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

Abstract

In 4‐h experiments, N‐depleted cells of Pyrocystis and Dissodinium species took up NO₃⁻ and NH₄⁺ at almost equal rates both day and night when preconditioned on 12 : 12 LD cycle at light intensities sufficient to saturate division (42–67 µEin m⁻² s⁻¹).Pyrocystis fusiformis but not Pyrocystis noctiluca took up NO₃ at a high rate when grown at 3 µEin m⁻² s⁻¹. There was no increase in the concentration of NH₄⁺ within the large vacuoles of Pyrocystis cells during uptake of NH₄⁺, but both NO₃ and NH₄⁺ were accumulated in the vacuole when external NO₃⁻ was supplied. In contrast, inorganic N did not accumulate in Dissodinium lunula cells during either NO₃⁻ or NH₄⁺ 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.