Il6 THE METABOLISM OF ALGAE 



this, the microscopical appearance of the cells changes, the 

 characteristic form at the beginning of the stationary phase 

 being large, with vacuolated protoplasm, extensive dark 

 green chromatophore, and thick walls. The composition as 

 well as the total amounts of the major chemical fractions 

 may show changes. Thus the end of the exponential phase 

 is marked by a replacement of hexose by disaccharide in the 

 soluble carbohydrate fraction and in senescent cultures there 

 is a tendency towards hydrolysis of starch with a corre- 

 sponding increase in disaccharide (see Fig. i6). The com- 

 position of the protein of C. 'vulgaris growing under the 

 same conditions does not change to any great extent after 

 the cessation of exponential growth; the most marked effect 

 being an increase in histidine content as cultures age.^'^ 

 Similar changes have been observed in other algae. In 

 Chlorella pyrenoidosa, growing in a purely mineral medium 

 which in the later stages of growth became nitrogen 

 deficient, the protein content has been observed to fall 

 from over 60 per cent of the dr}' weight in the exponential 

 phase to less than 10 per cent in the post-exponential phase. ^'^ 

 In another organism, Polytomella caeca, exponential in- 

 crease in ribonucleic acid continues for a while after the 

 cessation of exponential increase in protein with the result 

 that cells at the beginning of the stationary phase have a 

 high ribonucleic acid content.^^^ 



The accumulation of lipide in algae which have ceased 

 active growth has been noted frequently. Beijerinck first 

 showed by qualitative methods that deficiency of nitrogen 

 is the principal factor promoting fat accumulation in 

 diatoms-® but detailed quantitative information is so far 

 available only for Chlorella pyretioidosa.^^'^^ ^^^' ^" The lipide 

 content of this alga can be varied experimentally by changes 

 in any one of several environmental conditions, but the extent 

 of the variation is limited primarily by the concentration of 

 available nitrogen in the medium. The effect of the initial 

 concentration in the medium of ammonium nitrogen on 

 the lipide content of C. pyrenoidosa is shown in Fig. 17; 

 only if this concentration becomes reduced below o-ooi M 

 in the course of growth is lipide accumulation considerable. 



