78 THE METABOLISM OF ALGAE 



As assimilation continues, these fractions account for less 

 of the ammonia taken up, basic amino-acids, particularly 

 arginine, lysine and ornithine, being formed instead.^'®' ^^^ 

 Corresponding with the assimilation of ammonia there is a 

 disappearance of non-reducing sugar and of polysaccharide 

 from the cells (Fig. 13) and it is evident that the carbon 

 skeletons for the nitrogenous compounds formed are derived 

 from these substances. ^"^ When glucose is supplied, all the 

 ammonia nitrogen added to the cells is eventually converted 

 to insoluble nitrogen and the cells return to the nitrogen- 

 starved condition. When no glucose is supplied a large pro- 

 portion of the ammonia-nitrogen assimilated remains in 

 the cells in a soluble organic form.^'''^ 



The mechanisms whereby ammonia is incorporated into 

 organic substances in algae have not yet been investigated 

 in detail. In higher plants supplied with ammonia labelled 

 with heavy nitrogen, heavy nitrogen appears more rapidly 

 and extensively in glutamic acid than in any other amino- 

 acid, suggesting that there is ready exchange between 

 ammonia and its amino-group and that this is the principal 

 route of entry of ammonia into metabolism. ^^ Glutamic 

 acid, which evidently occurs in substantial proportions in 

 most algae (see Table 8) and which is more readily assimil- 

 ated than other amino-acids by algae such as Scenedesmus^ 

 and Eiiglena spp.,^^^ appears* to occupy a similar special 

 position in the nitrogen metabolism of these organisms and 

 that this is so for Nostoc muscoriun has been confirmed 

 using heavy nitrogen as a tracer. ^°^« Glutamic acid is formed 

 in plant tissues from oc-ketoglutaric acid by a reversible pro- 

 cess involving a reduction catalysed by glumatic dehydro- 

 genase: 



.41 



COOH COOH COOH 



C:0 1h5) C:NH +2H CH.NH2 



(CHo)o (CH2)2 (CHo)2 



I I I ' 



COOH COOH COOH 



a-Ketoglutaric a-Iminoglutaric Glutamic 

 acid acid acid 



(21) 



