504 FOOD REQUIREMENTS 



in experimental studies. Observations on the natural occurrence of 

 Protozoa indicate definite differences in oxygen requirements. Some 

 species appear to be strict aerobes, others are perhaps comparable to the 

 microaerophiles among the bacteria, and many intestinal parasites are 

 probably to be regarded as facultative anaerobes. Investigations on proto- 

 zoan respiration are discussed in another chapter of this volume (Chap- 

 ter VI). 



Growth in relation to oxygen requirements has been investigated for 

 only a few bacteria-free strains, and no attempt has been made to corre- 

 late definite oxygen tensions with growth rate. G. p'niformis, according 

 to Lwoff (1932), is incapable of growth under anaerobic conditions. 

 Likewise, Hall (1933b) found that under reduced oxygen tension 

 (Buchner pyrogallol method), growth of C. campylum was approxi- 

 mately 50 percent less than in aerobic controls in peptone medium. With 

 added dextrose, however, growth was greater than in the aerobic con- 

 trols in peptone medium. The results suggest a certain degree of simi- 

 larity between C. campylum and the facultative anaerobes among the 

 bacteria. Phelps (1936) demonstrated that aeration of flask cultures 

 in yeast autolysate produces a much heavier population of G. p'nijormis 

 than in unaerated flasks. These results are somewhat comparable to the 

 findings of Jahn (1936), who compared G. p}rJfor7n!s and Chilomonas 

 Paramecium with respect to effects of aeration. Growth of the ciliates 

 was most rapid at first in unaerated flasks, but after three days of in- 

 cubation the aerated flasks showed heavier populations. In C. Para- 

 mecium, however, growth was consistently more rapid in the unaerated 

 flasks. Rottier (1936a) has reported that the growth of Polytoma uvella 

 in flasks is more rapid than in tubes, after approximately forty hours of 

 incubation. Likewise, aerated tube cultures showed heavier growth than 

 unaerated ones. 



The Redox Potential 



As applied to culture media, the redox potential may be considered 

 an indication of the oxidizing or reducing power of such an oxidation- 

 reduction system. In other words, the more positive the redox potential, 

 the more highly oxidized is the medium; the more negative the po- 

 tential, the more highly reduced will be the medium. In effect, the 

 potential is a measure of intensity rather than of oxidizing or reducing 

 capacity, and hence is somewhat analogous to the pH, which gives no 



