10 THE METABOLISM OF ALGAE 



respiratory mechanisms to those of other forms of 

 Hfe. 



The oxidations which occur in the course of the tri- 

 carboxyHc acid cycle are accompUshed by means of dehy- 

 drogenases, the hydrogen being eventually transferred to 

 free oxygen. This terminal process often occurs through 

 cytochromes, hydrogen carriers which appear to be of 

 universal occurrence in aerobic organisms. ^^ The demon- 

 stration of cytochromes in algae is rendered difficult by the 

 presence of photosynthetic pigments but it has been accom- 

 plished for a red alga, Porphyra,^^ for VaucJuria}^^ which 

 is generally classified with the Chlorophyceae but which 

 in metabolism more resembles a member of the Xantho- 

 phyceae, for the flagellate, Euglena,^^^ and for the brown 

 seaweed, Fuciis.^^^ It must be noted, however, that in these 

 last three algae the cytochrome that has been found seems 

 to be part of the photosynthetic rather than the respiratory 

 mechanism.^^^ The respiration of Chlorella supplied with 

 sugar is affected by carbon monoxide in a manner char- 

 acteristic of oxidations involving cytochrome."^ The pres- 

 ence of cytochrome oxidase in Polytoniella caeca, a colourless 

 member of the Chlorophyceae, has been reported.^^^ These 

 are indications that the cytochrome system is of general 

 occurrence in algae but there is evidence that in many 

 species alternative oxidation mechanisms exist. Thus con- 

 centrations of cyanide sufficient to inhibit the oxidation of 

 substrates supplied exogenously (i.e. in the external medium) 

 are without effect on, or may even stimulate, endogenous 

 respiration (i.e. using substrates stored within the cell) in 

 various species of Chlorophyceae.'^^' ^^^' ^^*' ^°^' ^''*' ^^® 

 Cyanide is an inhibitor for enzymes containing heavy metals 

 and thus prevents the operation of the cytochrome system. 

 The cyanide-stable respiration of these forms may possibly 

 be carried on by enzymes of the flavo-protein type, which 

 are cyanide-insensitive and, although probably chiefly con- 

 cerned with intermediate hydrogen transfers, are, in some 

 cases, capable of transferring hydrogen to molecular 

 oxygen. ^^ Riboflavin, a component of the prosthetic group 

 of the flavo-proteins, has been demonstrated in all the algae 



