Phosphate utilization by an oceanic diatom in phosphorus‐limited chemostat culture and in the oligotrophic waters of the central North Pacific1

Thalassiosira pseudonana from the oligotrophic waters of the central North Pacific Ocean was grown in phosphorus‐limited chemostat culture. When the chemical composition of the cells, expressed as ratios, was compared with that of nitrogen‐limited cultures, several of these ratios varied in excess of a factor of five between the two systems. Among these, which emerged as diagnostic indicators of phosphorus‐starvation vs. nitrogen‐deficiency, were the C : P, N : P. P : Chl a and Chl a : ATP ratios.

The physiological responses of a population were determined by its preconditioning history. The minimum cellular phosphorus content, q_o, varied with the degree of phosphorus starvation. The rate of phosphate uptake at a saturating concentration of phosphate by phosphorus‐limited cultures of T. pseudonana exceeded the rates of uptake by nonphosphorus‐limited cultures by an order of magnitude; the K_s for phosphate uptake was not a function of phosphorus deficiency.

Phytoplankton in phosphorus‐starved chemostat cultures on medium made with central North Pacific Ocean water did not use naturally occurring dissolved organic phosphorus despite the presence of cell‐surface alkaline phosphatase. Phosphate uptake by natural phytoplankton assemblages in the central North Pacific was measured with ³³P. Uptake rates in the field were not significantly affected by light intensity. No circadian periodicity in uptake rates was detected in natural phytoplankton or algae cultured on a light/dark cycle.