ROLE OF AUTOTROPHIC BACTERIA IN NATURE 123 



The fact that the methane bacteria are capable of reducing carbon 

 dioxide to methane, shows that they have developed a mechanism which 

 avoids the intermediate formation of a carbohydrate (because the latter 

 would present an "energy barrier" which is insurmountable at ordinary 

 temperatures). Under these conditions, it seems possible that the 

 large-scale, exothermal reduction of carbon dioxide to methane may be 

 used by the methane-liberating bacteria to the same purpose as the 

 large-scale, exothermal oxidation of autoxidizable substrates is used by 

 the autotrophic bacteria, namely, to provide energy for the reduction of 

 a relatively small proportion of carbon dioxide to a carbohydrate. 



In the same class with the methane-producing bacteria may perhaps be placed the 

 species of Clostridium, which reduce carbon dioxide to acetic acid by means of hydrogen 

 (Wieringa 1936): 



(5.42) 2 H2 + 2 CO2 > 2 H2O -H CH3COOH + 68 kcal 



or by means of various purines {Clostridium acidi urici, Barker, Ruben and Beck 1940). 



We shall not continue with the enumeration of bacterial and other 

 biological systems which have been found capable of absorbing carbon 

 dioxide and incorporating it into organic matter. Although some of 

 them probably carry out a " chemosynthetic " reduction of carbon 

 dioxide, similar to that achieved by the microorganisms described above, 

 the most important examples worked out so far appear to belong to a 

 different type, that of enzymatic carboxylations. It is customary to 

 speak of "reduction of carbon dioxide" whenever this compound is bound 

 in an organic molecule. However, it is advisable to distinguish clearly 

 between true reduction of carbon dioxide and carboxylation, carhamination 

 and similar "additive" reactions of carbon dioxide with organic molecules. 

 Whether carboxylations should be called reductions at all, is a matter of 

 definition. In chapter 8 arguments will be presented in favor of not 

 using this designation. This convention would prevent misunder- 

 standings which have led to the use of expressions like "dark assimila- 

 tion," or even "dark photosynthesis" for processes in which carbon 

 dioxide was merely added to existing organic compounds. Carboxylation 

 is important from the point of view of photosynthesis, not as an analogy 

 to the main photosynthetic process, but as a possible way of entry of 

 carbon dioxide into the photosynthetic apparatus. It will therefore be 

 considered in detail in chapter 8, which deals with the immediate fate 

 of carbon dioxide in photosynthesis. 



C. The Role of Autotrophic Bacteria in Nature 



Bacterial metabolism is of great importance for the elucidation of the 

 chemical mechanism of photosynthesis. It indicates that photosynthesis 

 consists of two distinct stages, the reduction of carbon dioxide, and the 



