36 THE METABOLISM OF ALGAE 



with Nitzschia closterium, for example. ^^® The concentra- 

 tions of all forms of carbon dioxide available in freshwater 

 vary much more than those in seawater, but generally they 

 are such as not to be limiting for photosynthesis by phjto- 

 plankton except in the surface layers of waters poor in dis- 

 solved minerals under conditions of bright sunlight and 

 calm. With bulkier algae, e.g. Gelidium, in which the rate 

 of diffusion through the tissues up to the site of fixation is 

 limiting, the saturating concentration of carbon dioxide may 

 be considerably higher than that normally to be found in 

 seawater.^^* i 



THE FIXATION OF CARBON DIOXIDE 



The transformations undergone by intermediates in the 

 metabolic processes of plants are frequently extremely 

 rapid and until recently the nature of those involved in the 

 photosynthetic fixation of carbon dioxide remained a matter 

 for speculation, the available analytical techniques being 

 inadequate for the separation and detection of the minute 

 amounts of the substances concerned. Our present know- 

 ledge of the intermediates in photosynthesis is based 

 largely upon results obtained by the school of Calvin and 

 Benson and confirmed and extended by Gaff"ron and Fager, 

 using isotopic tracer techniques in conjunction with paper 

 partition chromatography and ion-exchange methods. In 

 these investigations the unicellular algae Scenedesmus and 

 Chlorella have been the principal materials for study. 



If Scenedesmus or Chlorella is supplied with radioactive 

 carbon dioxide in the dark it is found that malic, succinic, 

 fumaric, citric, glutamic and aspartic acids and alanine, 

 become labelled with radioactive carbon and together 

 account for 95 per cent of the total radioactivity of the algal 

 products. These compounds appear to be labelled by 

 reversible carboxylation reactions such as are well known 

 in other organisms and which occur independently of the 

 capacity for photosynthesis. Under anaerobic conditions 

 radioactive carbon fixed in this manner remains in the 

 water-soluble fraction, only when aerobic respiration takes 

 place is it transferred to polysaccharide, fat or protein. If 



