Nutrient- and light-mediated buoyancy control of the oceanic non-motile dinoflagellate Pyrocystis noctiluca Murray ex Haeckel (1890)

Abstract

It appears that nutrient concentrations rather than light intensity are the primary factors controlling the buoyancy of Pyrocystis noctiluca Murray ex Haeckel (1890) cells in the sea, and that changes in the buoyancy of P. noctiluca tend to concentrate it at subsurface depths. In the southern Sargasso Sea and the northern Caribbean Sea, vegetative cells of P. noctiluca had population maxima at depths between 60 and 100 m at the 15 to 2% isolumes and ≈0 to 70 m above the apparent depth of the nutricline. From the mixed layer to the depth of the vegetative cell population maximum, there was an increasing proportion of positively buoyant cells of P. noctiluca. The highest proportions of positively buoyant cells occurred from just below the depth of the vegetative cell maximum to ≈ 170 m depth. These results suggest that cells either above or below the depth of the vegetative cell maximum tend to use their buoyancy to return to that depth. Experiments to test what affected the cell buoyancy were carried out in chambers containing P. noctiluca and suspended below drifting buoys in the Sargasso Sea and the Caribbean Sea. In chambers with plankton netting for sides (having ambient nutrients), P. noctiluca populations became negatively buoyant when held in the mixed layer 5 days, but became positively buoyant when held in the thermocline at 110 m for 5 days. In chambers having solid sides P. noctiluca also became negatively buoyant in the mixed layer and at 110 m unless nutrients (including 8.8 μM NO 3 − and 0.36 μM PO 4 3−), were added to the sea water in the chambers. In the laboratory, nutrients rather than light intensity also appeared to be the primary factor controlling the buoyancy of P. noctiluca cells. In the laboratory, an increase in the proportions of sinking cells took place only after nutrient depletion. In cultures, light at saturating and subsaturating intensities for cell division rate apparently affected buoyancy by controlling the rates at which the cells used nutrients through growth processes. In both the sea and in cultures, cell buoyancy characteristics were apparently due to the balance of rates of nutrient accumulation due to uptake and loss due to growth.