230 Marine Microbiology 



Substrates Employed 



In a general but limited suney of this type, the choice of 

 potential substrates is important, since with so many isolates 

 it is impracticable to test a wide variety of substrates. Since 

 previous investigations showed that glucose was preferred for 

 heterotrophic growth by many freshwater diatoms (2), and that 

 lactate was readily oxidized by respiration in the dark ( 3 ) , tliese 

 two compounds were chosen as the substrates most likely to 

 support heterotrophic growth in marine diatoms. 



Most marine diatoms in illuminated cultures grow better 

 when the medium is supplemented not only with nitrate but 

 also with an organic nitrogen source, such as Tryptone. (Other 

 organic sources such as glycine or Casamino acids serve 

 equally well.) For those species stimulated by Tryptone in the 

 light, Tryptone was also added to the media in tests for hetero- 

 trophic growth in the dark. 



The general procedure was to transfer cells of each isolate 

 into: (a) sea- water medium enriched with nitrate, phosphate, 

 silicate, trace elements, vitamin Bi, vitamin B12, and generally 

 also with 0.1 per cent Tiyptone; (b) the same medium + 

 glucose (0.5%); (c) the same medium + lactate (0.2%). The 

 cultures were incubated both in the light and in the dark. At the 

 end of two weeks the presence or absence of growth in darkness 

 was noted; in most cases the result was unequivocal. 



RESULTS 



Species Capable of Heterotrophic Growth 



The species tested and the results obtained are summarized 

 in Table I, from which certain trends and generalizations emerge. 

 Half of the twenty-four species of littoral pennate diatoms tested 

 included strains capable of heterotrophic growth. Among the 

 fifteen species of centric diatoms tested, so far only one isolate 

 has been found capable of heterotrophic growth. This is a small 

 species of Cyclotella isolated from Martha's Vineyard, Mass., by 

 Dr. R. Guillard. None of the seven isolates of centric diatoms 

 obtained by Guillard from the Sargasso Sea grew heterotrophi- 

 cally. 



