COMPARATIVE BIOCHEMISTRY 83 



When carbon dioxide is supplied as the sole carbon source 

 in the light (in the presence of molecular hydrogen) the 

 products of fixation are different from those with acetate 

 and are similar to the products found with green algae. The 

 nature of these substances we shall discuss in the next 

 chapter (Chapter 6). Thus Kamen has suggested that light 

 in itself is not concerned with the process of carbon fixation 

 but only modifies a pre-existing dark carbon metabolism. 

 This is in agreement with the general view that has arisen 

 since it has been shown that chemosynthetic and hetero- 

 trophic organisms as well as photosynthetic plants are 

 capable of carbon dioxide assimilation. The fixation of 

 carbon dioxide requires energy and it appears probable that 

 the different types of organisms do not necessarily differ as 

 to the mechanism of fixation but rather in the mechanism 

 by which energy is supplied. The photosynthetic organisms 

 supply energy from light via some pigment complex, the 

 chemosynthetic from the oxidation of inorganic substances 

 and the heterotrophs from the oxidation of organic com- 

 pounds. According to the tenets of comparative biochemistry 

 we can advance our knowledge of carbon fixation in photo- 

 synthetic organisms by studying fixation in the other groups 

 such as the heterotrophs, and this is the subject we discuss 

 in the next chapter. 



THE CHEMOSYNTHETIC BACTERIA 



Finally we return to a brief discussion of the chemo- 

 autotrophs first discovered by Winogradsky. A wide range 

 of organisms of this type is now known which utilize the 

 energy of oxidation of a great variety of substances for the 

 assimilation of carbon dioxide. In addition to the 'nitrifiers' 

 and the sulphur-oxidizing bacteria already mentioned, 

 organisms are known which oxidize iron salts, thiocyanate, 

 hydrogen, and carbon monoxide. Many soil bacteria are 

 facultative heterotrophs living heterotrophically in the soil 

 but capable of growth in purely inorganic media when 

 hydrogen and carbon dioxide are supplied. As we shall see 

 in the last chapter of this book it is of interest to note that 

 in these organisms the energy produced by the oxidation of 



