62 METABOLISM 



and the sulphur to sulphate. The green sulphur bacteria also utilize HgS, oxidizing 

 it to sulphur. 



Stage 2.- — Energy and carbon compounds for assimilation are derived by utilization 

 of carbon compounds more reduced than CO 2, which is not assimilated ; by the assimila- 

 tion of nitrogen from simple sources {N^, NH^, NO3) the organisms can synthesize 

 their protoplasm. 



In this group are found a few species utilizing carbon monoxide, methane and 

 other hydrocarbons. Of particular interest is B. oligocarbophilus, which, though 

 capable of obtaining its energy by the oxidation of CO, can also utilize simple 

 organic compounds like formic, acetic and butyric acids (Lantzsch 1922), constituting 

 a transitional type between Stage 2 and Stage 3. 



Stage 3. — Energy and carbon compounds for assimilation are derived by utilization 

 of carbon compounds more reduced than CO2 ,' amino-acids are required for nitrogen 

 assimilation, some as specific components for the synthesis of protoplasm ; ammonia 

 cannot be used as a nitrogen source. 



Stage 4. — Energy and carbon compounds for assimilation are derived by utilization 

 of carbon compounds more reduced than CO^. An array of amino-acids are needed 

 for nitrogen assimilation, as specific components for synthesis of protoplasm. Accessory 

 growth-promoting substances are also required, some organisms requiring more than 

 one. 



Knight regards the nutritional series obtained by arranging bacteria in order 

 of increasing complexity of nutritional requirements as a possible model for the 

 evolutionary process that has produced the bacterial species at present in existence. 



The interpretation of species differences along evolutionary lines is of necessity 

 sj)eculative, and particularly so among the bacteria. 



The autotroph capable of obtaining its energy from simple carbon and nitrogen 

 compounds provides us with a possible model for the forms of life that first appeared 

 in an environment presumably consisting of little but what we now regard as 

 inorganic compounds. But it does not by any means follow that the original 

 bacteria had in fact a complex metabolism of this kind, capable of building com- 

 plex proteins, carbohydrates, fats and vitamins from the simplest materials. On 

 this basis, we might suppose that heterotrophs evolved from autotrophs by adapta- 

 tion to environments richer in organic matter, whereby they were able to dispense 

 with enzymes concerned in the synthesis of less complex compounds. With 

 increasing parasitism, the organisms would increasingly lose synthetic power, 

 until a stage was reached when the parasite was, perhaps, dependent on another 

 living organism not only for the supply of already synthesized complex food 

 materials, but for the maintenance of a strictly regulated environment to utilize the 

 food. Thus, we should progress from the bacteria in Stage 4 to stricter parasites 

 like Myco. leprce, and finally to the rickettsiae and the viruses, which need the specific 

 environment of the cytoplasm or nucleus of certain cells for their development, 

 and have few or no demonstrable metabolic processes. 



The evolutionary process may, in fact, have gone the other way, the highly 

 complex autotroph being the final stage of a process which started in a virus-like 

 organism, proceeding by the gradual acquisition of new assimilative and synthetic 

 powers. 



The study of natural and induced bacterial variation (Chapter 9) gives us 

 no valid indication of trends in nutritional evolution. Variation proceeds in both 

 directions ; fastidious strains of limited synthetic powers may be trained to utilize 



