External metabolites in the sea 143 



rigorously than has previously been possible the precise requirements of phytoplankton 

 organisms in nature and in culture. Meanwhile, Provasoli, Hutner, and their 

 colleagues at the Haskins Laboratories, have also very rightly undertaken fundamental 

 and precise experiments on the basic requirements for the growth of marine and fresh 

 water micro-organisms. Provasoli and Pintner ( 1 953), in their review of the ecological 

 implications of the nutritional requirements of algal flagellates, have brought together 

 a large number of references to the need of various micro-organisms for growth 

 factors and trace elements. To these they have added the striking results obtained in 

 their own laboratory. The evidence ranges from the early demonstration by Hutchin- 

 son (1943) of the actual existence of thiamin in natural waters, to the significant 

 requirement of various forms for cobalamin (vitamin Bio) in pure culture within 

 chemically defined media. Of particular importance is the rigorous technique, 

 necessary and adopted, in their work. All the results to date demonstrate aspects of 

 " non-predatory " relationships. As they say: 



" It is a reasonable assumption that if an organism requires a growth factor in vitro, 

 then this metabolite or its physiological equivalent should be found in significant 

 amount in the environment " (p. 845). 



" The water environment is the one in which metabolites are interchanged most 

 efficiently. It is to be expected that the interdependent growth of the different groups 

 of water organisms should sensitively reflect the excretion and consumption of 

 metabolites. Undaunted by new intricacies, we should envisage all the possibilities 

 in these relationships, and not hesitate to follow Lucas's lead in constructing theoretical 

 frameworks upon which to hang data. In the present paper, only a few aspects of 

 the nutrition of phytosynthetic forms are considered. It is possible, nevertheless, 

 to state more definitely some of the interdependencies based upon ' external meta- 

 bolites ': (1) the interchange of growth factors; (2) the lowering of inhibitory con- 

 centrations of several major mineral nutrients, especially PO4; and (3) the preferential 

 utilization of minerals, including trace metals, may condition waters, bringing their 

 concentrations into the optimal zones for succeeding forms. The practical aim — to 

 predict algal successions and blooms— may be achieved through a comprehensive 

 knowledge of vitamin cycles as well as mineral cycles. An immediate problem is to 

 trace the thiamine and cobalamin cycles " (p. 849). 



Droop (for example, 1954) is pursuing a similar course at Millport, in Scotland, 

 and has shown the need of several marine flagellates* for vitamin Bj... LEW'iN,in Canada, 

 has also found B12 essential for the growth of the alga Stichococcus in sea water (1954). 

 The presence of free B12 in natural sea water has been demonstrated, and its distribu- 

 tion is being examined in more than one laboratory. Droop is now developing an 

 assay for it in sea water. Along a rather diff'erent line Fogg (1952) has suggested that 

 some at least of the external metabolites (for instance, the polypeptides of blue-green 

 algae) may further communal growth, via chemical linkage, by making relatively 

 insoluble nutrients available in a form more suitable for assimilation. 



Turning now to the animal field, several workers have followed Allee in linking 

 aggregations with the release of metabolites (for example. Cole and Knight-Jones, 

 1949, and Knight- Jones, 1950). Again, there was the demonstration by Allison 

 and Cole (1935) that the feeding movements of barnacles can be correlated with the 



* Droop has now demonstrated this need in the diatom Skeletonema costattim (Droop 1955). 



