Light and dark uptake of nitrate and ammonium by large oceanic dinoflagellates: Pyrocystis noctiluca, Pyrocystis fusiformis, and Dissodinium lunula1

In 4‐h experiments, N‐depleted cells of Pyrocystis and Dissodinium species took up NO 3 − and NH 4 + at almost equal rates both day and night when preconditioned on 12 : 12 LD cycle at light intensities sufficient to saturate division (42–67 µ Ein m −2 s −1 ). Pyrocystis fusiformis but not Pyrocystis noctiluca took up NO 3 at a high rate when grown at 3 µ Ein m −2 s −1 . There was no increase in the concentration of NH 4 + within the large vacuoles of Pyrocystis cells during uptake of NH 4 + , but both NO 3 and NH 4 + were accumulated in the vacuole when external NO 3 − was supplied. In contrast, inorganic N did not accumulate in Dissodinium lunula cells during either NO 3 − or NH 4 + uptake. Nutrient‐replete cells of P. noctiluca, P. fusiformis, and D. lunula stored in their vacuoles as inorganic N ca. 30, 5, and 2% of their cellular‐N requirements.