no THE METABOLISM OF ALGAE 



It would appear that during exponential growth the dif- 

 ferent enzyme systems concerned must increase at equal 

 rates.i^^ This does not necessarily mean that the chemical 

 composition of an alga should remain the same as long as the 

 relative growth factor remains constant. In fact, the chemi- 

 cal composition of Chlorella vulgaris may vary during expo- 

 nential growth"-^ and during exponential growth of Anabaena 

 cylindrica the nitrogen content per unit volume of cell ma- 

 terial falls considerably.^^ Such variation may be partly due 

 to changes in amount of substances not directly concerned 

 in growth, particularly reserve products and cell wall con- 

 stituents. The variations both in the total amount and in the 

 relative proportions of various fractions of ribonucleic acid 

 which have been found to occur during exponential growth 

 of Polytomella caeca are less easily explained, particularly 

 since it has been reported that in yeast there is a linear rela- 

 tionship between relative growth rate and ribonucleic acid 

 content. ^^^ However, when exponential growth is continued 

 for long periods then a constant chemical composition of cell 

 material is approached. In an apparatus in which a constant 

 cell population per unit volume of medium is maintained 

 indefinitely in exponential growth by continuous automatic 

 dilution with fresh medium, cells are produced having 

 nearly constant chemical and physiological characteris- 

 tics.2^^» 2^2 The type of algal cell characteristic of exponen- 

 tial growth that has continued for some time is small, with 

 thin cell walls and little reserve material, and having 

 vacuoles in the protoplast either absent or inconspicuous 

 and the chromatophores relatively undeveloped. Such cells 

 have a high nitrogen content and it is evident that their 

 metabolism is chiefly directed to protein and protoplasmic 

 synthesis. ^^^ Chlorella pyrenoidosa in this state growing 

 under a set of conditions in which light intensity was limit- 

 ing has been found to have the following elementary com- 

 position: C 53-0 per cent, H 7-5 per cent, O 28-5 per cent, 

 N 10-8 per cent;^^^ this corresponds to a protein content 

 of about 67-5 per cent of the dry organic matter of the cells. 

 From this analysis, a 'formula' for Chlorella may be ob- 

 tained and, since extracellular organic products are negli- 



