Ammonium uptake and incorporation by Chesapeake Bay phytoplankton: short term uptake kinetics

Using ‘JN-labeled NH, I-, we assessed the time-course of NH4+ uptake, the parameters of saturation kinetics, and the rates of incorporation of NII, “ into trichloroacetic acid-insoluble material by Chesapeake Bay phytoplankton. The amount of NH4+ taken up by the phyto-plankton increased linearly with time for 2 h for all samples growing on or exposed to high concentrations (> 1 pg-atom N * liter-‘) of NH4, + but extrapolations to time zero did not inter-cept the origin. These results could be explained by rapid isotopic equilibration of internal and external NH,+ pools. For stations for which ambient NH, + bordered on detection limits, rapid but nonsaturablc uptake was observed in l- and 5-min incubations, whereas saturation was attained in 15-60-min incubations. The experiments on rates of NH4+ incorporation into TCA-insoluble material demonstrated that most (>85%) of the 15N label was incorporated into the macromolecular fraction by-15 min. These data thus demonstrate that rates of NH,+ uptake may become limited within minutes by rates of assimilation and incorporation into macromolecules and that reported kinetic parameters reflect these metabolic rates rather than transport rates. In the sea NH4+ is a major source of phytoplankton nitrogen and, despite low ambient concentrations, accounts for 30-90 % of the nitrogen used by phytoplank