234 Marine Microbiology 



by ammonia. Diatoms having such a double requirement for 

 reduced carbon and nitrogen sources presumably have less op- 

 portunity for heterotrophic growth in natural environments than 

 do species with only a single requirement. Tw^o pennate species 

 vi^ere found that could multiply in the dark on Tryptone alone, 

 although the cells growing under these conditions were unusually 

 thin and pale. 



We now know something about the dissolved organic sub- 

 stances that occur in marine environments. For instance, free 

 sugars (sucrose, glucose, fructose, galactose, arabinose, xylose) 

 have been demonstrated in deep-sea sediments (5), and organic 

 acids (acetic, formic, lactic, glycolic) have been demonstrated 

 in sea-water samples from the Pacific Ocean (1). It is still un- 

 known to what extent diatom cells can make use of such dis- 

 solved substances for heterotrophic growth in natural environ- 

 ments, where they must compete with obligately heterotrophic 

 micro-organisms such as bacteria, fungi, and colorless flagellates. 

 Mud samples collected from the ocean floor at depths as great 

 as 7,400 m contained diatoms, some still with cell contents (6, 7). 

 Some cells of pennate forms from shallow waters (60-120 m) were 

 actively motile, but it remains to be determined whether they 

 had been actually growing at these depths. ( See Chapter 22. ) 



ACKNOWLEDGMENTS 



The author is grateful to Dr. Robert Guillard, Woods Hole 

 Oceanographic Institution, for cultures of several marine centric 

 diatoms, and to Irma Pintner, Haskins Laboratories, who isolated 

 the strain of Coscinodiscus aster omphalus. 



REFERENCES 



1. KoYAMA, T. and Thompson, T. G.: Organic acids of sea water. Abst. 



from Intern. Oceanog. Congr. Reprints, p. 925-926. Am. Assoc, for 

 Adv. of Sci., 1959. 



2. Lewin, Joyce C: Heterotrophy in diatoms. /. Gen. Microbiol., 9:305- 



313, 1953. 



3. Lewin, Joyce, C: Silicon metabolism in diatoms. III. Respiration and 



silicon uptake in Naviciila pelliculosa. J. Gen. Physiol., 39:1-10, 1955. 



4. Lewin, Joyce C. and Lewin, Ralph A.: Auxotrophy and heterotrophy 



in marine littoral diatoms. Can. J. Microbiol., 6:127-134, 1960. 



