METHANE-PRODUCING BACTERIA 121 



that they, too, use it mainly or exclusively as a source of hydrogen and 

 energy, but prepare their cell matter by the reduction of carbon dioxide. 

 However, the metabolism of most of these bacteria is not sufficiently 

 known to allow one to assert that they do not use at least a part of the 

 organic substrate for direct heterotrophic assimilation — especially since 

 we know, from the example of the purple Athiorhodaceae, that synthesis 

 of carbohydrates by the reduction of carbon dioxide can often be coupled 

 with heterotrophic assimilation of one part of the reductant. 



Only one type of bacteria which uses organic substrates for the 

 reduction of carbon dioxide shall be described here, the methane-producing 

 bacteria, which were discovered in 1910 by Sohngen (who had previously 

 discovered the methane-burning bacteria). IMethane is produced by the 

 fermentation of many organic substrates; these processes have been 

 investigated, e. g., by Neave and Buswell (1930), Fischer, Lieske and 

 AVinzer (1931, 1932), Stephenson and Stickland (1933), Barker (1936i'2, 

 1937), and Barker, Ruben and Kamen (1940). One thinks, at first, 

 that methane must be the product of dismutation of an organic substrate, 

 as, for example, in the simplest case of the acetate fermentation: 



(5.34) CH3COOH > CH4 + CO2 - 6 kcal 



(Decarboxylation can be considered as a special case of dismutation in 

 which one part of the molecule is oxidized to carbon dioxide.) However, 

 Sohngen noticed that the same bacteria which cause methane fermenta- 

 tion of organic substrates, also reduce carbon dioxide to methane in the 

 presence of molecular hydrogen : 



(5.35) CO2 + 4 H2 > CH4 + 2 H2O + 62 kcal 



More recently, Barker (1936-) found a species of methane bacteria which 

 reduces carbon dioxide to methane by means of ethanol: 



(5.36) CO2 + 2 CaHsOH > CH4 + 2 CH3COOH + 21 kcal 



These examples make it probable that even in methane fermentations 

 which proceed with a net liberation of carbon dioxide, as in (5.34), the 

 way to methane leads through carbon dioxide. Arguments in favor of 

 this hypothesis were adduced by Barker (1936, 1937). He found, for 

 example, that in the gradual decomposition of butanol by methane 

 bacteria, the first stage conforms to the equation: 



(5.37) 2 C4H9OH + CO2 > 2 C3H7COOH + CH4 + 21 kcal 



and the second stage to the eciuation: 



(5.38) 2 C3H7COOH + CO2 + 2 H2O > 4 CH3COOH + CH4 + 9 kcal 



while, in the third stage, four molecules of carbon dioxide are liberated, 

 according to the over-all equation (5.34), thus giving a net production 



