66 THE METABOLISM OF ALGAE 



to pyruvate. The condensation of two molecules of acetic 

 acid to give succinic acid: 



CH3.COOH _,H CHo.COOH 



+ . I . . (16) 



CH3.COOH +-H CH2.COOH 



has been demonstrated in certain micro-organisms^®^' ^^ 

 and if a similar reaction occurs in algae pyruvic acid could 

 be formed via oxaloacetate produced by the operation of 

 the tricarboxylic acid cycle (Fig. 10). It is to be noted that 

 the effects observed in thiamine-deficient Prototheca (see 

 above) do not exclude the possibility that assimilation of 

 acetate occurs in this way. 



Some indication of the manner in which oxidation and 

 assimilation are related is given by the effects upon them 

 of cyanide. ^^'* In Chlorella vulgaris low concentrations of 

 cyanide have been found to stimulate endogenous respira- 

 tion. In the absence of cyanide one-eighth to one-ninth of 

 added glucose may be completely oxidized, most of the 

 remainder being assimilated to di- or polysaccharide. Low 

 concentrations of cyanide reduce the rate of oxidation of 

 added glucose to a level which does not, however, fall 

 below that of endogenous respiration under the same con- 

 ditions, but at the same time produce an increase in the 

 amount of oxygen taken up for a given amount of this 

 substance. That is, in the presence of low cyanide concen- 

 trations more of the added glucose is oxidized and less is 

 synthesized to more complex substances. The facts seem 

 best explained on the assumption that glucose may be 

 oxidized by either of two mechanisms, one of which is un- 

 affected, the other blocked, by cyanide. The assimilation 

 of glucose appears to be coupled with the cyanide-sensitive 

 system in such a way that for every molecule which is oxi- 

 dized by this path a certain fixed number of molecules are 

 synthesized to more complex substances. ^^^ This cyanide- 

 sensitive system is presumably one involving a cyto- 

 chrome, evidence for the presence of which in Chlorella 

 has already been mentioned (p. 10), with a phosphorylation 

 cycle as a means of energy transfer between the reactions 

 of oxidation and synthesis. 



A 



