SECT. 2] ORGANIC REGULATION OF PHYTOPLANKTON FERTILITY 179 



Table IV 



Extra-cellular Organic Nitrogen Produced by Fresh-Water Algae 

 (from Fogg and Westlake, 1955) 



of peptide N shows no apparent correlation with the trophic state of the lake 

 and the occurrence of peptide does not depend on any particular algae. 



We have no data on production of extra-cellular organic nitrogen in cultures 

 of marine algae. The high C/N ratios in the organic solutes found in sea-water 

 indicate, as mentioned, that in the sea the situation is quite different from that 

 in fresh waters ; however, we know nothing about their nature. Furthermore, 

 Duursma's results indicate that the release of organics in the sea occurs after 

 the algal blooms. 



C. Growth Factors 



Vitamin B12 (cyanocobalamin), thiamine and biotin are indispensable 

 requirements for many marine algae (Table VIII). Hence these growth factors 

 should be present in the environment : cyanocobalamin and thiamine have 

 been measured in sea-water. The search for the producers is going on, stimulated 

 by the pioneer work of Lochhead and collaborators (1951, 1957) on the growth- 

 factor relationships among the micro-organisms of the soil. Most of the work in 

 the sea centers around vitamin B12 and the family of cobalamins 1 ; cyanoco- 



1 A series of cobalamins is produced by micro-organisms ; these cobalamins differ one 

 from another largely in respect to the type of nucleotide portion of the molecule or its 

 absence (factor B). For cyanocobalamin the nucleotide is 5,6-dimethylbenzimidazole ; for 

 pseudo-Bi2, adenine ; for factor A, 2-methyladenine ; for factor H, 2-methylhypoxanthine ; 

 for factor III (factor I), 5-hydroxybenzimidazole — other cobalamins have been obtained 

 by biosynthesis with Escherichia coli (Bernhauer, 1956). The vitamin Bi2-requiring 

 organisms have characteristic patterns of response toward the cobalamins (see Table X, 

 and chapter on bioassays). It suffices here to mention that most algae respond only to 



