FOOD REQUIREMENTS 503 



In addition to the presumably direct influence upon growth rate, the 

 pH of the medium has been found to modify the effects of other fac- 

 tors. For example, ElHott (1935a) has shown that the maximal acceler- 

 ating effect of certain carbohydrates on the growth of C. striatum is 

 exerted below pH 7.0, with little or no acceleration above that point. 

 Some of Jahn's (1935b) results with E. gracilis in inorganic medium 

 also seem to show a correlation between pH and the effect of several 

 carbohydrates. Elliott noted also that the effects of sodium acetate and 

 butyrate varied with the pH. The former inhibited growth of C. striatum 

 more or less completely at pH 6.0 and lower, but produced moderate 

 acceleration near the neutral point. Butyrate was toxic at pH 6.5 or 

 below, but showed an accelerating effect at pH 7.0-7.5. Jahn (1934) 

 has suggested that such effects of acetate and butyrate may be explained 

 on the basis that only the undissociated organic-acid molecule is toxic. 

 A. Lwoff (1935a), in reviewing Elliott's work, stated that acetate and 

 butyrate inhibit the growth of Colpidium; this is true for only a certain 

 pH range. Another indirect effect of the pH is the influence on tempera- 

 ture relationships, as indicated in Jahn's (1933a) observation that the 

 susceptibility of E. gracilis to relatively high temperature is lowest at pH 

 5.0 and greatest above pH 7.0. The growth-accelerating effects of panto- 

 thenic acid on C. striatum (Elliott, 1935c) and of plant auxins on E. 

 gracilis (Elliott, 1938) are also dependent upon the pH of the medium. 



The evidence already accumulated shows that the pH relationships of 

 Protozoa are exceedingly complex, and that they vary not only with the 

 individual species but also with the composition of the medium and with 

 other environmental conditions. Furthermore, such relationships may 

 vary with time, since Jahn (1931) has observed that the optimal pH 

 differs in young and in old cultures of E. gracilis. To some extent, the 

 pH-growth relationships may be correlated with the activities of enzymes, 

 which may show characteristic pH optima; for example, the protease of 

 G. piriformis (Lawrie, 1937) shows maximal activity at pH 6.0. But 

 this may represent only one of many ways in which growth is related 

 to the pH of the medium. 



Oxygen Relationships 



That oxygen tension of the medium influences growth of Protozoa is 

 obvious, but relatively little detailed information has been accumulated 



