450 Physiology 



developed for Tetrahymena geleii W (280, 282). The simpler of these 

 media contained eleven amino acids (arginine, histidine, isoleucine, leu- 

 cine, lysine, methionine,^ phenylalanine, serine, threonine, tryptophane, 

 valine), glucose, eleven known vitamins, several inorganic salts, and in 

 the proportion of 1:10, a plant or animal tissue extract apparently con- 

 taining a nimiber of amino acids, certain unidentified growth factors and 

 known vitamins and several minerals. Further progress has made possible 

 the gradual substitution of known growth-factors (Table 8. 3) for supple- 

 ments of natural origin. This general type of medium, which has proven 

 satisfactory for several strains of Tetrahymena (121, 122, 494), is illus- 

 trated in Table 8, 1, although nucleic acid may be replaced by known 

 purines and pyrimidines. Such media are now almost completely defined 

 in a chemical sense and are potentially useful in the assay of certain vita- 

 mins and also such amino acids as histidine, isoleucine, lysine, and trypto- 

 phane (494). 



OXYGEN RELATIONSHIPS AND 

 OXIDATIONS 



Ecological distribution 



The distribution of Protozoa suggests that some species are obli- 

 gate aerobes, that others are microaerophiles (requiring only a little 

 oxygen), and that many intestinal parasites and some free-living types may 

 be obligate or facultative anaerobes. Natural waters containing much 

 putrefying material are anaerobic at their lower levels, and such artificial 

 environments as Imhoff sewage tanks also insure anaerobiosis beneath the 

 surface (304, 305). Species characteristic of such a fauna, sometimes 

 termed polysaprobic or sapropelic, are at least facultative anaerobes. They 

 include Ctenostomina and scattered species of other ciliates, as well as a 

 few flagellates and Sarcodina. Another practically anaerobic environment 

 is found near the bottom of deep fresh-water lakes, but very little is 

 known about this fauna. Other Protozoa, typical of clean waters with a 

 high oxygen content, are aerobes and some may be obligate aerobes. How- 

 ever, such ciliates as Coleps Jiirtus and Frontonia leucas, which are not 

 sapropelic, may survive anaerobically for several weeks (318). 



Oxygen relationships of parasites doubtless vary with the usual site of 

 infection. Species which invade the blood and other tissues probably 

 have access to about as much oxygen as the surrounding tissue cells and 

 may be predominantly aerobic. Oocysts of Eimeria stiedae and E. magna, 

 for instance, apparently cannot sporulate under strictly anaerobic con- 



^The reported ability of T. geleii to use homocystine, with supplementary "liver- 

 fraction," as a replacement for methionine (285) has been refuted (150) on the basis 

 that the observed growth in the presence of homocystine can be attributed to the 

 methionine content of the liver-fraction used by Kidder and Dewey. 



