SECT. 2] ORGANIC REGULATION OF PHYTOPLANKTON FERTILITY 167 



cally large volumes of resins are necessary, huge amounts of liquid must be 

 evaporated, and the resins give a large organic blank. 



Extraction with water-immiscible solvents or with solvents after evaporation 

 of sea-water is a simple and quick way to prepare organic concentrates without 

 altering materially the organic compounds. Phenol, 2,4-lutidine, ethyl acetate 

 and benzoyl alcohol remove substantial quantities of organic material. Even 

 though any one solvent separates only a fraction of the organic matter, this 

 method has been successful in extracting and identifying several organic 

 substances in sea-water. 



It might be interesting to try the method of "gel filtration", which acts as 

 a molecular sieve. Like dialysis, it separates large molecules from small, but 

 apparently is far more rapid and versatile. Columns or beds made of small 

 grains of cross-linked dextran [Sephadex G-25 and G-50 produced by Phar- 

 macia Laboratories (American address, Box 1010, Rochester, Minnesota)] form 

 a gel which retards and apparently traps small molecules in the intramolecular 

 spaces of the polymer, while large molecules do not enter the gel phase and flow 

 freely in the interstitial fluid (i.e. between the grains of polymer molecules). 

 This system has been employed to separate into two distinct fractions am- 

 monium chloride from the proteins in serum (Porath and Flodin, 1959), and 

 for the fractionation of polypeptides and proteins (Porath, 1959, 1960). It 

 offers several advantages over dialysis : it is as rapid as filtration and can be used 

 with the same results on a small or a large scale ; high recovery is achieved in 

 the presence of electrolytes ; the polymer is inert to charged groups and stable 

 in the pH range 2-10; and the gel beds retain their original properties even 

 after daily use for a period of months. Two grades of cross-linkages are available, 

 and perhaps others will be designed to trap or retard smaller molecules. 

 Chlathrates or other systems of molecular sieves may be even better. 



The direct chemical approach, hampered as it is by the inherent technical 

 difficulties, has contributed only a few data on some of the organic substances 

 present in relatively large quantities in sea-water. Better methods of extraction 

 could be developed if one could direct the marine chemist to specific compounds. 



The problem of the organic substances in waters and their fertility, being 

 fundamentally biological, has also been studied by biologists. The first isolated 

 attempts to solve the problem for algae have developed along several lines 

 which have contributed some valuable information : the nutritional approach 

 through culture of organisms, the biological analysis of sea-water, and the 

 chemical analysis of the organisms and their excretions. These independent 

 developments are all part of a comprehensive logical attack which, though 

 mainly tested on algae, could be applied to all water organisms. 



We can look at the problem in two ways : the origin and fate of the organic 

 compounds in waters, and the nutritional needs of the organisms. 



C. Origin and Fate of Organic Substances 



Obviously, almost the totality of organic substances is derived, in the present 

 state of our planet, from the activities of living organisms. 



