96 THE METABOLISM OF ALGAE 



Because of its special colloidal properties alginic acid finds 

 considerable use in industry.-®^ 



A polysaccharide yielding only xylose upon hydrolysis 

 has been reported as present in Rhodymenia palmata. This 

 xylan contains both i : 3 and i : 4 linkages. ^-'^ 



Other polysaccharides constituting the intercellular ma- 

 terial of marine algae are composed of residues esterified by 

 sulphuric acid. Because of this, a considerable proportion of 

 the ash constituents of marine algae are present in the living 

 plants in chemical combination with the intercellular ma- 

 terial. ^^^ Such sulphuric esters have so far only been found 

 in the Rhodophyceae and Phaeophyceae and in animals and 

 have not been reported as occurring in the bacteria or 

 M}^ophyceae. It has been suggested that the sulphate group 

 plays a similar part in the synthesis of these polysaccharides 

 as does the phosphate group in the synthesis of starch^^® 

 but there is as yet no evidence to show whether this is 

 actually so. 



Agar, or, more properly, agar-agar, is the most familiar 

 polysaccharide of this type, being extensively used as a 

 substratum for the culture of micro-organisms and for a 

 variety of medical and industrial purposes.-"^ Its structure 

 has not yet been fully elucidated. Many, but not all, Rhodo- 

 phyceae yield extracts which gel^^^ and the tendency is 

 to apply the term agar to all the substances concerned 

 although they certainly differ in chemical constitution. A 

 commercial agar, probably derived from Gelidium sp., has 

 been found to contain D-galactopyranose units linked by 

 I : 3 linkages together with L-galactose residues linked with 

 the others through Cj and C4 and esterified by sulphuric 

 acid at Cg.^^^ These two components are respectively: 



HO 



/ 



CH2OH 



O H 



HO 



H- and 



