BIOLOGICAL FORMATION OF METHANE 15 



family, which might well be called Methanobacteriaceae. 

 Within the physiological family, four genera can be recog- 

 nized on the basis of morphology (Table 4) . The known 

 methane bacteria are either rods or cocci. The rod-shaped 

 organisms may be further subdivided into two groups on 

 the basis of sporulation. The non-sporulating types are 

 placed in the genus Methanobacterium, and the sporulating 

 types in the genus Methanobacillus* The spherical organ- 

 isms whose cells occur singly, in pairs, or in irregular 

 masses are put in the genus Methanococcus, whereas the 

 organisms with more or less spherical cells arranged in 

 cubical sarcina packets are included in the genus Methano- 

 sarcina. The species in these four genera differ with respect 

 to size and arrangement of cells, presence or absence of 

 flagella, or the ability to utilize various substrates. Sub- 

 strate specificity has been of particular value in species 

 identification; therefore the substrates known to be attacked 

 by different species are included in Table 4 following the 

 species name. So far, a total of eight species of methane 

 bacteria have been described in these four genera. These 

 species have been obtained entirely from sewage sludge and 

 black mud. Undoubtedly many more species will be dis- 

 covered as more adequate isolation methods are developed, 

 and as the organisms associated with other habitats, such 

 as the digestive tracts of ruminants, are systematically 

 investigated. 



Chemistry of Methane Fermentations 



The early studies on the chemistry of methane fermenta- 

 tions were done entirely with enrichment cultures contain- 

 ing a single substrate and a mixture of organisms. The 



# The one species of this genus that has been studied extensively, 

 Mbac. omelianskii, has previously been included in the genus Methano- 

 bacterium. 



