INTRODUCTION 9 



known to be concerned in the cycle. ^^ The respiration of 

 Chlorella,^^^ of Myelophyciis and of Gelidium'^^^ is reduced 

 in the presence of maionate, which is a specific inhibitor 

 for succinic dehydrogenase, an essential for the tricarboxylic 

 acid cycle represented in Fig. 2. However, maionate has 

 been found not to inhibit the respiration of Ulva^^^ or 

 Cylindrospermum?^^ This evidence points to the existence 

 of some alternative mechanism, as yet unidentified, for the 

 oxidation of succinic acid in these algae, but it should 

 be noted that, as has happened in experiments with 

 Chlorella,^^' ^^^ the medium used for Ulva may not have 

 been sufficiently acid to secure adequate penetration of 

 malonic acid into the cells (see p. 55). There is also some 

 evidence that, in addition to the tricarboxylic acid system, 

 Chlorella contains a pyruvic dehydrogenase by means of 

 which the direct oxidation of this substance can take place. ^* 

 It may be concluded that while there is evidence that reac- 

 tions concerned in the tricarboxylic acid cycle occur in 

 algae there is as yet no conclusive proof of the participation 

 of this system in normal algal respiration. 



Pyruvic acid enters the tricarboxylic acid cycle after 

 oxidative decarboxylation to yield acetic acid which then 

 condenses with oxaloacetic acid to give citric acid (see Fig. 2 

 and ref. 19). The presence of thiamine (vitamin B^), in the 

 form of its pyrophosphate, co-carboxylase, is essential if 

 this reaction is to take place. Thus, the rate of oxidation of 

 pyruvic acid by thiamine-deficient Prototheca is greatly 

 enhanced by the addition of thiamine. ^^ The presence of 

 thiamine may also be necessary for other decarboxylations 

 involved in the tricarboxylic acid cycle. Thiamine has been 

 demonstrated in all of the numerous algae in which it has 

 been sought, including species of Chlorophyceae, Phaeo- 

 phyceae, Rhodophyceae,^^^ Bacillariophyceae^^^ and Myxo- 

 phyceae,^'*^' ^^^ and those forms which are unable to 

 synthesize it or one of its constituent parts for themselves 

 must be supplied with the appropriate substance if they 

 are to grow (see Chapter V). There is thus no reason to 

 doubt that thiamine plays an essential part in the meta- 

 bolism of all algae and this suggests a similarity of their 



