AME 27:275-284 (2002)  -  doi:10.3354/ame027275

ABSTRACT: We measured the time-course of dissolved organic nitrogen (DON) release and inorganic nitrogen assimilation in the freshwater phytoplankton Scenedesmus quadricauda (Turp.) Brébission and Microcystis novacekii (Kom.) Comp. by the ¹⁵N tracer method. The algae were cultivated under nitrogen-limited or -replete conditions. We estimated the total DON release, release of recently assimilated nitrogen (RANR) and net nitrogen assimilation (NNA), using ammonium chloride or sodium nitrate as the nitrogen source. In the ¹⁵N tracer incubation of S. quadricauda prior to 1 h and in M. novacekii prior to 3-6 h, minimal release of total DON was observed under the nitrogen-limited conditions. This result suggests that nitrogen-limited cells efficiently use incorporated nitrogen. Appreciable release of total DON was observed in the ¹⁵N incubation of nitrogen-replete S. quadricauda and M. novacekii. RANR in both nitrogen-limited and -replete algae was smaller than, and lagged behind, release of total DON, indicating that newly assimilated nitrogen was less easily released. NNA by nitrogen-limited S. quadricauda increased from the onset of the tracer incubation, showing that ambient nitrogen was needed for nitrogen-limited cells from the beginning of the incubation. Nitrogen-replete S. quadricauda showed no NNA during the first 1 h of the tracer incubation, suggesting that the alga did not require external nitrogen immediately after being transferred from nitrogen-replete conditions. This study shows that nitrogen release and assimilation are markedly affected by the nutritional states of the algae.

KEY WORDS: Ammonium · Nitrogen assimilation · Organic nitrogen release · Microcystis novacekii · Nitrate · Scenedesmus quadricauda