120 THE METABOLISM OF ALGAE 



a more restricted range of hydrogen ion concentra- 

 tion (e.g. ^^' 2^). The autoinhibitor produced by Nitzschia 

 palea appears specifically to block mitosis without affecting 

 assimilation.^^ This does not mean that the capacity to 

 assimilate carbon does not decrease as cultures age. A con- 

 tinuous increase in the minimum quantum requirement 

 has, in fact, been shown to occur as cultures of Chlorella 

 3ggi63 2^^(^ ^]^g maximum rate of photosynthesis of which 

 Chlorella cells, taken in the stationary phase, are capable 

 has been found to be from a quarter to less than one-tenth 

 of that which cells in exponential growth can achieve. ^^^' ^^^ 

 A water-soluble substance inhibiting photosynthesis, which 

 may perhaps be the agent bringing about this decrease in 

 efficiency, has been extracted from cells from old cultures of 

 Chlorella?^^ Nevertheless, the results of many different 

 workers (e.g. ^^^' ®^' ^°^' ^®'*) show that photosynthesis and 

 chemotrophic assimilation of carbon continues in cultures 

 in which all cell-division has ceased. 



It has already been emphasized that assimilated carbon 

 enters directly into a system of intermediates common to 

 the major metabolic cycles and that therefore it may be used 

 directly for the synthesis of carbohydrate, lipide or protein. 

 When cell-division is no longer possible, carbon which 

 would otherwise be used for the synthesis of protoplasmic 

 constituents will flow along other channels and accumulate 

 as reserve products. In cells in which grouth has ceased as 

 a result of deficiency of nitrogen, the reserve materials 

 formed will necessarily be carbohydrate or lipide rather 

 than protein and, in general, protein synthesis appears to 

 be more sensitive to adverse conditions than synthesis of 

 lipide or carbohydrate. 



It may be supposed in the first instance that in those 

 forms which characteristically store lipide, lipide-synthe- 

 sizing enzymes are more effective in competing for the pri- 

 mary products of photosynthesis than are those which 

 synthesize carbohydrate. Nevertheless, there is the con- 

 tradictor\' fact, already commented upon (p. 113), that 

 the photosynthetic quotient under conditions of relative 

 nitrogen deficiency of algae of this type, e.g. diatoms, 



