438 Physiology 



sodium, apparently needed by C. paramechnn (423), is uncertain. Sulfur, 

 as a constituent of several vitamins and amino acids, is presumably essen- 

 tial. A variety of inorganic salts, cystine, glutathione, and cysteine are 

 satisfactory sources for C. Paramecium (392). Silicon stimulates growth of 

 C. Paramecium, (398) and prolongs life in a phosphate-deficient medium 

 (396). Whether this effect is attributable to silicon or to impurities (73) 

 in the silicate used, remains to be determined. Vanadium also seems to 

 accelerate growth of C. Paramecium (25). A need for copper, apparently 

 a component of ascorbic and phenol oxidases, became apparent in Tetra- 

 hymena pyriformis when natural products were replaced by purified con- 

 stituents of culture media (287). Zinc, apparently involved in aldolase, 

 carbonic anhydrase, and uricase activity, has often been included in cul- 

 ture media on the assumption that it is essential to protozoan growth. 

 Cobalt, as a constituent of vitamin Bjo (cyano-cobalamin), is required by 

 Euglena gracilis (230) and probably various other Protozoa. Molybdenum, 

 which accelerates nitrogen-fixation by bacteria and seems to be essential 

 for certain molds, needs investigation as a protozoan requirement. 



In summary, fragmentary evidence now indicates that, in addition to 

 carbon, hydrogen, oxygen, and nitrogen, at least twelve other elements — 

 calcium, cobalt, copper, iron, magnesiimi, manganese, phosphorus, potas- 

 sium, silicon, sodium, sulfur, and vanadium — are either stimulatory or 

 essential to growth of certain Protozoa. In addition, there are reasons for 

 believing that others, such as molybdenum and zinc, may be important. 

 Highly purified chemicals and the newer techniques of investigation may 

 expand the list of required trace elements, and should clarify the status 

 of some of them in protozoan metabolism. Even so, a complete list of the 

 basal requirements apparently remains unobtainable with the inorganic 

 materials and culture vessels now available. 



Vitamin requirements^ 



The requirements of many Protozoa, although incompletely known, 

 are probably comparable to those of Metazoa. Colpoda steinii (duo- 

 denaria) needs more than five vitamins, TetraJiyfnena pyriformis needs 

 at least nine or ten, and not less than six are important in the metabolism 

 of malarial parasites. At the other extreme, a few phytoflagellates have 

 been grown in media apparently free from vitamins. There is every reason 

 to believe that such differences depend upon the ability or inability to 

 synthesize particular vitamins. 



Among the phytoflagellates, Chilomonas Paramecium, in an acetate, 

 inorganic salt and thiamine medium, synthesizes nicotinic acid and the 

 diphosphopyridine nucleotide (DPN, or coenzyme I) which contains nico- 

 tinamide and adenine (223). Microbiological assays of comparable cul- 

 tures have confirmed the synthesis of nicotinic acid and demonstrated that 



* Several reviews of the earlier literature arc available (99, 180, 347, 348). 



