PHOSPHORUS EXCHANGE IN PHYTOPLANKTON 



81 



or of a combination of the two. When exchange 

 occurs, the specific activity, or ratio of radio- 

 active to total phosphorus atoms, of the medium 

 is changed. In this experiment only nonradio- 

 active phosphorus could initially be exchanged 

 from the cells since the cells had previously grown 

 in medium containing only nonradioactive phos- 

 phorus. That the observed differences in the 

 two curves could have been caused by exchange 

 seems improbable, when the accuracy of the 

 colorimetric method is considered. With this 

 method small changes cannot accurately be 

 determined. This is especially so when the 

 medium contains large amounts of phosphorus 

 requiring dilution to obtain satisfactory color 

 intensities for measurement. 



Even if all the difference between the two 

 absorption curves could be attributed to ex- 

 change, which it cannot, the total amount of 

 phosphorus exchanged would still be very small. 

 From this experiment it can be concluded that, 

 since the metabolic need for phosphorus by divid- 

 ing cells is so great and the errors of the colori- 

 metric method for measuring phosphorus are so 

 large, any movement of phosphorus due to ex- 

 change cannot be detected with a reasonable 

 degree of accuracy. 



CONDITIONS AFFECTING PHOSPHORUS EX- 

 CHANGE BY CELLS WHEN FILTER-WASHED 



Another way to detect exchange is to incorpo- 

 rate radioactive phosphorus into the cells before 

 placing them in a medium which is continually 

 renewed and which contains only nonradioactive 

 phosphorus. If an exchange of phosphorus occurs, 

 the radioactive phosphorus leaving the cells is 

 removed before it can be reabsorbed by the cells. 

 The appearance of radioactive phosphorus in the 

 medium would indicate that phosphorus was being 

 exchanged between cells and medium. Since 

 the washing medium in these experiments con- 

 tained the same concentration of nonradioactive 

 phosphorus as the medium in which the cells had 

 been growing, no differentiation could have been 

 made between phosphorus originally in the 

 washing medium or in the cells without the use of 

 radioactive phosphorus. 



In experiments conducted by Gest and Kamen 

 (1948) on exchange by fresh-water algae, washings 

 were made by centrifuging the cells containing 

 radioactive phosphorus from the medium, decant- 



ing the medium, resuspending the cells in new 

 medium, and repeating the process several times. 

 This method was first used as a means of washing 

 in these investigations, but it was later found that 

 filter-washing was more suitable. Goldberg, et al. 

 (1951) found that 50 percent of the radioactive 

 phosphorus in the diatom Asterionella japonica is 

 lost when the organism is washed while suspended 

 on a filter. However, it will be shown that several 

 factors may influence the amount of phosphorus 

 exchanged from phytoplankton when filter- 

 washed. 



In the following experiments, Nitzschia cells in 

 two different physiological states, nondeficient and 

 phosphate-deficient, were used to follow phos- 

 phorus exchange. The nondeficient cells were 

 grown in a north window in culture medium 

 containing phosphorus, while the phosphate- 

 deficient cells were grown for several days in the 

 culture cabinet in medium containing no added 

 phosphorus, following methods developed by 

 Ketchurn (1939b). The deficient cells continue 

 to divide in the absence of phosphorus in the 

 medium until the phosphorus in each cell is 

 greatly reduced and a condition is reached where 

 further division without phosphorus is impossible. 

 Exchange by nondeficient cells 



The required number of cells were centrifuged 

 from stock culture medium containing phosphorus. 

 These cells were then resuspended in new culture 

 medium and divided into two equal cultures. To 

 one culture enough radioactive and nonactive 

 phosphorus was added to give a concentration of 

 45.5 /igAP/L, and enough to give 1,137.5 ugAP/L 

 was added to the remaining culture. To each 

 culture, radioactive phosphorus was added in 

 sufficient amounts to give a specific activity of 

 0.1624 ttc/ngAP. Both cultures were placed in 

 the culture cabinet in a constant source of light 

 for 20 hours. During this time the cells took up 

 phosphorus and became radioactive. At the end 

 of 20 hours the cells from 50 cc. of each of these 

 cultures were filtered by means of a BaS0 4 filter, 

 and a radioactive count was made on them to 

 determine the total radioactive phosphorus ab- 

 sorbed. To measure the loss of phosphorus, cells 

 from another 50-cc. portion were then filtered 

 and washed five times with 50-cc. portions of 

 culture medium containing the same concentration 

 of phosphorus as the medium in which these cells 



