BIOLOGICAL FORMATION OF METHANE 23 



that the methyl group of methanol, like that of acetate, is 

 transferred intact into methane. The lower percentage of 

 deuterium in the methane could be attributed to a prefer- 

 ential utilization of the mass 1 hydrogen from the water. 



4CH 3 OH -2$. 3CH 3 D + C0 2 (14) 



The specific reactions involved in methane formation 

 from carbon dioxide, acetate, or methanol are not known. 

 The simplest mechanism proposed for the conversion of 

 carbon dioxide to methane is a stepwise reduction involving 

 formate or carbon monoxide, formaldehyde, and methanol 

 as successive intermediates. A faint suggestion of such a 

 sequence was provided by Stephenson and Stickland, 29 who 

 claim to have isolated a bacterium capable of reducing 

 carbon dioxide, carbon monoxide, formate, formaldehyde, 

 and methyl alcohol to methane using hydrogen as a reduc- 

 tant. However, their experiments do not provide any 

 substantial evidence for a sequential reduction of these 

 compounds. Carbon monoxide, formaldehyde, and meth- 

 anol were reduced far too slowly to be intermediates. 

 Formate was shown to be converted to hydrogen and 

 carbon dioxide more rapidly than to methane; conse- 

 quently carbon dioxide actually appeared to be an inter- 

 mediate in formate reduction rather than the reverse. 



Kluyver and Schnellen 14 later reinvestigated the fermen- 

 tation and reduction of one-carbon compounds in the 

 presence of hydrogen by pure cultures of three species of 

 methane bacteria. Cell suspensions of Mbac. omelianskii 

 were unable to reduce any one-carbon compound at a sig- 

 nificant rate, except carbon dioxide, which reacted rapidly. 

 Methanosarcina barkerii, which ferments methanol, ace- 

 tate, and carbon monoxide, was unable to reduce formate 

 or formaldehyde. A mixture of carbon monoxide and 



