ON NITROGEN DEFICIENCY IN TROPICAL PACIFIC OCEANIC PHYTOPLANKTON. II. PHOTOSYNTHETIC AND CELLULAR CHARACTERISTICS OF A CHEMOSTAT‐GROWN DIATOM1

Abstract

Cells of the tropical Pacific diatom, Chaetoceros gracilis, were grown in a nitrogen‐limited chemostat at varying percentages of the maximum growth rate, harvested, and photosynthetic and cellular parameters measured. Assimilation ratio (photosynthesis at light saturation per unit chlorophyll) increased with increasing growth rate. Cellular C : chlorophyll ratios also decreased with increasing growth rate, but carotenoid : chlorophyll ratios showed no obvious trend. The C : N ratio decreased and chlorophyll : cell increased with increasing growth rate. Steady‐state cell numbers were not constant at different growth rates but decreased as the growth rates increased. Growth rates seemed to be controlled by internal supplies of nitrogen and the apparent half saturation constant, K ′ s , decreased with increasing growth rate.