CHAPTER IX 



FOOD REQUIREMENTS AND OTHER FACTORS 



INFLUENCING GROWTH OF PROTOZOA 



IN PURE CULTURES 



R. P. Hall 



It is obvious that the growth of Protozoa is influenced by many differ- 

 ent factors. The importance of some of these is well recognized and the 

 relationships to growth are partially understood in a few instances, but 

 there is little or no detailed information bearing on other factors. Here 

 and there, investigations have suggested possible solutions to certain 

 problems, but just as frequently have uncovered new problems which 

 in turn must be solved in the approach to an understanding of protozoan 

 growth. The present lack of information extends to such questions as 

 the list of essential elements, the nature of the simplest organic foods 

 adequate for various species, "growth factor" or vitamin requirements, 

 and the combined effects of various environmental factors on growth. 

 Furthermore, Protozoa in cultures constitute populations and presumably 

 are subject to general laws of population growth. Hence the final inter- 

 pretation of many experimental results demands further knowledge of 

 the behavior of populations. 



From the experimental standpoint, several types of protozoan popu- 

 lations may be distinguished. (1) The pure culture contains a single 

 protozoan species with no other microorganisms. In most cases such 

 cultures have been started from pure lines and are thus genetically homo- 

 geneous. The number of bacteria-free strains now in existence is un- 

 certain, although an estimate of 100 may be fairly accurate. Many strains 

 of Phytomastigophora are maintained by Pringsheim (1930), while ad- 

 ditional species belonging to various groups of Protozoa are to be found 

 in several other laboratories. (2) The species-pure culture contains a 

 single protozoan species, usually in pure line, with bacteria, algae, or 

 other microorganisms as sources of food. Populations of this type have 

 been maintained on known species of microorganisms (e.g., Oehler, 



