Coupling and uncoupling between nitrate uptake and growth rate in Prorocentrum minimum (Dinophyceae) under different frequencies of pulsed nitrate supply

Overview & Synopsis

Nitrate uptake and growth rates of the red-tide dinoflagellate Prorocentrum minimum were measured in a chemostat culture system in which nitrate was added in the same total amounts every 1, 2 or 3 d In comparison with continuous nitrate supply, the rate of cell division was not affected by the 1 or 2 d pulse treatments, whereas it fell drastically when a nitrogen source was added only every 3 d. Delayed steady uptake rates were reached during the 1 or 2 d pulse phases, which reflected a mid-term adaptation of the cell uptake process under discontinuous nutrient supply. This adaptation permitted P. minimun~ to maintain a steady growth rate under these regimes. During the 3 d pulse treatment, the maximal uptake rate measured during each pulse experiment increased considerably. which reflected a long-term adaptation, but was not sufficient to maintain the initlal growth rate. For low frequencies of nitrate supply, uptake and growth rate became largely uncoupled. It is concluded that P. minimum is a species able to form a large internal pool of nitrogen which constitutes a competitive advantage. This is discussed in the light of in situ observations.