PHOSPHATE-LIMITED GROWTH OF PAVLOVA LUTHERI (PRYMNESIOPHYCEAE) IN CONTINUOUS CULTURE: DETERMINATION OF GROWTH-RATE-LIMITING SUBSTRATE CONCENTRATIONS WITH A SENSITIVE BIOASSAY PROCEDURE(1).

The relationship between steady-state growth rate and phosphate concentration was studied for the marine prymnesiophyte Pavlova lutheri (Droop) J. C. Green grown in a chemostat at 22°C under continuous irradiance. A bioassay procedure involving short-term uptake of 10 picomolar spikes of (33) P-labeled phosphate was used to estimate the concentration of phosphate in the growth chamber. The relationship between growth rate and phosphate was well described by a simple rectangular hyperbola with a half-saturation constant of 2.6 nM. The cells were able to take up micromolar spikes of phosphate at rates two to three orders of magnitude higher than steady-state uptake rates. The kinetics of short-term uptake displayed Holling type III behavior, suggesting that P. lutheri may have multiple uptake systems with different half-saturation constants. Chl a:C ratios were linearly related to growth rate and similar to values previously reported for P. lutheri under nitrate-limited conditions. C:N ratios, also linearly related to growth rate, were consistently lower than values reported for P. lutheri under nitrate-limited conditions, a result presumably reflecting luxury assimilation of nitrogen under phosphate-limited conditions. C:P ratios were linearly related to growth rate in a manner consistent with the Droop equation for growth rate versus cellular P:C ratio.