PLANKTON 37 



The lipids represent one of the most important components of micro- 

 scopic algae. 223 Many investigators have confirmed Beijerinck's observa- 

 tion in 1921 that fat droplets accumulate in diatoms as the cultures be- 

 come old and the available fixed nitrogen decreases. 40 - 41 This occurs, as 

 we have indicated earlier, because nitrogen deficiency limits growth and 

 favors the building of reserve materials. The ratio of lipid to carbohydrate 

 is variable; it depends far less on algal genetic traits than it does on 

 environmental factors such as nitrogen supply and enzyme systems. Also 

 noteworthy is the fact that algal fat metabolism is but little reflected in 

 gas exchanges. 



The lipid content differs not only from one species to another (the 

 blue-greens have very little), but also may vary greatly within the same 

 species with changes in cultural conditions. Chlorella generally contains 

 20-25 per cent fat, yet nitrogen starvation can increase the amount to 

 as high as 86 per cent. Even during World War II, the Germans, cul- 

 turing Chlorella, NHzschia, and Scenedesmus as potential sources of needed 

 fat, attained lipid yields of 40-70 per cent. These comprised chiefly the 

 triglycerides of stearic, oleic, and linoleic acids. 



Most of the fat in phytoplankton is unsaturated, with a high propor- 

 tion in the 16 and 18 carbon series. 41 Detailed analyses of Chlorella, for 

 example, show 54-67 per cent unsaturated C-18 and 18-29 per cent C-16 

 acids. Unsaturation ranges from 1.60 to 2.25 double bonds per molecule. 

 Palmitic is the main saturated acid, while stearic constitutes less than 4 

 per cent of the total fatty acid. As the lipid content of Chlorella rises, 

 the unsaponifiable fraction decreases and the fatty acid portion increases ; 

 at the same time there is a significant fall in the degree of unsaturation of 

 the fatty acids. Chlorella fat resembles other unsaturated plant lipids and 

 falls into the chemical classification of "drying oils." 



Sterols have been obtained from most of the algal groups, excepting 

 the blue-greens. Klosty and Bergmann 224 reported in 1952 that chondril" 

 lasterol is the principal sterol in Scenedesmus obliquus, while ergosterol 

 predominates in Chlorella pyrenoidosa. Chondrillasterol may be remem- 

 bered as having been favorably considered a few years ago as a starting 

 point for cortisone synthesis. Table V offers analyses of the various lipids 

 in several samples of Chlorella. 



Phytoplankton, like the macroscopic seaweeds, has a high vitamin con- 

 tent. It has been determined that most of the known vitamins are present 

 in Chlorella and related microalgae. The quantities are eminently ade- 

 quate for the algae to be used as food (for man or animals), but not if 



