CHAPTER V 

 HETEROTROPHIC ASSIMILATION 



Heterotrophic organisms are unable to synthesize the full 

 range of organic substances necessary for life and are conse- 

 quently dependent on exogenous sources for one or more 

 essential metabolites. In the algae autotrophism is evidently 

 the primitive condition, the heterotrophic habit being 

 derived from it by specialization leading to loss of particular 

 synthetic mechanisms. 



There are many indications that a requirement for specific 

 organic growth factors is of frequent occurrence among 

 algae but few definite instances have been recorded. Partly, 

 this is because the isolation of algae has usually involved 

 'enrichment culture'* in a mineral medium, a procedure 

 which tends to select species without organic growth sub- 

 stance requirements and which results in our present know- 

 ledge of algal metabolism being biased in favour of com- 

 pletely autotrophic forms.^'® There is also the technical 

 difficulty of establishing requirements for substances which 

 may be active at exceptionally high dilutions. ^^^ Many 

 reports of growth factor requirements among flagellates 

 cannot be confirmed^^^ and these will not be discussed here. 



THE AMINO-ACID REQUIREMENTS OF ALGAE 



Twenty or more different a-amino-acids appear to be 

 essential in living matter and, if any part of the mechanism 

 for the synthesis of these from ammonia and appropriate 

 carbon residues is absent, the organism concerned must 

 obtain the particular amino-acids from exogenous sources. 



Eiigletia deses can only grow if provided with a suitable 

 amino-acid such as aspartic acid.^^^ Here the requirement 



* An enrichment culture is one, inoculated with a mixture of 

 species, in which particular conditions favour the development of 

 certain organisms whilst hindering that of others (see ref. 241). 



84 



