NITRATE UPTAKE KINETICS BY 2 MARINE DIATOMS USING THE RADIOACTIVE-TRACER N-13

The response of the nitrate uptake system to nitrate concentrations in the range 1 to 500 muM was examined with the radioactive isotope N-13 for two marine diatoms (Skeletonema costatum and Thalassiosira weissflogii) under various states of nitrogenous nutrition. Classical saturation kinetics were observed in the lower range of concentrations (1-40 muM), but a further increase in uptake was observed above 60 muM. Intracellular nitrate concentrations between 5 and 17 mM indicate that it is an active uptake system rather than a diffusion phenomenon which is responsible for uptake at those high external nitrate levels.