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We measured the time-course of dissolved organic nitrogen (DON) release and inor- ganic nitrogen assimilation in the freshwater phytoplankton Scenedesmus quadricauda (Turp.) Brebission and Microcystis novacekii (Kom.) Comp. by the 15 N tracer method. The algae were culti- vated 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 chlo- ride or sodium nitrate as the nitrogen source. In the 15 N tracer incubation of S. quadricauda prior to 1h and in M. novacekii prior to 3-6 h, minimal release of total DON was observed under the nitro- gen-limited conditions. This result suggests that nitrogen-limited cells efficiently use incorporated nitrogen. Appreciable release of total DON was observed in the 15 N 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 incu- bation, 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 trans- ferred from nitrogen-replete conditions. This study shows that nitrogen release and assimilation are markedly affected by the nutritional states of the algae.

Release of dissolved organic nitrogen from Scenedesmus quadricauda (Chlorophyta) and Microcystis novacekii (Cyanobacteria)