54 THE METABOLISM OF ALGAE 



providing the carbon skeletons necessary for the synthesis 

 of further protoplasm and thus support chemotrophic 

 growth. There is, in fact, good evidence that this is so in 

 certain algae. In a study of the metabolism of ProtothecUy in 

 which about seventy compounds, including fatty acids, 

 other organic acids, carbohydrates, alcohols, ketones and 

 nitrogen-containing compounds were used, a close correla- 

 tion was found between the growth that could be obtained 

 with a particular substrate and the value of the same sub- 

 stance as a substrate for respiration.^^ In a more limited 

 investigation with Chlorella a good correspondence has like- 

 wise been found between the values as substrates for 

 respiration and for growth of a number of organic acids. ^"' ^^ 

 It must be noted, however, that such a correlation has not 

 been found in all algae that have been examined. Obligate 

 phototrophs, to be discussed on p. 59, are able to oxidize 

 a number of substances which they are not able to utilize 

 as substrates for chemotrophic growth, and, although 

 Namcula pelliculosa is apparently only able to use glucose 

 for growth in the dark, nevertheless citrate, acetate, pyru- 

 vate, succinate and lactate are stimulatory to respiration.^"^ 



If it is supposed that any substance which is an inter- 

 mediate in metabolism or readily convertible into one 

 should be utilizable as a substrate for chemotrophic gro\\th 

 provided that it is of sufficient potential chemical energy, 

 then it is necessary to explain how it is that some substances 

 which are apparently of this kind cannot be assimilated by 

 certain algae and how it is that different species vary so con- 

 siderably in their requirements for chemotrophic growth. 



One possibility is that a substance which would otherwise 

 be metabolized may not be able to enter the cell. This is 

 obviously so in the case of bulky molecules, such as those 

 of proteins and polysaccharides, which are unable to pene- 

 trate the plasma membrane unless they are first broken 

 down into smaller units. Thus most algae able to assimilate 

 glucose are unable to utilize glucose polymers such as starch 

 or glycogen supplied externally although they may store 

 and utilize the same materials within their cells. The pro- 

 duction of extracellular enzymes by means of which such 



