BIOLOGICAL FORMATION OF METHANE n 



a methane fermentation. This is true in the sense that 

 these substrates are decomposed by crude cultures with the 

 formation of methane and carbon dioxide. However, since 

 these compounds are rapidly decomposed by many common 

 bacteria that do not produce methane, it is virtually certain 

 that the conversion of these substrates to methane is a two- 

 or multistage biological process in which various bacteria 

 that cannot form methane convert the substrates to volatile 

 fatty acids, alcohols, and other common fermentation 

 products which are then transformed by methane bacteria 

 to their characteristic products. 



Species Substrate Specificity. In addition to the 

 apparently severe limitations of the methane bacteria as a 

 group with respect to utilizable substrates, each species 

 characteristically is restricted to the use of a few com- 

 pounds. A few examples of the substrate specificity of 

 different species are shown in Table 3. Methanobacterium 



TABLE 3 



Substrate Specificity of Some Methane Bacteria 



Species Oxidizable Substrates 



Methanobacterium for micicum H2, CO, formate 



Methanobacillus omelianskii H2, ethanol, primary and secondary 



alcohols 

 Methanobacterium suboxydans Butyrate, valerate, caproate 



Methanosarcina barkerii H2, CO, methanol, acetate 



formicicum 9 oxidizes only hydrogen, carbon monoxide and 

 formate. Methanobacillus omelianskii cannot oxidize car- 

 bon monoxide or formate but specializes in the decom- 

 position of primary and secondary short-chain aliphatic 

 alcohols 17 and hydrogen. 18 Methanobacterium suboxydans 

 cannot attack any of the previously mentioned compounds 

 except possibly hydrogen which was not tested, but spe- 



