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 SO 4 2− at ⅛th, and Ca 2+ and Mg 2+ 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 + , Ca 2+ , Mg 2+ , or Cl − in cell sap, but cells grown at high light intensities had more K + and Mg 2+ in their cell sap than cells grown in dim light. Nutrient‐replete cells were more positively buoyant than nutrient‐depleted cells. NH 4 + stored in cell sap of nutrient‐replete cells lowered its density by about 0.2 mg · ml −1 . Vegetative cells killed with HgCl 2 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.