Positive buoyancy through ionic control in the nonmotile marine dinoflagellate Pyrocystis noctiluca Murray ex Schuett 1

In culture, vegetative cells, which are positively buoyant, have vacuoles that are about 95% of their volume. They may rise at 0–20 cm · h−1 and have cell sap 2.0–3.1 mg · ml−1 less dense than their seawater medium. Isotonic cell sap has SO42− at ⅛th, and Ca2+ and Mg2+ at ½ seawater concentrations (these ions are replaced by NaCl) which accounts for most of its lower density. There were no day to night differences in the concentrations of Na+, K+, Ca2+, Mg2+, or Cl− in cell sap, but cells grown at high light intensities had more K+ and Mg2+ in their cell sap than cells grown in dim light. Nutrient‐replete cells were more positively buoyant than nutrient‐depleted cells. NH4+ stored in cell sap of nutrient‐replete cells lowered its density by about 0.2 mg · ml−1. Vegetative cells killed with HgCl2 sank at 30–40 cm · h−1. In nature, nutrients and light intensity may act as counteracting factors to keep Pyrocystis noctiluca suspended at a depth favorable for growth.