102 THE METABOLISM OF ALGAE 



connexion it should be noted that lysine, methionine and 

 tyrosine, reported as absent in Ulva, are present in Chlorella 

 vulgaris, the other species of Chlorophyceae examined, and 

 similarly cystine and lysine, reported as absent in Phormi- 

 diuni, are present in another member of the Myxophyceae.^^ 

 It thus seems most probable that there is a general similarity 

 in amino-acid composition between the proteins of the 

 various classes of algae and of higher plants. This similarity 

 cannot, of course, be taken as implying that the individual 

 proteins of these various forms are similar. 



Certain amino-acids other than those mentioned in Table 8S 

 may be characteristic of particular classes. All representa- 

 tives of the Myxophyceae which have been so far examined, 

 i.e. Anabaena cylindrica, Oscillatoria sp. and Mastigocladus 

 laminosus, have been found to contain a-£-diaminopimelic 

 acid, COOH.CH(NH2).(CH2)3.CH(NH2).COOH. This 

 amino-acid appears to occur otherwise only in certain bac- 

 teria and has not been found in representatives of the 

 Chlorophyceae, Xanthophyceae, Bacillariophyceae, Eugle- 

 nineae, Phaeophyceae and Rhodophyceae in which it has 

 been sought. ^^^ The presence of diiodotyrosine has been 

 demonstrated in Laminaria spp. Diiodotyrosine accounts 

 for only a small proportion of the iodine to be found in 

 these brown seaweeds, the bulk evidently being in inor- 

 ganic form.^^^" 



Water-soluble peptides occur in certain marine species 

 of Chlorophyceae, Phaeophyceae and Rhodophyceae, in 

 amounts comprising up to 0-73 per cent of the dry weight 

 of the alga.124, 126, 129, 120, e? j^. ^g not clear whether these 



substances are comparable in origin with the extracellular 

 polypeptides produced by blue-green algae (see p. 90). 

 They are not present in all marine algae and there is 

 evidence that they may arise as a result of lack of balance 

 between carbon and nitrogen metabolism. ^^^ The composi- 

 tion of these peptides varies from species to species, e.g. 

 in Corallina officinalis a pentapeptide of aspartic acid is 

 present^^^' ^^^ whereas C. squamata contains a peptide con- 

 taining alanine and arginine.^^^ Only for Pelvetia fastigiata 

 has the constitution of the peptide been established, the 



