RATE OF PHOSPHORUS UPTAKE BY PHAEODACTYLUM TRICORNUTUM

Portions of a phosphorus-deficient culture of Phaeodactylum tricornutum Bohlin were dispensed into fresh media containing phosphate concentrations from 8 to 80 µM. The instantaneous initial phosphate uptake rates were 12 to 24 x 10-17 mole/cell·min. The concentrations of phosphorus in the cells extended from a high of 66 x 10-15 mole/cell after 12 hours' exposure to phosphate-rich (32 and 80 µM) media to a low of 2 x 10-15 mole/cell when PO4 depletion of the medium limited further growth. In another experiment Phaeodactylum was prepared with varying intracellular P concentrations in media with very low PO4 concentrations. Radioactive phosphate was then added, the time course of P32 distribution was followed, and the rate of phosphate uptake was calculated. The initial rates ranged from 10-17 to 3 x 10-17 mole/cell·min. These were about one order of magnitude less than the uptake rates during the first two hours of the first experiment, even though the PO4 concentrations were two to four orders of magnitude lower. Radioisotope analysis showed that Phaeodactylum decreased the phosphate in the medium to as little as 7.2 x 10-10 M, a concentration much below the limit of sensitivity of the chemical analytical method.