Nature of NH₄⁺ uptake kinetics of Gracilaria tenuistipitata var. liui and Ulva pertusa.

Nonlinear time courses of NH₄⁺ depletion from the mediumimply that the rate limiting step for ammonium uptake changes over time.NH₄⁺ uptake by the starved macroalgae Gracilaria tenuistipitata var.liui and Ulva pertusa were studied from the depletion of NH₄⁺ after single additions of NH₄⁺ to cultures.The experiments were carried out by the combination of two different experimental setups——the "multiple flask" and the "perturbation" technologies.Uptake rate was nonlinear with time and decreased in V_m as incubation time increased (i.e., V_max^0~15, V_max^15~30, V_max^30~60, V_max^90~120 and V_max^120~240).The corresponding maximum uptake rates, the half saturation constants and the V_max/K_s (a) ratios (the affinity for ammonium uptake at low substrate concentrations) in U.pertusa were all higher than those of in G.tenuistipitata.Three distinct succeeding phases of NH₄⁺ uptake were identified: (1) Surge uptake phase, (2) internally controlled uptake phase, and (3) externally controlled uptake phase.The nature of NH₄⁺ uptake rate of decline is species specific and may be due to feedback inhibition from the filling of an internal pool.