Organic Sources of Nitrogen for Marine Centric Diatoms 97 



added aseptically to autoclaved nitrogen-free medium. Stock solu- 

 tions of glutamate, glutamine, and urea were sterilized by filtra- 

 tion through a membrane filter ("Millipore," 0.45 /- pore size). 

 The entire medium containing uric acid was filter sterilized be- 

 cause of the low solubility of uric acid. Sterility tests of algal stock 

 cultures, nitrogen stocks and certain experimental flasks were 

 made with % strength nutrient broth and agar (Difco), made 

 with both distilled water and 80 per cent sea water and with 

 medium STP of Provasoli et al. ( 19 ) . Three weeks of incubation 

 at the temperatures of the algal cultures were allowed for bac- 

 terial growth. 



The species of algae studied are shown in Table 2, with the 

 locations from which they were isolated. Coscinodiscus astcrom- 

 phalus was isolated by Miss Irma Pintner, and Coccolithus Jnix- 

 leyi (clone 92-A) by Dr. Mary Parke; the others were isolated 

 by the author. All clones were bacteria-free except Chaetoceros 

 lorenzionus. Some flagellates were included to compare with dia- 

 toms of the same habitats. Clones BT-5 and BT-6 (Coccolithus 

 luixlet/i) were of particular interest because they are present 

 throughout the year in the Sargasso Sea (13). 



RESULTS 



Table 2 shows the vitamin requirements of the different 

 strains and result of experiments in which growth rates were 

 observed in media having different salinities. A critical salinity 

 interval, when found, is given for each species. Methods are 

 discussed elsewhere (9, 22). 



Growth Without Added Nitrogen 



The aged Sargasso Sea water used had about 2.8 /'M/L 

 of total inorganic nitrogen and an unknown amount of residual 

 organic nitrogen. The increase in cell numbers without added N 

 ranged from 0.5 to 5.3 per cent of the increase brought about 

 by 100 /'M nitrate or ammonia. The average was 2.6 per cent, 

 which coiTcsponds to the measured amount of inorganic nitrogen. 

 This agrees with Harvey's observation ( 12 ) that in freshly col- 

 lected sea water the amount of nitrogen that was used by two 



