SECT. 2] ORGANIC REGULATION OF PHYTOPLANKTON FERTILITY 191 



the normal marine flora of Great South Bay when pollution from duck farms 

 accumulated because of poor flushing of the bay are resistant to ammonia 

 (1 mg atom N/1.) and utilize urea and other organic sources of N (Ryther. 

 1954a). Amino acids and other organic N are in general poorly utilized, if at all, 

 by photosynthetic marine algae. An exception is Hemiselmis virescens which 

 requires glycine ; colorless species might be expected to utilize and even to 

 require amino acids as do Gyrodinium cohnii (Provasoli and Gold, 1962) and 

 Oxyrrhis marina (Droop, 1959). Inorganic phosphate and also certain forms of 

 organic P are good P sources. 



This summary of the inorganic requirements shows that only the trace metals 

 and their status in sea- water (as particles or solutes) can be a cause of "good' : 

 or of "bad" waters, when inorganic N and P are not deficient. 



B. Organic Requirements 



As mentioned, photosynthetic marine algae have slight, if any, ability to 

 utilize organic nitrogen. 



Glycerophosphoric, adenylic, cytidylic and guanylic acids and perhaps 

 other nucleotides serve as P sources for all the marine species tested (extending 

 the data of Chu, 1946, and Harvey, 1953). The widespread ability to utilize 

 these organic compounds of P suggest that the P fraction of the organic solutes 

 in sea-water should be closely measured and that P need not be mineralized 

 to be available to phytoplankton organisms (see review in Provasoli, 1958, 

 p. 294). This may explain why turnover of P is more rapid than expected and 

 why in the early spring one diatom species after another can bloom so rapidly. 

 In preliminary experiments, we noted that during the logarithmic phase of 

 growth of Navicula pelliculosa, P becomes at first undetectable in the medium, 

 but later significant amounts of P are released in the medium, and this before 

 any portion of the population has become senescent or dying. 



Organic acids and sugars are in general not utilized as carbon sources by 

 photosynthetic marine algae. At times small concentrations of organic C are 

 stimulatory, but the increase in growth is too little to suspect utilization as a 

 C source. Similarly, very low concentrations (0.1-1 mg %) of amino acids and 

 purines or pyrimidines are stimulatory ; this may be due to chelation of trace 

 metals and also to sparing of some substance along the pathways of bio- 

 synthesis. 



The above remarks on the ability to utilize organic substances are derived 

 from data on algae living in the photic littoral zone and may not apply at all 

 to algae living in other zones of the sea, especially the shallow and deep bottoms 

 and deep waters. Many pennate diatoms live close to, and, in the coastal muds : 

 11 of 26 species of diatoms isolated from such habitats not only utilize exo- 

 genous carbon sources but grow in darkness if supplied with glucose, acetate 

 or lactate (Lewin and Lewin, 1960). A colorless marine diatom, Nitzschia 

 putrida, has been found and is also heterotrophic (Pringsheim, 1951); it is 

 probable that more colorless species exist in muds. This heterotrophic ability 



