444 Physiology 



only a few cases. Among the phytoflagellates, experimental evidence is 

 available for Chilomonas paromec'nim (182, 207, 224) and Polytoma 

 oceJlatitm (182). There is also some presumptive evidence in the case of 

 heteroautotrophs which have been grown without exogenous thiamine. 

 Thiazole can replace thiamine in stimulating growth of Polytoma cauda- 

 tum (352) and P. ocellatum (353), while the thiazole and pyrimidine 

 components together replace the vitamin for PoJytomeUa caeca (351). 

 Several substituted thiazoles and pyrimidines also can be utilized instead 

 of the natural components of thiamine (348). Although actual synthesis 

 by a phytoflagellate has not been demonstrated, it is assumed that some 

 species can produce thiamine from simple raw materials while others need 

 thiazole or both components. In investigating such problems, composition 

 of the medium must be considered carefully since the supply of trace 

 elements, such as iron (348), and the nature of the substrate may be 

 important factors in a potential synthesis. The significance of the sub- 

 strate is suggested by failure of Chilornonas Paramecium to grow on 

 pyruvate without added thiamine, although the flagellate grows slowly on 

 acetate in a thiamine-free medium (73). This is an interesting parallel 

 to Prototheca zopfi which can oxidize acetate in a thiamine-deficient 

 medivnn but apparently requires thiamine for utilization of pyruvate (4). 

 In general, the burden of proof seems to rest upon those who would deny 

 that phytoflagellates can synthesize a variety of vitamins. For the higher 

 Protozoa, such assumptions are not justified because these organisms have 

 been grown almost exclusively in chemically undefined media. So long 

 as materials of natural origin are included, it is unsafe to assume that a 

 particular vitamin has been eliminated from a culture medium. 



Synthesis of thiamine from its thiazole and pyrimidine components has 

 been reported for Acanthamoeba castellaTiii (345), and from unspecified 

 intermediates in the case of Tetrahymena pyriformis (277). However, 

 A. castellanii was grown in peptone media of unknown vitamin content, 

 and the interpretation of the earlier data for T. pyriformis has been 

 questioned (183, 184). Although more recent data have been supplied for 

 the ciliate (278, 282), there is no conclusive evidence that either A. castel- 

 lanii or T. pyriformis can synthesize thiamine. 



The Trypanosomidae which need exogenous hematin presumably are 

 unable to synthesize the porphyrins necessary to the formation of heme. 

 Others, which possess the cytochrome system but do not require ready 

 made porphyrins, obviously synthesize heme from simpler constituents 

 of culture media. The problem of obtaining suitable raw materials is a 

 minor one because such a substrate as acetate (450) may serve as a starting 

 point. Syntheses of this nature may be assumed for Chilomonas Para- 

 mecium, Polytoma uvella. Astasia klebsii, Euglena gracilis, and Tetra- 

 hymena pyriformis, for example. As a source of direct evidence, the 



