132 THE METABOLISM OF ALGAE 



This flexibility is understandable when it is considered 

 that the reactions in which the intermediates of metabolism 

 are involved are mostly reversible and that the major meta- 

 bolic processes share common intermediates. That this is 

 true for carbohydrate, acid and nitrogen metabolism, has 

 become apparent from studies of other organisms, but the 

 idea that photosynthesis is not apart from the rest of meta- 

 bolism but intermeshes with other processes at an early 

 stage has arisen as the result of studies with algae. Material 

 may be introduced into the metabolic system at many points 

 and does not follow any unique pathway, but appears as 

 ultimate products the nature of which varies according to 

 the conditions to which the system has been exposed in 

 the past as well as to those to which it is exposed at the 

 moment. The variation in chemical composition of the cell 

 material which results is perhaps more marked in algae 

 than in comparable bacteria or fungi because algae have a 

 marked tendency towards conservation of material within 

 their cells rather than of excretion of surplus substances 

 into the surrounding medium. 



While many algae are thus able to utilize a wide range of 

 substrates and are capable of synthesizing for themselves 

 all the metabolites which they need, others have more 

 limited metabolic capacities and are dependent upon the 

 presence of particular substances in their environment 

 either as energy sources or as growth factors. An inability 

 to utilize a given substrate as energy source may be due to 

 impermeability of the plasma membrane towards the sub- 

 stance, to the absence of the specific enzyme necessary for 

 its entry into metabolism, or to less well-defined causes. 

 Obligate phototrophy, an inability to maintain growth ex- 

 cept when carrying out photosynthesis which is perhaps 

 common among algae, is possibly due to the absence of a 

 mechanism for the synthesis of a specific substance but 

 has not yet been satisfactorily explained. In the other 

 direction, the capacity for photosynthesis may be lost and 

 many colourless organisms absolutely dependent on organic 

 substances as energy sources have clearly been derived from 

 photosynthetic algae by loss of chromatophores. Loss of 



