BIOLOGICAL FORMATION OF METHANE 25 



would possess several types of chemical machinery for 

 making methane from different substrates. 



A schematic representation of the possible pathways of 

 carbon in methane formation, which combines the infor- 

 mation obtained from studies of the acetate and methanol 

 fermentations with the carbon dioxide reduction theory of 

 van Niel, is presented in Fig. 1. Carbon dioxide is postu- 



C0 2 + XH — *• XCOOH 



CH3OH + XH 



CH3COOH +XH4- XCH 3 



XH + CH 4 



Fig. I. Possible Pathways of Methane Formation. 



lated to combine with an unidentified organic compound 

 XH to form a carboxylated derivative of X which is then 

 reduced by three successive steps to a methyl derivative of 

 X which on further reduction yields methane and regener- 

 ates the carbon dioxide acceptor XH. Methanol and 

 acetate are postulated to react also with XH to give the 

 intermediate XCH 3 of the carbon dioxide reduction path- 

 way. The direct formation of this intermediate from the 

 organic methyl donors might well inhibit carbon dioxide 

 utilization by lowering the concentration of XH or in other 

 ways and thus account for the absence of carbon dioxide 

 reduction with these substrates. 



