PHOSPHORUS EXCHANGE IN PHYTOPLANKTON 



85 



cells in a short enough period of time to prevent 

 much from being combined into organic compounds, 

 there is greater possibility of detecting exchange 

 by measuring the loss of radioactive phosphorus 

 from the cells. It must also be considered that, 

 since each cell takes part in the exchange process, 

 the total amount of radioactive phosphorus 

 exchanged from the cells will depend upon the 

 number of cells used. If a large number of 

 dividing cells are used, a high concentration of 

 phosphorus in the medium is needed. Further, 

 if the concentration of phosphorus in the medium 

 is not sufficient, the radioactive phosphorus 

 exchanged from the cells to the medium will be 

 reabsorbed in too large an amount to permit a 

 detection of exchange. 



A culture of Nitzschia with 22.7 X 10 7 cells per 

 liter in medium containing 2 microcuries of radio- 

 active phosphorus per 100 cc. and a phosphorus 

 concentration of less than 0.5 MgA/L was prepared. 

 It was found that cells could absorb this amount 

 of phosphorus within 20 hours and yet retain 

 most of the radioactive phosphorus in the inor- 

 ganic state. The cells were then centrifuged from 

 this medium and resuspended in new medium 

 containing 2,000 /igAP/L of nonradioactive phos- 

 phorus. It has been found that very little reduc- 

 tion in the division rate of Nitzxchia occurred in 

 this concentration. This new culture was placed 

 in the culture cabinet in the light. Aliquot 

 samples of 25 cc. each were removed periodically, 

 and the cells were filtered from the medium so 

 that radioactive measurement could be made on 

 the culture medium. The sampling was con- 

 tinued over a period of 14 days (table 5). 



Table 5. — Loss of radioactive phosphorus to medium by 



photosynlhesi zing and dividing cells 



[The cells of 25 cc. of medium contained 9,728 counts per minute) 



Phosphorus exchange did occur, since radio- 

 active phosphorus from the cells appeared in the 

 medium. The increase in radioactivity in the 

 medium was rapid at first and then decreased 

 with time. Thus, it has been shown that cells, 



while absorbing and converting phosphorus into 

 organic compounds, do exchange phosphorus with 

 the medium. The radioactive phosphorus in the 

 medium continued to increase until 6.97 percent 

 of the total radioactive phosphorus contained in 

 the cells had been exchanged to the medium at the 

 end of 14 days. 



Exchange in the dark 



Ketchum (1939 b) has demonstrated that 

 phosphate-deficient cells placed in the dark will 

 continue to absorb phosphate only for about 10 

 hours. He also showed that the amount of phos- 

 phate absorbed in the dark by phosphorus- 

 deficient cells depends on the length of time the 

 cells have been grown in the light in me dium 

 containing no phosphate. From these findings it 

 can be concluded that any uptake of radioactive 

 phosphorus by cells can be attributed to exchange 

 if radioactive phosphorus is added to the medium 

 after the phosphate deficiency of the cells has 

 been replaced. 



Exchange determined with radioactive phos- 

 phorus in the medium. — A culture of 45 X 10 7 cells 

 per liter was prepared in medium containing 

 46 /igAP/L. The cells used to prepare this culture 

 came from a medium containing phosphorus. 

 However, to be sure that the cells had no phos- 

 phate debt, they were placed in the dark for 16 

 hours in medium containing phosphorus. The 

 cells were then centrifuged from this medium and 

 resuspended in identical medium to which 0.45 

 microcuries of radioactive phosphorus per 100 cc. 

 had been added. The culture was again placed 

 in the dark and only removed periodically for 

 sampling. The cells were filtered from aliquot 

 samples of medium, and counts were made for 

 radioactivity in the cells. Inorganic-phosphorus 

 determinations were also made on the medium 

 from each sample. Exchange does occur, as 

 shown by radioactive phosphorus appearing in 

 the cells (table 6). Although the uptake of 

 radioactive phosphorus is small, it is believed to 

 be representative of the amount exchanged, since 

 corrections for contamination were made. This 

 was accomplished by filtering the medium con- 

 taining the same quantity of radioactive phos- 

 phorus and determining the activity retained on 

 the filter, in the BaS0 4 precipitate, and on the 

 filter paper. The counts appearing in table 6 

 were then obtained by subtracting the counts 



