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FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



had been growing but to which no radioactive 

 phosphorus had been added. 



The 45.5 and 1,137.5 MgAP/L in these experi- 

 ments will be referred to as low and high phos- 

 phorus concentrations. It is realized that the 

 45.5 ;ugAP/L is not a low concentration when 

 compared with concentrations occurring in nature. 

 However, the quantity of phosphorus available 

 per cell is more important than the concentration 

 per unit volume of medium. Since some popula- 

 tions of cells used are often many times greater 

 than those ordinarily occurring in nature, the 

 conditions in these cultures with the low con- 

 centrations may after all be very similar to natural 

 conditions. 



In all filter-washing experiments, the activity 

 in the washings plus the activity still remaining 

 in the cells after washing always amounted to at 

 least 90 percent of the total activity contained 

 in the same quantity of cells before washing. 

 Thus, never more than 10 percent of the radio- 

 active phosphorus was unaccounted for even 

 though six different activity readings were made 

 in the washing procedure. 



Although the medium with the high phosphorus 

 concentration contained 25 times more phosphorus 

 than that of the low concentration, it also con- 

 tained 25 times more radioactive phosphorus, 

 specific activity being the same in both cultures. 

 Like amounts of radioactive phosphorus absorbed 

 or exchanged in either culture would therefore 

 represent the same amount of phosphorus. 



Loss of radioactive phosphorus to the medium 

 showed that nondeficient cells grown in both the 

 low and high phosphorus concentrations ex- 

 changed phosphorus with the medium when 

 filter-washed (table 1). The cells grown in the 

 high concentration not only absorbed more phos- 

 phorus from the medium in which they were grown 

 but also exchanged more phosphorus with the 

 medium when filter-washed. Ketchum (1939 a) 

 has previously shown that Nitzschia cells absorb 

 more phosphorus when grown in medium con- 

 taining high phosphorus concentrations. Since 

 the amount of phosphorus entering the cells is 

 proportional to the concentration in the medium, 

 it necessarily follows that any phosphorus entering 

 the cells in excess of that which the cells can con- 

 vert into the organic state, will remain in the in- 

 organic state. This may explain the observation 

 that, when other conditions are similar, the 



concentration of phosphorus in the medium in 

 which cells are grown controls the amount of 

 phosphorus in the cells available for exchange. 



Table 1. — Exchange by nondeficient cells grown in low and 

 high phosphorus concentrations for 20 hours before filter- 

 washing 



[Initial cell count was 46X10' cells per liter. Specific activity was 0.1624 

 jic/jjgAP in both media] 



Exchange by phosphate-deficient cells 



Ketchum (1939 b) has shown that phosphate- 

 deficient cells contain only a small fraction of the 

 phosphorus found in nondeficient cells, and that 

 more phosphorus is absorbed by deficient cells 

 when again placed in medium containing phos- 

 phorus. Since the physiology of phosphate- 

 deficient cells must be different from that of 

 nondeficient cells, the amount of phosphorus ex- 

 changed by this type of cell when filter-washed 

 was tested. The effects of low and high phos- 

 phorus concentrations on the amount of phospho- 

 rus exchanged was also determined. 



The phosphate-deficient cells used in the exper- 

 iment were prepared by suspending cells in 

 medium to which no phosphorus had been added. 

 After placing the culture in the cabinet in the 

 light for 3 days, the population had increased from 

 45 X 10 7 to 82 X 10 7 cells per liter. Since the 

 culture medium contained only a trace of phos- 

 phorus, the increase in the number of cells prob- 

 ably had reduced the amount of phosphorus within 

 each cell to a point where further division was 

 impossible. From this culture the cells were cen- 

 trifuged, resuspended in medium containing radio- 

 active phosphorus, and then filter-washed as de- 

 scribed for the nondeficient cells. 



Phosphate-deficient cells grown in the higher 

 concentration of phosphorus absorbed more phos- 

 phorus than those grown in the lower concentra- 

 tion (table 2). Those grown in the higher con- 

 centration also lost a larger percentage of phos- 

 phorus when filter-washed. Thus, the phosphorus 

 concentration of the medium in which the cells 

 are grown controls the amount of phosphorus 



