SECT. 2] ORGANIC REGULATION OF PHYTOPLANKTON FERTILITY 169 



chances are better there of finding larger quantities of the products of their 

 metabolism. Besides, one can determine the conditions under which some 

 substances are produced and released into the environment, study the chemical 

 composition of the organisms, employ them as food organisms, or let them die 

 and be broken down by micro-organisms. This experimental simplification does 

 not mean that the task is simple. 



In effect, what we are looking for is the chemical relationships between 

 organisms mediated by the water environment; of the hundreds of organic 

 substances present in the waters only a few may be important for a particular 

 receiver species. 



D. Nutritional Requirements 



The complementary approach to the problem is the study of the nutritional, 

 morphogenetic and physiological requirements of the organisms. Obviously, 

 the nutritional requirements can be identified only in the absence of any other 

 organisms. 1 Axenic cultures are also indispensable in discerning the chemical 

 relationships between the organisms of a given biocenosis. The lack of these 

 exogenous contributions often results in no, or poor, growth, in abnormal 

 morphogenesis, incomplete life cycles, mortality of larvae, difficulties in 

 moulting, lack of copulation and ovideposition, etc. To impute these events to 

 the artificiality of in vitro experimentation is to reject the opportunity of finding 

 the cause and to miss potential bioassays. 



The methodology and principles of nutritional studies are well established. Briefly, 

 when attempting the isolation in axenic culture of the organism, it is advantageous to 

 reproduce as far as possible the environmental conditions, both physical and chemical. 

 Since often we do not have a precise knowledge of the environment, or we are resorting to 

 culturing as a means to learn more about the biotic factors of the environment, we have 

 to break new ground. We cannot afford to neglect any scrap of information or hint, 

 experimental or observational. For instance, in dealing with an exacting alga, knowledge 

 of the nutrition of other algae may be inadequate or irrelevant; one may have to resort to 

 information on the nutrition of higher plants, plant-tissue culture, and animal nutrition. 

 This applies to inorganic and organic requirements as well as to how to solve particular 

 technical problems such as pH, rH, trace-metal buffering, emulsifiers, etc. In any case it 

 is also advisable to try a variety of natural products and extracts rich in unknowns (soil, 

 algal extracts ; powders, infusions or hydrolysates of various organs ; blood, coconut milk ; 

 etc.). If any of these proves useful, substitution of the natural products with their known 

 major components will follow and may lead to a complete substitution with chemically 

 defined compounds or even to a new growth factor or regulator (Hutner, Cury and Baker, 

 1958; Hutner, Provasoli and Baker, 1961). 



Nutritional studies, in defining the factors needed for growth, indicate to 

 the marine chemist and the ecologist which are the important parameters of 

 the environment. The assumption is, if an organism requires absolutely a 



1 This is a wholly unnatural condition but so is the light-and-dark-bottle method. 

 Somehow we seem to understand more easily the mechanism of a watch when we tear it 

 apart, even if unable to reassemble the parts, let alone reconstitute a functional unit. 



