Abstract:
Using 15N-labeled NH4+, we assessed the time-course of NH4+ uptake, the parameters of saturation kinetics, and the rates of incorporation of NH4+ into trichloroacetic acid-insoluble material by Chesapeake Bay phytoplankton. The amount of NH4+ taken up by the phytoplankton increased linearly with time for 2 h for all samples growing on or exposed to high concentrations (> 1 μg-atom N • liter-1) of NH4+, but extrapolations to time zero did not intercept the origin. These results could be explained by rapid isotopic equilibration of internal and external NH4+ pools. For stations for which ambient NH4+ 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 NH4+ 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.
