286 POPULATIONS OF THE SEA 



New York, by Droop in Millport, and Sweeney at La Jolla (for 

 literature, see Droop, 1957) information has been obtained on the 

 vitamin requirements of representatives for marine diatoms, dino- 

 flagellates, and chrysophyceans, especially as to \-itamin Bi and 

 Bi2. These results have been reached by means of an improved 

 technique which we shall consider before presenting some of the 

 findings of their research into the necessity of organic compounds 

 for growth. 



The new technique is the result mainly of the work by Pro\'asoli 

 and his collaborators (see Provasoli et al., 1957) on the composi- 

 tion of media, and of the success of various workers in establishing 

 bacteria-free cultures. Compared with the older synthetic media 

 the new ones are characterized by the following improvements: 



(1) a uniform and satisfactory supply of micronutrients by means 

 of chelating agents as EDTA (ethylenediamine tetraacetic acid), 



(2) buffering of the medium by means of a compound which is not 

 used during the growth of the alga, as TRIS (hydroxymethyl- 

 aminomethane), and (3) addition of defined quantities of organic 

 compounds required by the alga, as vitamin Bi and Bi-., which 

 were previously supplied by the natural sea water and the soil 

 extract (Sweeney, 1954). 



The use of these compounds has made it possible not only to 

 obtain satisfactory growth in synthetic media with a general 

 composition similar to that of sea water but also to make essential 

 changes in the ionic composition of the medium for a study of the 

 relative importance of the various components of sea water, as 

 made by Provasoli et al. (1954). Droop (1958b) has used this 

 technique in a study of euryhaline species from littoral pools and 

 of the pelagic diatom Sceletonema costatiim and has drawn the 

 tentative conclusion that the different salinity response of the 

 various species is due mainly to their special requirements for the 

 concentration of sodium ions in the medium. 



One of the main drawbacks with the Allen technique was the 

 presence of bacteria and colorless flagellates in the persistent cul- 

 tures. Schreiber (1927) devised a special apparatus for washing 

 diatoms in order to get rid of bacteria. Jenkin (1937) used a treat- 

 ment with iodine, suggested by Allen, for destroying at least the 



