288 POPULATIONS OF THE SEA 



requires B12. One of the dinoflagellates, the nonphotosynthetic 

 Oxyrrhis marina, requires both, uhereas the two photosynthetic 

 species probably have no need for Bi in the medium. The chrys- 

 ophyceans require the pyrimidine component of thiamine, while 

 the dinoflagellate Oxyrrhis uses the thiazole component. 



The possible ecological implications of these results we shall 

 leave for a brief consideration later, but there is another principal 

 feature which may be stressed here. Although only a few repre- 

 sentatives of the various taxonomic groups ha\'e been investigated, 

 the results indicate a diversity in the requirements of various 

 species within the same taxonomic group, as shown for the dia- 

 toms and the dinoflagellates. This is still more clearly demonstrated 

 by the response to various compounds of the B 12 group as presented 

 by Provasoli (1958). As will be seen from Table III, there are in 

 chrysomonads and in dinoflagellates differences as to their ability 

 to utilize the various cobalamins, two types being represented 

 among the chrysomonads and two within the dinoflagellates, 

 while the diatoms add to this diversity. 



These results make it evident that generalizations of results 

 obtained for this feature of the metabolic pattern, would be un- 

 warranted. They may also be taken as a sign that great variations 

 may occur not only in this aspect but also in the general response 

 of the various species of a taxonomic group to the composition of 

 the medium and other environmental factors. This was also indi- 

 cated by the experiments on the salinity factor which have been 

 referred to previously. What the situation \\\\\ be if we consider 

 ecological groups, established by field observations, is a question 

 that cannot be answered until more species have been examined. 



Provasoli and Pintner (1953), Provasoli (1958), and Droop 

 (1957) have discussed the possible importance of the auxotrophic 

 demands of the various species for their occurrence in the sea. 

 Undoubtedly interesting ecological implications suggest them- 

 selves while the available information on the occurrence of vita- 

 mins in the sea continues to be rather scanty (Cowley, 1956), so it 

 would seem premature to draw definite conclusions. The work on 

 vitamin requirements is, howe\'er, of paramount importance for 

 experimental studies on cultures. In addition to the other im- 



