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287 



major part of the bacterial population, but did not obtain truly 

 pure cultures. During recent years bacteria-free cultures of several 

 species of marine diatoms and flagellates have been obtained. 

 Two methods have been used: (1) simple washing w4th sterile 

 medium and (2) treatment with antibiotics, with minor variations 

 in the procedure (for literature, see Spencer, 1952; Droop, 1954). 

 If these methods are combined with the use of a varied selection 

 of media, there should now, as indicated by Droop, be a far greater 

 chance of success in establishing bacteria-free cultures of the 

 autotrophic members of the marine plankton, although obstacles 

 may still be encountered (Kain and Fogg, 1958). Special efforts 

 ought to be made to improve still further the isolation technique. 

 We may now' return to the results which have been obtained 

 by the use of this new technique. Droop (1957) has summarized 

 the results on the vitamin requirement of marine phytoplankton 

 species up to 1957. Table II shows that the four chrysophyceans 

 which were studied in bacteria-free cultures have the same re- 

 quirements for vitamin Bi and B12, while the two diatoms, 

 Phaeodactyliim tricornutum and Sceletonema costatum, differ. 

 Phaeodactylum requires neither of them, whereas Sceletonema 



Table II. Vitamin Requirements in Bacteria-Free Cultures (Droop, 1958a) 



Portion of 



Thiamine 



B12 Thiamine Required 



Chrysophyceans 

 Monochrysis lutheri 

 Prymnesium parva 

 Syracosphaera elongata 

 Microglena arenicola 



Diatoms 



Phaeodactylum tricornutum 

 Sceletonema costatum 



Dinoflagellates 

 Oxyrrhis marina 

 Glenodinium Joliaceum 

 Peridinium trochoideum 



+ 

 + 



4- 

 + 



-h? 



Pyrimidine 



Thiazole 



