6 THE METABOLISM OF ALGAE 



Apart from determinations of the rate of the process in 

 various species (for references see 40) general studies of 

 algal respiration appear to have been made only by Gene- 

 voisii^ and by Watanabe.^^^. 295, 296 



The respiration of a hexose sugar, which substance may 

 for convenience be considered to be the immediate sub- 

 strate for the process, takes place by a sequence of reactions 

 which may in the first place be separated into two stages, 

 one of breakdown or glycolysis, and one of oxidation of the 

 products of glycolysis. The mechanism of glycolysis is 

 known in considerable detail from studies on muscle and 

 yeast^®^' '^^' ^^ and involves the phosphorylation of hexose 

 followed by splitting of the molecule into C3 compounds 

 which after a series of transformations give rise to pyruvic 

 acid, generally regarded as the end product of the process 

 (Fig. i). Under anaerobic conditions, pyruvic acid is not 

 removed by the oxidative mechanism and fermentation 

 occurs, the intermediates of glycolysis then giving rise by 

 mutual oxidation-reduction to end products the nature of 

 which varies according to the organism and the conditions 

 to which it is exposed. The mechanism of glycolysis in algae 

 does not appear to have been investigated in detail, but there 

 is indirect evidence that it follows essentially the same 

 course as that just outlined. Thus, substances such as 

 pyruvic acid, phosphoglyceric acid and triose and hexose 

 phosphates, which are known to be intermediates in yeast 

 and muscle glycolysis, have been shown to be present in 

 the green algae, Chlorella and Scenedesmus.'''^ Pyruvic acid 

 has been found to be a suitable substrate for respiration in 

 Chlorella,^"^ Prototheca}^ and Ulvar^'^ among the Chloro- 

 phyceae, in Myelophyciis, a brown alga,^^^ and in Gelidium, 

 a red alga,^^^ but not, however, in a blue-green alga, 

 Cylindrospermum, even under conditions apparently favour- 

 able for the penetration of this acid into the cells. ^^® Under 

 anaerobic conditions various species of Chlorophyceae have 

 been found capable of fermentations of a mixed acid type^^^ 

 and glucose is fermented by Prototheca to give lactic acid 

 as the only product as in muscle. ^^ 



In the aerobic respiration of vertebrate tissues pyruvic 



