88 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



an equilibrium to be reached than if the phospho- 

 rus were exchanged only from the surface. 



The amount of phosphorus on the surface of the 

 cell may also be directly related to the manner in 

 which phosphorus enters the cell. Two ways in 

 which phosphorus may enter a cell are discussed 

 by Kamen and Spiegelman (1948). One method 

 is believed to be diffusion through the cell mem- 

 brane of phosphorus as inorganic orthophosphate 

 to combine with the intracellular orthophosphate. 

 Inorganic orthophosphate is assumed to be the 

 source of phosphorus for the various organic phos- 

 phates in the cell. The other method is the entry 

 of phosphorus into the cell through esterification 

 at the cellular interface. Intracellular inorganic 

 orthophosphate would then arise primarily from 

 the breakdown of organic phosphate. From their 

 experimental data with yeast these investigators 

 concluded that the primary mechanism of the 

 entrance of phosphate is by esterification. 



Phosphorus probably enters algae by one or a 

 combination of these two methods. The mecha- 

 nism of entry is of importance in phosphorus- 

 exchange studies only if it requires that phosphorus 

 remain on the surface of the cells for any period 

 of time. If the entry of phosphorus into the cell 

 is by diffusion instead of by esterification, it might 

 be very difficult for much phosphorus to be ad- 

 sorbed on the cell surface, since there would be a 

 tendency for any phosphorus coming in contact 

 with the cell surface to be taken into the cell. 

 The process of esterification, on the other hand, 

 would probably require that phosphorus remain 

 on the surface of the cell for a longer time. Since 

 a population of unicellular algae presents a large 

 surface area, exchange can take place between any 

 phosphorus adsorbed on the surface of the cells 

 and the medium. As far as I can determine there 

 has not been sufficient consideration of the part 

 adsorption plays in phosphorus exchange in micro- 

 organisms. It is impracticable to distinguish the 

 phosphorus exchanged from the cell surface from 

 that exchanged from inside the cells. Thus, ex- 

 change demonstrated in these studies should be 

 considered to include phosphorus both from the 

 cell surface and from inside the cell. 



The amount of phosphorus in phytoplankton 

 varies with the concentration of phosphorus in the 

 medium and some investigators have considered 

 that the minimum phosphorus which can exist in 

 the cell is "bound" phosphate, presumably organic 



phosphorus. Several observations were made in 

 this investigation which possibly show that all of 

 the minimum phosphate in the cell does not neces- 

 sarily exist as an organic fraction. It was observed 

 in the phosphorus-absorption experiment that a 

 trace of phosphorus remained in the medium. 

 For this phosphorus to remain in the medium 

 would require that some phosphorus, which could 

 be exchanged with the medium, still remain in the 

 cells. Since a sufficient amount of all other 

 nutrients were present to allow the complete 

 utilization of all the phosphorus, it appears that 

 some phosphorus was still present in the inorganic 

 fraction of the cells. It is well known that cells 

 placed in culture medium containing no phos- 

 phorus will continue to divide until they become 

 phosphorus deficient. These cells then could con- 

 tain only a minimum amount of phosphorus. Cells 

 containing radioactive phosphorus and placed in 

 medium containing only a trace of phosphorus 

 continued to divide and reduce the inorganic- 

 phosphorus fraction for several days. However, 

 after all division had stopped some radioactive 

 phosphorus remained in the inorganic fraction. 

 Thus it appears that some of the minimum phos- 

 phorus of the cell may remain in the inorganic 

 fraction. 



Hutchinson and Bowen (1950) added radioactive 

 phosphorus at the surface of Linsley Pond and 

 studied its movement over a 4-week period in 

 summer. It appeared that practically all the radio- 

 active phosphorus entered the phytoplankton 

 immediately and that the rate of regeneration was 

 rapid. This continual release of phosphorus from 

 decomposing organisms, which could be mistaken 

 for exchange, is one of several factors in operation 

 tending to complicate exchange studies under 

 natural conditions. Of course, phosphorus lost in 

 this manner could only remain in the water and 

 subsequently be mistaken for exchange when the 

 concentration of phosphorus in the water was 

 already greater than the demands of the organ- 

 isms. If a lack of phosphorus limited cell division, 

 any phosphorus returned to the water through 

 either regeneration or exchange would be absorbed 

 immediately and any possibility of its detection 

 would be eliminated. However, with cultures 

 grown in the laboratory it is possible to reduce 

 decomposition of cells to an insignificant point 

 and thus eliminate the effect of regeneration. Also, 

 it is possible to control the phosphorus concentra- 



