56 THE METABOLISM OF ALGAE 



enzyme responsible for the phosphorylation of glucose and 

 fructose.^®®' ^' ^^^' ^^ Polytornella caeca, for instance, stores 

 a starch-like substance which it is able to synthesize from 

 glucose- 1 -phosphate but not from glucose, maltose or 

 sucrose. ^^^' ^^ Whereas the hexose phosphates are inter- 

 mediates in metabolism, the sugars themselves are not and 

 the absence of the enzymes necessary for their phosphoryla- 

 tion does not impair metabolic activity although it prevents 

 direct utilization of these substrates. 



It is unlikely that all the vagaries of algae in the assimila- 

 tion of organic substrates can be accounted for along these 

 lines. The metabolic system of an organism is not infinitely 

 adaptable and the adjustment of the proportions of the 

 different enzymes necessary to cope with a particular sub- 

 strate may in a given species not be compatible with the 

 general economy of the cell. The limits within which adjust- 

 ment is possible probably vary considerably from species to 

 species and may be expected to be narrower the greater the 

 morphological complexity of the organism. Adaptation in 

 bacteria has received considerable attention and it has been 

 shown that, within limits, an organism may be trained to 

 attain maximum growth rate on a substrate which the 

 original material was unable to utilize.^*^ A few cases of 

 similar adaptation are known among algae. For example, 

 Scenedesmus costulatus var. chlorelloides exhibits a lag period 

 before beginning exponential growth with maltose as a sub- 

 strate whereas no such lag is shown in media containing 

 glucose, sucrose or glycerol. This lag period may be inter- 

 preted as the period needed for the development of maltase 

 necessary for the conversion of this sugar to glucose. ^^*^ A 

 further example is afforded by a strain of Chlorella vulgaris 

 which has been found to grow slowly at first in the presence 

 of cellobiose in the light but, after several transfers in the 

 light in a medium containing this substance, to grow almost 

 as rapidly as it does under otherwise comparable conditions 

 when supplied with glucose. ^'^'^ Adaptation evidently is 

 dependent upon suitable environmental conditions. Thus 

 Eugletia gracilis cannot utilize glucose for grovvth under the 

 usual conditions of culture, but in an atmosphere containing 



