170 I'ROVASOLI [chap. 8 



substance in vitro, this metabolite, or its physiological equivalent, must be in 

 the environment : most of the algal flagellates of the littoral zone need vitamin 

 B12 and indeed this vitamin is measurable in sea-water. However, the nutri- 

 tional studies, to have ecological meaning, should be extensive and not limited 

 to growth factors (which are, after all, only one of many important variables). 



As explained recently (Provasoli, 1958), one needs to know — besides the minimal mineral 

 and organic requirements — the idiosyncrasies, tolerances and abilities to utilize a host of 

 N, C, and P compounds. The success of an organism is controlled by the quantity and 

 quality of metabolites ; deficiency or excess are both detrimental. 



Similarly, emphasis on the limiting factors of the environment implies belief 

 in a simplification that can rarely be true. Just as interactions between or- 

 ganisms are sometimes fundamental, so are interrelations between chemical 

 and physical factors. 



Thermophilic bacteria, Ochromonas malhamensis, Euglena gracilis, etc., can be grown 

 above the normal "lethal" temperature limit of the species if the vitamins and metals are 

 increased manyfold; temperature is a nutritional variable (Baker et al., 1955; Hutner, 

 Baker et al., 1957 ; Hutner, Aaronson et al., 1958). Braarud (1945) noted that some boreal 

 diatoms (Chaetoceros spp.) bloom in the warmer water (18°-20°C) of the Oslo Fjiord, but 

 only in polluted areas. Temperature and salinity interact affecting the growth of marine 

 fungi (Ritchie, 1957, 1959). Gradients of light and temperature markedly affect production 

 and the ratios of the photosynthetic and accessory pigments of algae (Halldal, 1958). 

 Similarly, the folic acid requirement of some organisms can be spared or substituted by 

 p-amino-benzoic acid, the B12 requirement by other cobalamins, methionine or deoxy- 

 ribosides ; some photosynthetic organisms can grow in darkness on exogenous carbon 

 sources. 



Axenic cultures of ecologically important organisms are the most fruitful 

 biological approach to the problem of fertility of waters. In the course of 

 determining nutritional needs of organisms one often detects the indispensability 

 of organic substances which act at concentrations so low that they are not easily 

 assayable chemically. The organism which has this need, or others more 

 technically suitable because of their rapid and abundant growth and because 

 of their specificity, can be employed for the quantitative determination of 

 the active principle (see Chapter 9). Axenic cultures are also necessary, as 

 mentioned, to determine the production of "external metabolites" and excre- 

 tions which might affect growth and morphogenesis of other organisms. Their 

 chemical composition can be analyzed giving information on the nutritional 

 needs of their predators. 



E. Biological Analysis of Sea-Water 



Another interesting approach to the fertility of waters is the so called "bio- 

 logical analysis". Selected organisms can serve to differentiate water-masses 

 into "good" or "bad" by their growth responses or other measurable physio- 

 logical events (hatching, setting of larvae, etc.). Once established, one can try 

 with different treatments to see whether the "bad" waters lack some nutrients 



