24 BACTERIAL FERMENTATIONS 



hydrogen was converted to methane according to equa- 

 tion 15. 



CO + 3H 2 —> CH 4 + H 2 (15) 



However, convincing evidence was obtained that this re- 

 action involves two steps with carbon dioxide being an 

 intermediate (equations 16 and 17) . Methanobacterium 



CO + H 2 — *- C0 2 + H 2 (16) 



C0 2 + 4H 2 — > CH 4 + 2H 2 (17) 



formicicum ferments both carbon monoxide and formate, 

 and reduces carbon dioxide with hydrogen, but is unable 

 to reduce formaldehyde or methanol. These observations 

 of Kluyver and Schnellen clearly exclude formate, carbon 

 monoxide, formaldehyde, and methanol as intermediates 

 in carbon dioxide reduction despite the fact that all of these 

 compounds, except possibly formaldehyde, can be metab- 

 olized by some species. Therefore it is virtually certain 

 that the intermediates in methane formation are not one- 

 carbon compounds. 



The only available information which seems to provide 

 evidence concerning the path of carbon dioxide reduc- 

 tion to methane is derived from the previously mentioned 

 studies on the acetate and methanol fermentations, which 

 curiously enough do not involve carbon dioxide. The argu- 

 ment might be made that these fermentations are entirely 

 unrelated processes and have no bearing on the path of 

 carbon dioxide reduction. However, there is a good 

 reason for rejecting this view, namely, the fact that the 

 Methanosarcina species which ferments acetate and meth- 

 anol can also reduce carbon dioxide to methane with 

 hydrogen; 9 it seems highly improbable that one organism 



