SECT. 2] ORGANIC REGULATION OF PHYTOPLANKTON FERTILITY 209 



deficient in total trace metals, and availability is less important ; a suitable 

 assay could tell. 



Since trace-metal utilization and deficiency may depend either on total 

 quantity, on availability of trace metals, or on both, the presence or absence in 

 sea-water of organic solubilizers becomes a primary factor. Many organic 

 substances (amino, hydroxy and nucleic acids) which have been detected or 

 quite likely exist in sea-water can chelate the heavy metals in various degrees. 

 In fresh waters the "humic" and "yellow" acids are probably the more im- 

 portant solubilizers quantitatively ; autochthonous substances like the poly- 

 peptides produced by blue-green algae may also be important. A similar 

 situation, though quantitatively less significant, may exist in the sea-waters 

 exposed to the influence of soil and large rivers, and may be partially responsible 

 for the fertility of bays, estuaries and, perhaps, banks where the winter mixing 

 more easily enriches the surface waters with the organic substances of the 

 muddy bottoms. Perhaps other substances of autochthonous origin, i.e. pro- 

 ducts of marine organisms, operate in the sea. Since fertility may depend in 

 large degree on the availability of trace metals, and chemical extraction of 

 organic substances is rather difficult, we have to resort provisionally to other 

 means. 



The C/N ratio may give some indications, particularly if substances similar 

 to the "humic" and "yellow" acids (which have a very high C/N ratio) are 

 present in sea-water. If we assume that plankton has a C/N ratio of about 6, 

 higher values would indicate the presence of some substances of the humic type 

 and lower values the presence of proteins, amino acids or amines. There is a 

 possibility that either very low or very high C/N ratios may coincide with the 

 chelating ability of sea-water. 



Clearly, chemical or biological methods are needed for measuring the che- 

 lating power in sea-water. Perhaps chelating power may be measurable by 

 employing one of the ions which are preferentially bound, like Cu or Hg, if a 

 way is found to detect them when they appear in sea-water in free form. 



A biological way of detecting the presence of chelators would be to measure 

 chelation as a function of its ability to remove the toxicity caused by poisonous 

 ions of high stability constant such as Cu and Hg. Fogg and Westlake (1955) 

 have demonstrated the chelating power of the polypeptides produced by blue- 

 green algae by comparing the toxic action of graded amounts of Cu in the 

 presence or absence of polypeptides on the motility of a filamentous blue-green 

 alga. The rate of movement of the filaments was perceptibly reduced at a con- 

 centration of Cu of 0.5 mg/1. whereas a similar effect in the presence of the 

 polypeptide was observed at 8-16mgCu/l. Euglena gracilis grows to high 

 densities in a medium visibly blue by the addition of Cu sulfate if it is over- 

 chelated by EDTA. 



A more complex type of assay than the one used by Johnston could be 

 useful : samples of sea- water uniformly enriched with N, P, Si and vitamins, 

 filter sterilized, could be enriched with aseptic additions of graded low con- 

 centrations of (a) a chelator ; (b) a non-chelated trace-metal mixture of Fe, 



