The physiology of nitrogen acquisition was determined for Gracilaria gracilis (Stackhouse) Steentoft, Irvine et Farnham in a series of perturbation experiments with the aim of examining uptake kinetics in response to transiently variable N. Experiments were designed to determine how variables such as history of exposure to nutrients, NO3−-N and NH4+-N concentrations and interactions, temperature and water motion affect parameters of linear and Michaelis-Menten models. A third ‘Michaelis-Menten parameter’ (α) is introduced here and used to extract additional ecological relevant information from the model. Ammonium-nitrogen uptake was best described by a linear, rate-unsaturated response, with the slope increasing with N-limitation, indicating that Gracilaria is more efficient at acquiring nutrients when internally stored N pools were impoverished. Temperature also affected the slope of the linear regression in the case of N-repletematerial. Nitrate-nitrogen uptake was suppressed by approximately 38 % in the presence of NH4+-N at concentrations above 5 μM, and the seaweed displayed a higher affinity for NH4+-N than for NO3−-N at low temperatures. Nitrate-nitrogen uptake followed a rate-saturating mechanism best described by the Michaelis-Menten model. Increased temperature enhanced the affinity for NO3−-N only in N-limited thalli, while nutrient limitation enhanced affinity irrespective of temperature. The maximal velocity of uptake (Vmax) and the half saturation constant (Ks) appeared to vary with experimental conditions, but these differences were not statistically significant. Water motion was shown to reduce ‘diffusion transport limitation’ experienced by the alga under conditions of low external dissolved inorganic nitrogen (DIN) concentrations, so that the rate of N uptake responds with a 4.5-fold increase under conditions of enhanced water motion. All results suggest that Gracilaria gracilis is well suited to remain productive in an upwelling environment dominated by the transient availability of DIN through the use of a high affinity system for NO3−-N and non-saturable uptake of NH4+-N. Water motion interacts strongly with nutrient concentration, and may alleviate N limitation by reducing boundary-layer resistance to diffusion. Practical application of the results of this study is discussed in terms of significance to mariculture.
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