202 PROVASOLI [CHAP. 8 



but is shared by many organic compounds, e.g. amino acids, nucleotides and 

 hydroxy acids, which have been found or could occur in sea-water (see Section 2 

 of this chapter and Chapter 9). Some components, then, of the organic 

 matter in the sea perform the dual role of nutrients and of solubilizers of the 

 indispensable trace metals. The experiments of Johnston show that the quality 

 of waters for phytoplankton may depend largely on the presence or absence of 

 trace-metal solubilizers. 1 The search for these substances is, then, of paramount 

 importance. 



Johnston, besides enriching samples of sea-water containing their living 

 phytoplankton, did two other types of assay in his attempts to assess the bio- 

 logical properties of the waters. In one assay (No. 1), filtered sea-water samples 

 were enriched with one-fifth volume of medium S36, then autoclaved, and 

 inoculated with a bacteria-free culture of Skeletonema costatum (Droop's strain) ; 

 medium S36 was developed for the same strain by Droop (1955a). In the second 

 assay (No. 2), filtered sea-water samples were enriched with N, P, Si, and a 

 chelated trace-metal mixture, autoclaved, and inoculated with unialgal 

 cultures (i.e. with unknown bacterial flora) of S. costatum and Peridinium 

 trochoideum. These two assays show that the quality of sea-water varies from 

 place to place, with depth and season. However, the unialgal cultures of 

 Peridinium and Skeletonema gave a different assessment of sea-water quality 

 indicating that the quality of sea-water acts differently on different classes of 

 organisms (confirming the previously noted contrast between diatoms and dino- 

 flagellates). Hence we should always specify for which organisms waters are 

 good or bad. The assay response obtained with bacteria-free S. costatum (No. 1) 

 does not correlate with temperature, salinity, oxygen, blue fluorescence, 

 phosphate, phytoplankton and zooplankton abundance or dominance ; it hints 

 that waters of superior quality for S. costatum are recently mixed oceanic and 

 neritic waters, and that waters collected after the bloom of spring diatoms are 

 poor. The assay response with unialgal cultures (No. 2) correlates with plankton 

 classification. 



Earlier experiments had detected a very important difference between the 

 assay with bacteria-free and with unialgal Skeletonema. Bacteria-free Skeleto- 

 nema grew very poorly or not at all in 215 samples of sea-water collected in 

 different seasons and localities of the northern seas when vitamins were omitted 

 from the enrichment (N, P, Si, with and without chelated trace-metal mixture). 

 Samples of sea-water similarly enriched but inoculated with unialgal (i.e. 

 bacterized) cultures of Skeletonema supported poor to good growth depending 

 upon the sea-water samples. Evidently the bacteria of the unialgal cultures 



1 In fresh waters some terminal products of microbial metabolism, like the "humic 

 substances", perform this action. The yellow organic acids extracted by Shapiro (1957) 

 are good trace -metal chelators and so are the polypeptides produced extracellularly by 

 blue-green algae (Fogg and Westlake, 1955). Their ecological importance is discussed in 

 these papers and by Fogg (1958). Similar substances have not yet been extracted from 

 sea-water, though the "Gelbstoff " of Kalle (1949), which may be similar to the yellow acids 

 of Shapiro and the "water humus" of Skopintsev (1959), may perform the same function. 



