carbon dioxide and to convert 

 all three carbon atoms of propionate to both methane and 

 carbon dioxide in varying degrees. Part of these results can 

 be interpreted in terms of the reactions already discussed 

 in combination with a secondary decomposition of acetate. 

 However, some additional reaction would be required to 

 account for the preferential conversion of the a carbon of 

 propionate to methane and a substantial conversion of the 

 /? carbon to carbon dioxide. These results could be caused 

 by the formation of a symmetrical intermediate like suc- 

 cinate from propionate. This would allow a randomization 

 of the a and ft carbons of propionate and therefore would 

 account for the similarity in their behavior. Other expla- 

 nations are also possible. This type of problem probably 

 ( nnot be resolved until pure cultures of all of the organ- 

 i ms involved are available. 



The results which I have mentioned, as well as others, 

 show conclusively that, with many substrates and at least 

 several species of methane bacteria, methane is formed 

 mainly or wholly by reduction of carbon dioxide. How- 

 ever, at least two compounds, methyl alcohol and acetate, 



