414 THE ALGAE 



SO long as readily available combined nitrogen sources (e.g. am- 

 monium salts) are lacking or in very low concentration. The process 

 also apparently requires the presence of traces of molybdenum (see 

 also p. 381). 



Respiration 



Detailed studies of the respiratory process have shown that it 

 follows the presently accepted normal channels in Chlorella^ Scene- 

 desmus, the colourless Prototheca^ Ulva, the Phaeophycean Myelo- 

 phycus and the Rhodophycean Gelidium. There would seem to be 

 no reason to suppose that it is othervdse in the uninvestigated 

 members of the Chlorophyceae, Phaeophyceae and Rhodophyceae. 

 Respiration does not, however, appear to follow the normal path in 

 Cylindrospermum and it is clear that further work with bacteria-free 

 cultures of Myxophyceae is highly desirable. The enzymes nor- 

 mally associated with respiratory reactions have been isolated from 

 a ntmiber of algae in addition to those mentioned above. There is 

 also evidence that at least some of the acids associated with the 

 Krebs cycle occur in the algae that have been thoroughly studied, 

 though again Cylindrospermum is a marked exception. 



Oxidative assimilation, in which a portion of the organic material 

 being respired is partly built back up into cell material, has been 

 demonstrated in the colourless Prototheca, and also in Chlorella 

 pyrenoidosa, C. vulgaris and Scenedesmus quadricauda. Since this 

 phenomenon also occurs in higher plants it is probably more wide- 

 spread among the algae, but at present the number investigated is 

 not great. 



Vitamins 



Most algae are able to build up vitamin B (thiamine) for them- 

 selves, though there are a few which do not possess the necessary 

 mechanism, but these can apparently absorb sufficient from the 

 thiamine dissolved in natural waters. The Eugleninae (see p. i) 

 are the forms most affected but there is a mutant of Chlamydomonas 

 moewusii which cannot synthesize thiamine. There is also some 

 evidence that the growth of marine algae may be dependent on 

 organic substances present in sea water, though the nature of these 

 substances is not known. Surface sea water, for example, has sub- 

 stances necessary for the germination of sporelings of Ulva and 



