PHOSPHORUS EXCHANGE IN PHYTOPLANKTON 



87 



nonradioactive phosphorus as that of the medium 

 in which the cells had previously grown. The 

 amount of phosphorus exchanged with the medium 

 when filter-washed in this manner was found to 

 vary with the physiological condition of the cells. 

 Phosphate-deficient cells grown for the same 

 period of time in medium containing radioactive 

 phosphorus not only absorbed more phosphorus 

 than nondeficient cells, but also exchanged a 

 smaller percentage of that absorbed when filter- 

 washed. Regardless of the condition of the cells, 

 the amount exchanged varied with the phosphorus 

 concentration of the medium in which the cells 

 were grown. Those grown in a high phosphorus 

 concentration not only absorbed more phosphorus 

 but also exchanged more phosphorus with the 

 washing medium. 



The percent of radioactive phosphorus ex- 

 changed by the cells also depends on the length 

 of time the cells have been grown in radioactive 

 phosphorus. In the filter-washing experiments, 

 nondeficient cells grown for 7 days in the presence 

 of radioactive phosphorus exchanged a smaller 

 percent of radioactive phosphorus than the same 

 type of cell grown in a similar medium for 20 hours. 

 This was the result of more radioactive phosphorus 

 being converted into organic or nonexchangeable 

 compounds in cells grown for the longer period of 

 time. Thus, a smaller percent of the radioactive 

 phosphorus in the cells was exchanged with the 

 medium even though the total phosphorus ex- 

 changed in both instances possibly was the same. 



In cultures of rapidly dividing unicellular 

 forms, the organic matter increases at a fast rate, 

 quickly removing both radioactive and nonradio- 

 active phosphorus from the medium and thus 

 complicating the measurement of exchange. It 

 is less difficult to detect exchange by measuring 

 the movement of radioactive phosphorus from the 

 cells to a medium continually renewed and con- 

 taining only nonradioactive phosphorus, provided 

 the specific activity of the exchangeable phos- 

 phorus of the cell is high. These were the con- 

 ditions in the filter-washing experiments. Since 

 under most conditions some radioactive phos- 

 phorus remains in the inorganic fraction of the 

 cell and the addition of large amounts of phos- 

 phorus to the medium causes an increase in the 

 amount of phosphorus exchanged by the cell, 

 exchange of radioactive phosphorus from the cell 

 can also be detected by increasing the phosphorus 



concentration of the medium instead of continually 

 renewing it. By following this procedure, phos- 

 phorus exchange was measured between dividing 

 cells maintained in the light and the culture me- 

 dium. While exchange is easily demonstrated by 

 filter-washing, the experiments conducted in the 

 light and in the dark may be more representative 

 of normal exchange. 



In the exchange experiment in the dark the 

 cells were in equilibrium with phosphorus in the 

 medium before the radioactive phosphorus was 

 added. The actual amount of phosphorus added 

 with the radioactive phosphorus in terms of 

 figAP/L was insignificant. Since the cells had 

 remained in the culture cabinet long enough for 

 any phosphate deficiency to have been repaid 

 and, therefore, any absorption of phosphorus 

 from the medium to have stopped, the appearance 

 of radioactive phosphorus in the cells indicated 

 that phosphorus exchange had occurred between 

 cells and medium. In some instances it is only 

 possible to measure exchange over short periods 

 of time; however, it should not be concluded that 

 exchange ceases when it is no longer possible to 

 detect it, but rather that both the radioactive 

 and nonradioactive phosphorus of the medium 

 and the cells have reached equilibrium. Thus, 

 neither n further absorption of phosphorus from 

 the medium nor exchange would alter the specific 

 activity, without which exchange cannot be 

 demonstrated. 



In the experiments conducted in the light and 

 in the dark the amount of phosphorus exchanged 

 was very small. It may be questioned whether 

 there was not an exchange between the phosphorus 

 of the medium and that adsorbed on the surface 

 of the cells instead of the phosphorus from inside 

 the cells. That some of the phosphorus exchanged 

 in these experiments came from inside the cells is 

 supported by several observations. In relation to 

 the total amounts of phosphorus adsorbed, the 

 amounts exchanged in the filter-washing experi- 

 ments were too large to have been exchanged only 

 with the phosphorus adsorbed on the cell surface. 

 Also it was possible to measure exchange for as 

 long as 24 hours in some experiments and for a 

 longer period of time in the experiment tested for 

 14 days. It is possible that exchange between 

 intracellular phosphorus and the medium is com- 

 plicated by biological or other processes of the 

 cell, resulting in a longer time being required for 



