BIOLOGICAL FORMATION OF METHANE 



ing the ability of CI. perfringens and other species to form 

 methane. 



Compounds Fermented. Methane bacteria specialize 

 not only in the chemical mechanisms of energy metabolism 

 but also in respect to the types of substrates which they 

 utilize. For some obscure reason, all of the methane 

 bacteria whose substrate requirements have been studied 

 effectively by the use of pure or nearly pure cultures are 

 unable to decompose the more usual substrates for bacteria 

 such as carbohydrates and amino acids. Early reports of 

 cellulose- or glucose-fermenting methane bacteria have not 

 been confirmed; probably they were based on observations 

 with mixed cultures containing both carbohydrate-ferment- 

 ing and methane-producing species. Of course the possi- 

 bility that still unknown species of carbohydrate- or amino 

 acid-fermenting, methane-producing bacteria do exist can- 

 not be excluded, but convincing evidence for such organisms 

 is still lacking. 



As a group the methane bacteria appear to be restricted 

 to the utilization of relatively simple organic and inorganic 

 compounds, many of which are products of the better- 

 known types of bacterial fermentations. Table 1 gives a 

 list of the oxidizable substrates that have been shown by 

 studies with pure or nearly pure cultures to be decomposed 

 by methane bacteria. These substrates fall into three 

 groups, the lower normal fatty acids containing from one 

 to six carbon atoms, the normal and iso alcohols contain- 

 ing from one to five carbon atoms, and three inorganic 

 gases, hydrogen, carbon monoxide, 14 .and carbon dioxide. 

 In Table 2 are listed some additional compounds that are 

 probably decomposed by methane bacteria, judging from 

 observations made with enrichment cultures. These com- 

 pounds include several long-chain fatty acids, dicarboxylic 

 acids, aromatic compounds, acetone, and butylene glycol. 



