SECT. 2] ORGANIC REGULATION OF PHVTOPLANKTON FERTILITY 201 



pronounced effect, reducing photosynthesis almost to the level of the un- 

 enriched controls; all other omissions, except Si, gave the same high 14 C 

 uptake as the complete enrichment ; omission of Si resulted in about half the 

 amount of 14 C uptake of the complete enrichment. Ryther and Guillard do not 

 mention the species composition of the phytoplankton present in the various 

 samples. Two interesting additional observations were made. The mixing of 

 deep waters (1000 m) with the surface waters of the Sargasso results in a photo- 

 synthesis as high as the surface water with complete enrichment, defining 

 experimentally the elements contributed by the deep waters which impart 

 renewed fertility after mixing. Even more interesting is that the surface waters 

 of the Sargasso are extremely poor in nitrate and phosphate yet the addition 

 of these two elements had no effect on photosynthesis (growth in complete 

 enrichment minus N or P = growth in complete enrichment). The authors 

 comment "... that the phytoplankton may be dependent upon the rate of 

 regeneration of these elements [N and P] and/or their presence as dissolved 

 organic compounds and more-or-less independent of their instantaneous con- 

 centrations as inorganic salts". It would seem necessary to measure organic P 

 and N of these deep waters. 1 



Johnston went a step further ; he found why the addition of trace metals is 

 necessary. The trace-metal mixture employed by him (PI Metals, see Provasoli 

 et ah, 1957) is over-chelated ; the ratio chelator /trace metals in milliequivalents 

 is about 2:1, raising the possibility that either the chelated (1:1) trace metals 

 or the excess free chelator of the mixture could be responsible for improving 

 "bad" waters for phytoplankton. In fact, the addition of chelators alone, as 

 EDTA and DTPA, and in a lesser degree of NTA and EDDHA, 2 is as effective 

 as the addition of the chelated trace-metal mixture. These results fit perfectly 

 with the ecological situation : at the normal pH of sea- water, iron, manganese 

 and probably the other trace elements are extremely insoluble. The total Fe 

 in sea-water varies between 1-60 \xg atoms/1, (see results of various authors in 

 Harvey, 1955, and Goldberg, 1957); in the offshore waters off the Norwegian 

 coast the total Fe is 3-21 [j.g atoms/1. (Braarud and Klem, 1931). However, 

 Cooper (1937) calculated that no more than 10 -8 \xg atoms/1, of Fe can remain 

 in solution at pH 8.0-8.5. Therefore the deficiency of trace metals depends 

 quantitatively far more on the physical status governing their availability to 

 the cells than on the total amount. Growth promotion by metal chelators is 

 due to their solubilizing power at the pH of sea-water, making available the 

 trace metals which are present but largely unavailable. Indeed, Johnston 

 found that most of the "bad" waters for phytoplankton became fertile upon 

 addition of chelators, and some "good" waters became poor. The latter happen- 

 ing may be due to very high total trace metals in the samples. The ability to 

 form soluble metal complexes is not at all restricted to the artificial chelates, 



1 See Addendum, page 210. 



2 EDTA = ethylenediamine tetraacetic acid; EDDHA = ethylenediamine-di-(o-hydroxy- 

 phenylacetic acid) ; DTPA = diethylenetriamine pentaacetic acid ; NTA = nitrilotriacetic 

 acid. 



