HETEROTROPHIC ASSIMILATION 87 



bacterial species, -^"^ a requirement for it in an alga appears 

 so far only to have been found in an artificially induced 

 mutant of Chlamydomonas moewusUy'^^ ^'^ In the nutrition 

 of this mutant ^-amino-benzoic acid may be replaced by 

 aniline at an efficiency of about i per cent, but it is remark- 

 able that aniline does not have a corresponding neutralizing 

 effect on the inhibition of the wild-type cells by sulphan- 



ilamide.1^6, 177 



UNIDENTIFIED GROWTH FACTORS 



Besides these few instances in which growth factors have 

 been identified, it may be as well to mention briefly examples 

 in which a requirement for less completely characterized 

 growth factors has been found, as an indication of the 

 extent to which heterotrophism occurs among the algae. 



There are several reports indicating that the growth of 

 marine algae may in some cases depend on organic sub- 

 stances present in seawater.^^^ Thus, Ditylium brightwelli 

 has been found to require for vigorous growth in artificial 

 seawater two organic substances, or groups of substances, 

 in addition to mineral salts. One of these factors may be 

 replaced by organic compounds containing sulphur in the 

 grouping, — S— CH2.CH(NH2)COOH. The other sub- 

 stance, or group of substances, is present in extracts of 

 algae and of yeast. ^^^ Seawater from the surface, particularly 

 that from the littoral zone, has been found to contain sub- 

 stances necessary for the germination of sporelings of 

 Enteromorpha and Ulva spp., which do not appear to be of 

 an inorganic nature and which are absent from water taken 

 from a depth of 35 to 40 metres. ^^^ A marine dinoflagellate, 

 Gymnodinium sp., cannot be subcultured indefinitely in the 

 absence of an organic factor or factors occurring in soil 

 extract.^'^^ The addition of soil extract is frequently neces- 

 sary to obtain growth of soil and freshwater algae in cul- 

 ture. ^'^^ Thus, Cryptomonas ovata and Synura uvella will 

 not grow in a medium found suitable for other planktonic 

 algae, until an extract of soil or lake sediment is added. 

 Ashing destroys the activity of these extracts.^^* Soil ex- 

 tract presumably contains a wide variety of substances 

 7 



