THE PHOTOTROPHIC ASSIMILATION OF CARBON 33 



hydrogen donor in photosynthesis, although if elementary 

 hydrogen is present this is used preferentially.^^* This use 

 of hydrogen sulphide by algae has not yet been investigated 

 in detail, but there are indications that it may be a wide- 

 spread phenomenon, especially among Euglenineae and 

 other flagellates. ^^° 



Photosynthetic processes utilizing water, elementary 

 hydrogen or hydrogen sulphide, are included under the 

 heading of photolithotrophy in the classification of 

 nutritional types given in the first chapter. An example of 

 photo-organotrophy, in which an organic substance acts 

 as hydrogen donor, is provided by hydrogen-adapted 

 Scenedesmus which will utilize substances such as glucose 

 in preference to hydrogen for this purpose. ^^^ No case of 

 obligate photo-organotrophy in an alga appears to have 

 been recorded so far. 



THE ABSORPTION OF CARBON DIOXIDE 



The majority of algae live in an aquatic environment in 

 which carbon dioxide is present in various forms in equili- 

 brium with each other: 



C02+H20^H2C03^HC03-+H+?=^C03--+2H+ . (4) 



The concentrations of these several forms depend on a 

 number of factors including hydrogen ion concentration, 

 the amount of base in excess of the equivalent of the strong 

 acid radicals present, the partial pressure of carbon dioxide 

 in the atmosphere and the temperature.^*^' ^^^ In consider- 

 ing the activities of algae, whether in culture or in fresh- 

 water or seawater, it is often important to know which of 

 these forms is acting as the immediate source of carbon 

 dioxide. It is necessary to bear in mind, of course, that the 

 form in which carbon dioxide enters the cell is not neces- 

 sarily the same as that which forms the immediate substrate 

 for photosynthesis. - 



It is generally held that unionized molecules penetrate 

 cell membranes more rapidly than ions and in agreement 

 with this there is evidence that undissociated carbon dioxide 

 is utilized by certain algae whereas bicarbonate and carbon- 

 ate ions are not. Thus in Chlorella the rate of photosynthesis 



