THE NUTRITION OF THE PROTOZOA

Summary The principal feature of recent studies on the nutrition of the Protozoa has been the development of culture media suitable for the maintenance of several score of species in ‘pure culture’. As investigators approached precise definition of the constitution of the culture media the significance of ‘trace elements’, accessory food factors and the like became more apparent. The types of culture apparatus, the methods of sterilization of organism and medium, the procedures of estimating growth of populations, and the exact composition of reagents have all been found to be of greater importance than originally realized. Within the phylum there exist chlorophyll‐bearing, plant‐like forms and numerous intermediary types up to and including those requiring living particulate food, thus providing the full range of nutritional types. The older systems of physiological classification have been extensively revised, but none of those proposed is adequate. In some instances the relations between variations in cytoplasmic structure (plastids) and nutritional requirements have been demonstrated. Many species of chlorophyll‐bearing and colourless flagellates have been studied. Intensive work on Polytoma, Chilomonas and Euglena has revealed notable species and strain variations with respect to factors such as fatty acid metabolism, nitrogen requirements, the effects of light on nutrient requirements and on response to plant hormones and vitamin constituents. The effect of systematic modification of chemical structure on the utilization of vitamin components has been studied for some species. A few dozen of the normally phagotrophic (holozoic) forms have been grown in the absence of other living forms (pure liquid culture), while certain forms have been grown on single species of food organisms (pure mixed culture) or on dead food organisms. With the exception of certain amoebae the salient work on pure culture has been carried out with the smaller ciliates, particularly Glaucoma. The media employed (protein hydrplysates, tissue autolysates, etc.) are chemically and physically highly complex. Such media contain labile constituents susceptible to important changes when subjected to minor procedural variations. Analyses of the media before and after the growth of organisms have given some indications of the nature of the nitrogenous fractions metabolized. The accessory growth‐factor requirements of these forms are complex. The very specialized nutrients required by parasitic forms have demonstrated interesting evolutionary relationships. An increasing number of biochemical experiments is being reported on the existence of various enzyme systems in the Protozoa. Our knowledge, however, of the metabolism of representative Protozoa remains fragmentary.