20 THE METABOLISM OF ALGAE 



40 carbon atoms, most of which are united into a long 

 polyene chain, i.e. one having alternating double and single 

 bonds. Carotenoid pigments are of two sorts, the carotenes, 

 which are hydrocarbons, and the xanthophylls, which are 

 oxygen derivatives of carotenes. The phycobilins are char- 

 acterized by having a metal-free linear tetrapyrrolic chromo- 

 phoric group linked to a protein of the globulin type. 

 Unlike the other photosynthetic pigments they are soluble 

 in water and insoluble in fat solvents. The differences 

 between the two types of phycobilin, the phycocyanins and 

 phycoerythrins, are not clearly marked, but the former are 

 generally blue whereas the latter are generally red (for an 

 absorption spectrum of a phycobilin see Fig. 7). Phyco- 

 bilins are strongly fluorescent. More detailed accounts of 

 the chemistry and properties of these three classes of pig- 

 ments are to be found elsewhere.^-- ^*'^» ^*^' ^'^^' ^'^ 



The principal pigments occurring in the different classes 

 of algae are shown in Table 2. Chlorophyll a is the most 

 abundant chlorophyll in all types of algae, as it is in higher 

 plants. Chlorophyll h, which is also to be found in higher 

 plants, occurs in the Chlorophyceae and Euglenineae but 

 is absent from the other classes, in which, however, chloro- 

 phylls c, d or e, may be present, ^-carotene is the most 

 abundant carotene in all classes, but a-carotene, character- 

 istic of Chlorophyceae and higher plants, is absent from 

 some classes and is replaced in the Bacillariophyceae by 

 e-carotene. The xanthophylls are numerous and each algal 

 class possesses its characteristic sorts. Those of the Chloro- 

 phyceae are similar to those to be found in higher plants. 

 It is to be noted that the Siphonales differ somewhat from 

 other orders of the Chlorophyceae in their carotenoids. In 

 the Siphonales a- and not /5-carotene is more abundant and 



• indicates the principal pigment of its group, 3 a pigment 

 comprising less than half the total pigments of its group, O a 

 pigment comprising only a small fraction of the total pigments of 

 its group, — absent, • incompletely examined, and ? uncertain. 

 The Cryptophyceae, which show diverse coloration and regarding 

 the pigments of which nothing is known, are omitted from this 

 table. 



