Effects of Phosphate Concentration on Cell Division Rates and Yield of a Tropical Oceanic Diatom ,

Cell division in batch cultures of the tropical oceanic diatom, Chaetoceros gracilis, was measured in artificial medium containing different initial amounts of phosphate. The rate of growth was limited by phosphate concentrations below approximately 0.22 µg.at./l. and appeared to be a linear function of concentration rather than a hyperbolic function. Ks in the hyperbolic relationship equating growth rate and concentration would be about 0.12 µg.-at./l. The results substantiate an earlier suggestion that low phosphate did not limit the rate of phytoplankton growth in the northeastern tropical Pacific Ocean; and they also verify previous use of a linear nutrient relationship in productivity equations given by other authors. Pauses in cell division at low phosphate concentrations are either attributed to the induction of phosphatases which can act on stored intracellular phosphorus or to low extracellular concentrations. Final cell numbers were a linear function of initial phosphate concentration up to about 0.8 µg.-at./l. The amount of phosphorus per cell is 2 x 10-9 µg.-at./cell or 10-11 µg.-at./cubic micron. A requirement for boron by this diatom is also suggested by increased growth upon the addition of this nutrient.