the accumulation of the cesium. Since this study was conducted in enriched 

 natural sea water medium no attempt was made to determine the effect of 

 potassium concentration on the accumulation of cesium. The medium contained 

 a concentration of 1.5 \xg of cesium/ liter (Smales, 1955) and 388 mg/ liter of 

 potassium from the sea water and 47.6 mg/ liter of potassium from the added 

 nutrients. This high potassium concentration undoubtedly accounts for the 

 low concentration factors. These experiments should be repeated in defined 

 medium where potassium salts are not used for enrichment. 



5 . The loss of Cs-Ba-*--^^ from Various Phytoplankton . 



To study the loss of cesium-barium isotope to the medium, cells of 

 G. s imp lex and N. seriata were grown in medium containg Cs-Ba-137 for 72 

 hours then were resuspended in fresh medium according to the procedure 

 described in the Methods section. After 24, 48, and 72 hours samples were 

 removed and the loss of radioactivity was calculated. 



The cdlls of G. simplex showed a weight loss during the first 24 

 hour period which was followed by steadily increasing weight thereafter. 



Figure 7 shows a plot of the radioactivity from Cs-Ba-137 in average 

 counts per minute per milligram of cell material starting 24 hours after 

 the radioactive cells were introduced into the fresh medium. This figure 

 also shows the calculated values for the loss in radioactivity due to the 

 increase in cell weight and the difference between the two values (observed 

 and calculated). This shows that there is a loss of radioactivity from 

 the cells during the first 24 hour period of active cell growth, and that 

 a steady state seems to be established after that. 



The results from the study with N. seriata showed that there was an 

 increase in the amount of radioactivity per milligram of cell material. 

 This would indicate that the actively growing older cells continued to 



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