Nutrition 



113 



their energy from inorganic reactions such as oxidation of iron or sulfur; (2) 

 phototrophic cells use sunlight; and (3) heterotrophic organisms oxidize organ- 

 ic compounds of varying degrees of complexity. Some heterotrophs can use 

 lower fatty acids such as acetate, but most of them need sugars or higher carbo- 

 hydrates. Similarly, with respect to nitrogen sources, organisms are autotrophic 

 if they can use inorganic nitrogen, nitrate or ammonia; they are mesotrophic if 

 they can rely on single amino acids (or ammonia and an organic acid); and are 

 metatrophic if they require many amino acids either in organized compounds 

 such as peptones or as simpler mixtures. All animals except green flagellate 

 protozoans are ultimately dependent on photo-autotrophic plants for their 

 carbon sources of energy. Whether the green flagellates are plants or animals 

 is a matter of definition. In evolution autotrophic organisms precede and are 

 required by heterotrophic organisms. 



There is no reasonable hypothesis as to how autotrophs originated. It has 

 been argued ^*'' ^"^ that a vast array of carbon-nitrogen-oxygen compounds 

 were formed by chance combination prior to the appearance of living organ- 

 isms, and that the first organisms were really heterotrophs, using pre-existing 

 compounds. Then, as these compounds became depleted, synthesizing mech- 

 anisms which appeared by mutation had survival value, and thus autotrophic 

 organisms evolved from hetero-metatrophic ones. There are difficulties with 



TABLE 19. FORMS OF MINIMAL NITROGEN AND CARBON REQUIREMENT 



IN PROTOZOANS OF VARIOUS SYNTHESIZING CAPACITIES 



Modified from Doyle,'" Dusi,-' Hutner et al., ''' Johnson,"- Lwoff," and Needham.'* 



this hypothesis, such as the small amount of atmospheric oxygen present before 

 there were green organisms. Whatever the sequence, certainly the basic cellu- 

 lar needs, particularly for amino acids and coenzymes, were established very 



