632 YU. I. SOROKIN 



In many heterotrophic bacteria there exist enzyme systems which activate 

 molecular hydrogen and catalyse its liberation from organic compounds [43]. 

 Most heterotrophic bacteria eflFect the transformation of the oxidized mineral 

 compounds of sulphur into reduced ones by the formation of SH-groups from 

 S04~ ~. The capacity of bacteria to reduce nitrogen in the form of NOa" to amino 

 groups, and to produce ammonia, is almost universal. The process of oxidation 

 of ferrous ions forms the basic enzymic reaction of biological oxidation. The 

 metabohc formation of methane is inherent in some heterotrophic bacteria. 



AH these reactions, which are carried on by heterotrophic bacteria with the 

 consumption of energy and reductants, when channelled in the reversed direc- 

 tion, will generate energy and reductants. Hence the reactions of constructive 

 metabolism of heterotrophic bacteria could have been converted into those of 

 energy metabolism of autotrophic bacteria. This may have been caused by the 

 appearance of free oxygen in the atmosphere when large quantities of reduced 

 products of anaerobic metabolism were present. 



The capacity for assimilating CO2 is likewise inherent in most heterotrophic 

 micro-organism.s [4]. However, in heterotrophic organisms the assimilation of 

 CO2 is not the main basis of carbon nutrition. The principal function of CO2 

 assimilation by heterotrophic bacteria is associated with the synthesis of final 

 products of oxidative and anaerobic fermentations. Since heterotrophic micro- 

 organisms are less elaborate living systems than chemosynthetic organisms, they 

 are devoid of certain mechanisms which enable chemoautotrophs to synthesize 

 the substances of their body wholly from the carbon of CO2. 



Micro-organisms must have passed through the following stages in the course 

 of transition from heterotrophic to autotrophic metabolism. At first, when 

 getting into an environment characterized by an excess of a certain reduced mineral 

 compound, the heterotrophic organisms acquired the capacity to oxidize it by 

 means of the enzymes in their possession, and to utilize the energy of oxidation 

 as an additional source of energy for constructive metabolism. Such organisms, 

 existing under definite conditions, are already relatively independent from 

 organic matter to meet their energy requirements. Therefore their evolution 

 tends to develop further in these organisms the pre-existing function of hetero- 

 trophic assimilation of CO2 so as to Uberate the constructive metabolism like- 

 wise from the requirement for organic carbon. It is this evolutionary develop- 

 ment in micro-organisms capable of oxidizing mineral substances (products of 

 anaerobic metabolism of heterotrophs) of the capacity for heterotrophic assimi- 

 lation of CO2 that led to the appearance of chemoautotrophic bacteria. 



After the discovery of the capacity of heterotrophic bacteria for assimilating 

 CO2, and of chemoautotrophs for assimilating organic substances, some investi- 

 gators were inclined to deny the existence of chemoautotrophs as a separate 

 physiological group of bacteria [4]. Umbreit thought it possible to include into 

 the group of chemoautotrophic bacteria only two species of thiobacteria and two 

 species of nitrifiers [3]. It is quite evident that this viewpoint is untenable. There 

 is just as much sound reason to single out a physiological group of chemoauto- 

 trophic bacteria as there is for the group of photoautotrophic organisms. Repre- 

 sentatives of both groups of bacteria can feed autotrophically. In this case the 



