12 THE METABOLISM OF ALGAE 



catalysing this reaction, similar to those found in the 

 tissues of higher animals and plants, have been reported as 

 present in Polytomella caeca^^^'^^ and in the blue-green 

 alga Oscillatoria princeps}^^ 



The available evidence thus suggests that the mechanism 

 of respiration in algae is of the same general pattern as that 

 in other organisms, but that certain of the enzyme systems 

 involved may, at least in some species, be different from 

 those found in yeast or vertebrates. It will, however, be 

 noticed that much of the evidence which has been quoted 

 relates to species of Chlorophyceae and that information 

 about the mechanism of respiration in algae belonging to 

 other classes is meagre. This illustrates a point which 

 deserves particular emphasis. Our present knowledge of 

 algal metabolism is based upon the examination of very few 

 types out of the great variety which exists, about half of 

 the investigations referred to in this book having, in fact, 

 been carried out with species of Chlorococcales. Species 

 belonging to other classes have received a little attention 

 but it cannot yet be said that a representative sample of 

 the various types has been examined with any degree of 

 thoroughness. Considerable differences in metabolism have 

 been found to exist between closely related species of 

 algae^^^ and such results make it clear that generalizations 

 based upon observations on one form only should be 

 treated with reserve. Table i shows the distribution among 

 the several classes of those genera of algae mentioned in 

 this book. 



As a framework for the consideration of the variety of 

 types of metabolism which exists among the algae a classi- 

 fication appears best which recognizes that the manner in 

 which an organism obtains the energy necessary for the 

 maintenance of its life and the organism's powers of syn- 

 thesis are independent. Organisms have for a long time 

 been classified broadly into autotrophs, those capable of 

 growth on inorganic nutrients only (e.g. photosynthetic 

 organisms), and heterotrophs, those needing organic sub- 

 stances for growth. The inadequacy of this classification 

 was made apparent by the discovery that certain species, 



