Physiology 437 



metal deficiencies. Therefore, they might be considered of little value to 

 the organism unless allowance is made for chelating activity. Intentional 

 induction of an inhibitory metal deficiency makes it possible, by trial and 

 error, to identify various trace elements which seem to be essential (229). 

 After preliminary qualitative observations, individual requirements can 

 be analyzed quantitatively by adding an excess of all needed elements ex- 

 cept one and then determining the amounts of this element which will 

 compensate for graded increases in the chelating agent. A curve plotted 

 from such data and then extrajiolated to zero chelate should give a fairly 

 good approximation of the basal requirement for a particular metal. 

 There remains to be considered the situation in which a trace element 

 minutely contaminating a supposedly required metal may actually be the 

 essential factor (229). 1 his conqjlication can be eliminated, if elimination 

 is possible, only by the use of highly purified metal sources. However, 

 the failure of progressive purifications to alter the apparent requirements 

 quantitatively would suggest that the original indications were valid. 



The older techniques were adequate only to the extent of indicating 

 qualitative requirements for certain metals in a few species. Calcium — 

 apparently needed by Eiiglena annbaena (109), E. stellata (105), Hyalo- 

 gonium klebsii (457), Chilomonas Paramecium (395), Oikomonas termo 

 (192), and Tetrahymena pyriformis (184) — may prove to be a general 

 requirement. Magnesium, necessary for Chilomonas Paramecium (395) 

 and Tetrahymena pyriformis (292), is a component of carboxylase and 

 should be a general requirement. As a constituent of chlorophyll, mag- 

 nesium also is obviously essential to green flagellates. Iron was foimd to 

 he a requirement of Chilomonas Paramecium (220), Eudorina elegans 

 (108), Euglena anabaena (109), Polytoma obUisum and P. uvella (355), 

 and Tetrahymena pyrijormis (184, 287). Since this metal is a constituent 

 of cytochromes, cytochrome oxidase, catalase, and peroxidase, it may be 

 impossible to find Protozoa which do not need iron unless it can be 

 shown that obligate anaerobes have absolutely no iron requirements. 

 Phosphorus, essential for Chilomonas Paramecium (423) and Tetrahy- 

 mena pyriformis (292), is obviously a general requirement for phosphory- 

 lation of metabolites and vitamins. Observations on the phosphate cycle 

 in T. pyriformis (123, 497) indicate that during the lag phase of growth 

 there is a rapid liberation of inorganic phosphate from organic sources, 

 whereas uptake of inorganic phosphate is more characteristic of logarith- 

 mic growth. Manganese, which favors growth of Euglena anabaena in 

 inorganic media (109, 174), and apparently participates in oxidation of 

 pyruvate and other metabolites by Plasmodium gallinaceum (535), may 

 be generally needed as an activator of phosphorylases and peptidases and 

 possibly other enzymes. Potassium, required by Chilomonas Paramecium 

 (423) and T. pyriformis (292), seems to be needed in certain phosphoryla- 

 tions and probably is a general requirement. The possible significance of 



