BIOLOGICAL FORMATION OF METHANE 13 



can be said with certainty only for those species which 

 have been studied in pure culture. These species all grow 

 satisfactorily in media containing the usual nutritive salts, 

 carbon dioxide, a reducing agent, a single oxidizable com- 

 pound suitable for the organism, and an ammonium salt 

 as a source of nitrogen. 8 One species, Mbac. omelianskii, 

 has been shown to utilize nitrogen gas. 19 The general 

 physiological properties of the methane bacteria suggest, by 

 comparison with other nitrogen-fixing bacteria, that the 

 ability to fix nitrogen may be a common characteristic of 

 the group. The addition of extracts containing amino 

 acids, growth factors, and other nutritional supplements to 

 synthetic media does not have a beneficial effect on the 

 rate or magnitude of growth of the few species that have 

 been studied in this respect. 



Special attention should be drawn to the carbon dioxide 

 requirement of methane bacteria. Qualitatively, there is 

 nothing unusual in this since all microorganisms require 

 small amounts of carbon dioxide to initiate and maintain 

 growth. However, several species of methane bacteria need 

 a large amount of carbon dioxide which they use as a major 

 substrate. 20 The specific role of carbon dioxide in the 

 energy metabolism of methane bacteria is considered in a 

 later section. 



pH Range. Another environmental factor that is im- 

 portant for the methane bacteria is the hydrogen ion 

 concentration. As a general rule these bacteria are most 

 active in the pH range from 6.4 to 7.2. Below pH 6 and 

 above pH 8 the growth rate of methane bacteria falls off 

 rapidly. At the high pH this may be the result of the 

 greatly diminished concentration of free carbon dioxide. 

 There is one exception to the generalization that an alka^ 

 line medium is unfavorable. The formate-fermenting 

 species Mc. vannielii grows best between pH 8 and 9. 10 



