Physiology 431 



to one method in a natural environment. Chlorophyll-bearing species are 

 often saprozoic and some can grow in darkness. A number of the green 

 flagellates also ingest solid food, while typically holozoic organisms also 

 may carry on saprozoic nutrition. The relative importance of one method 

 or another depends largely upon environinental conditions. 



The ecological classification of Kolkwitz and Marsson (301) divides 

 Protozoa into katharobes, living in water containing almost no organic 

 matter; and saprobes, found in water containing appreciable quantities of 

 organic matter. Saprobes are divided into oligosaprobes, mesosaprobes, 

 and polysaprobes, living in the presence of small, moderate, and large 

 amounts of organic matter. This classification involves oxygen relation- 

 ships as well as food supply, since katharobes are typically aerobic, while 

 polysaprobes are more probably anaerobes or facultative anaerobes. 



The earlier results with pure cultures soon demonstrated that these 

 older concepts were inadequate. For instance, it became clear that the 

 term, autotroph, could no longer be applied automatically to any green 

 flagellate that occurs naturally in fairly pure water exposed to light. In 

 fact, it is not yet certain that the existence of complete autotrophs, as 

 originally defined, has been demonstrated. Furthermore, some of the 

 "saprozoic" flagellates proved to be "autotrophic" with respect to nitrogen 

 sources, although needing organic foods as a source of energy. This situa- 

 tion furnished the stimulus for several more modern classifications of 

 protozoan nutrition (99, 176, 343, 358, 459). Although such classifications 

 were a distinct improvement over the older systems and were a conven- 

 ience in discussions, it now appears that even the more modern classifica- 

 tions have a tendency to oversimplify protozoan nutrition. From present 

 indications, food and vitamin requirements of Protozoa will show many 

 variations from species to species, so that much more information will be 

 needed before definitive classification can be attempted. 



The determination of food requirements 



Two general methods have been followed in the study of proto- 

 zoan food requirements. In one procedure the experimental media have 

 been the simplest ones which would support giowth in serial transfers. 

 In such media, growth is often at a minimum and the organisms do little 

 more than maintain themselves in successive transfers. In the other gen- 

 eral procedure, media have been devised to maintain growth at a maxi- 

 mum and any constituent which cannot be omitted is considered essential 

 to growth. This method is based upon the generalization that growth of 

 any species will reach a maximum when all conditions are optimal — 

 qualitative and quantitative aspects of the substrate, concentrations of 

 stimulatory and essential growth-factors, concentrations of essential min- 

 erals, and non-dietary environmental factors. 



On a theoretical basis, the first procedure might seem to offer the more 



