JOHN H. RYTHER 393 



exception, soil extract as their common ingredient (Barker, 1935; 

 Gross, 1937; Braarud, 1951; Sweeney, 1951). 



Sweeney found that this substance best supported the growth of 

 Gymnodinium splendens when aged for 4-6 weeks, and it was rela- 

 tively ineffective when freshly prepared or aged for more than two 

 months. Neither Barker nor Sweeney was able to replace soil extract 

 with trace element mixtures. The latter author concluded that the 

 active ingredient is probably organic in nature, and later (Sweeney, 

 in press) succeeded in replacing soil extract with vitamin Bio. 



King (1950) cultured Gymnodinium simplex in a medium consisting 

 of McClendon's artificial sea water, Hoagland's trace element mixture, 

 and yeast extract. Again, she was unable to obtain growth with the 

 inorganic constituents alone, but was able to replace yeast extract 

 with a mixture of amino acids added as pure chemicals in the same 

 ratio as they occur in the yeast extract. 



Provasoli and Pintner (1953) synthesized an artificial medium for 

 the growth of Gyrodinium californicum which contained, in addition 

 to the common inorganic nutrients and trace metals, a chelating agent 

 (EDTA), NaH glutamate, dl-\ycine, JMeucine, and vitamin B12. 



Although the growth-promoting ingredients of soil extract have not 

 been identified, it would appear likely, in view of the preceding evi- 

 dence, that they include growth factors such as vitamin Bio. The 

 organisms may also derive some benefit from humic acid and other 

 ingredients which may act as chelating agents, reducing the concen- 

 trations of some one or more of the trace metals to a nontoxic or 

 noninhibitory concentration. 



In inshore waters dinoflagellates are often abundant, and in many 

 parts of the world their populations may develop bloom proportions. 

 Here the close association of the plankton with land masses and the 

 contribution of runoff water to their enviromnent provide what may 

 be considered as a natural "soil extract," and, in the vicinity of heavily 

 populated areas, frequently a source of organic pollutants. As men- 

 tioned earlier, Braarud (1945) observed a heavy growth of dino- 

 flagellates in the highly polluted regions of the Oslofjord. Braarud and 

 Pappas (1951) later found that the addition of small amounts of raw 

 sewage to the medium stimulated the growth of Peridinium trique- 

 trum,. 



