476 FOOD REQUIREMENTS 



1916, 1919; Philpott, 1928; Geise and Taylor, 1935; D. F. Johnson, 

 1936; W. H. Johnson, 1933, 1936; Loefer, 1936d) or on mixtures of 

 bacteria. (3) Mixed populations, as described in the work of Gause 

 (1935), contain two species of Protozoa feeding on other microorgan- 

 isms, or perhaps one upon the other. This technique presents interesting 

 possibilities. (4) Wild populations are mixtures of species as obtained 

 from natural sources. Such populations have been studied particularly 

 in relation to succession of species in cultures (e.g., Woodruff, 1912). 

 The present discussion deals primarily with investigations on pure 

 cultures, which, with their obvious advantages, afford favorable ma- 

 terial for the study of many problems. With the exclusion of other 

 microdrganisms, it is possible to control the food supply and to deter- 

 mine, more accurately than by other methods, the relation of environ- 

 mental factors to growth. Detailed investigation of metabolic activities 

 is possible with pure cultures, whereas allowance must be made for 

 other microorganisms when bacteria-free material is not used. The pure- 

 culture technique and scrupulous cleanliness of glassware are essential 

 in studies on food requirements. This is true particularly of investiga- 

 tions on autotrophic nutrition, since protein contamination, to the ex- 

 tent of one part in millions, may influence growth. Likewise, pure cultures 

 are a prerequisite to investigations on specific growth factors, or vitamins. 

 Some of the methods used by various investigators have been described 

 elsewhere (Pringsheim, 1926; Hall, 1937a). The technique is not par- 

 ticularly difficult and, while the preparation of glassware is somewhat 

 laborious and constant precaution against contamination must be exercised, 

 the results more than justify the additional time and effort. 



Food Requirements of Protozoa 



Food requirements of the various groups of Protozoa differ in certain 

 general respects. The chlorophyll-bearing species may utilize carbon 

 dioxide, while other types require a more complex carbon source. Nitro- 

 gen requirements also vary. Some forms thrive on ammonium salts or 

 on nitrates; growth of other species is supported by nothing simpler than 

 an amino acid; while that of a third group is dependent upon peptones 

 or comparable protein-cleavage products. On the basis of such criteria, 

 a number of different methods of protozoan nutrition have been recog- 

 nized (Lwoff, 1938a; Pringsheim, 1937d). A somewhat simplified classi- 

 fication (Hall, 1939b) is presented below : 



