204 PROVASOLI [chap. 8 



It is evident that comprehensive biological assays of sea-water with unialgal 

 and bacteria-free cultures of varied organisms will be extremely useful. 



7. Prospects 



It is evident that the biological approach to the problem of "bad" and 

 "good" waters for phytoplankton has been quite successful; two variables, 

 vitamins and trace metals, have emerged and seem to be, with N and P, the 

 important parameters of the environment for phytoplankton. In the absence 

 of chemical methods, the biological approach (i.e. nutritional requirements, 

 organics produced and excreted, and biological analysis of water) may also be 

 the way to attack similar problems for the marine herbivores and carnivores. 

 Preliminary work on the nutrition of Crustacea in bacteria-free culture seems 

 promising. 



The nutritional studies reveal new important cycles. One of them, the B12 

 cycle, can now be roughly sketched : the main producers are the micro-organisms 

 (mostly bacteria) though it is not excluded that photoautotrophic algae may 

 be as important either as direct producers of vitamins or, after their death, as 

 food for vitamin-producing micro-organisms. Micro-organisms (bacteria and 

 unicellular algae, as far as we know) are the main consumers ; possibly animals 

 are also important consumers. Filter-feeding organisms — if the scant knowledge 

 on Tigriopus can be extrapolated — may absorb vitamins directly as solutes. 

 Perhaps animals with extensive gill systems absorb vitamins or other organic 

 micro -nutrients in these highly permeable organs. The non-living particles 

 (clay, organic and inorganic micelles, detritus) absorb large quantities of 

 vitamin B12 and on ingestion may supply additional vitamins. How much the 

 removal of vitamins from the particles affects vitamin cycles is unknown ; we 

 do not know whether these particles fix the vitamins in a stable way or only 

 transiently ; does partial elution maintain a certain level of vitamins as solutes 

 during high consumption of vitamins by phytoplankton? Elution in deep muds 

 might fertilize upwelling waters. 



Consumption seems rarely to bring to zero the soluble B^-like compounds 

 in sea-water (Table XII). Though data are scarce, clearly coastal and bay 

 waters, because of the influence of soil, are richer in cobalamins than open 

 waters (Kashiwada et al., 1957a; Droop, 1955; Lewin, 1954; Cowey, 1956). 

 Surface waters show clearly a seasonal variation (Cowey, 1956) and growth of 

 the spring diatoms (the dominant species is Skeletonema costatum, a B12 re- 

 quirer) in Long Island Sound is responsible for a sharp drop in B12 level (Vish- 

 niac and Riley, 1959, 1961). There is more B12 in deep waters (Kashiwada 

 et al, 1957; Daisley and Fisher, 1958). * 



Vitamin Bi 2 -like growth factors, therefore, behave like the other ecologically 

 significant nutritional variables — but are the cobalamins limiting, and where? 

 Ecological judgment is uncertain because most of the measurements have been 

 done with bioassay organisms which are not specific for true B12 (E. gracilis and 



1 See Addendum, page 210. 



