CHANGES IN CELL COMPOSITION AND LIPID-METABOLISM MEDIATED BY SODIUM AND NITROGEN AVAILABILITY IN THE MARINE DIATOM PHAEODACTYLUM-TRICORNUTUM (BACILLARIOPHYCEAE)

The effects of nitrogen starvation in the presence or absence of sodiu m in the culture medium were monitored in batch cultures of the marine diatom Phaeodactylum tricornutum Bohlin. During nitrogen starvation i n the presence of sodium, cell nitrogen and chlorophyll a decreased, m ainly as a consequence of continued cell division. These decreases wer e accompanied by decreases in the rates of photosynthesis and respirat ion There was no change in either cell volume or carbohydrate, but bot h carbon and lipid increased. During nitrogen starvation in the absenc e of sodium, cell division ceased. Cell nitrogen and chlorophyll a rem ained constant, and respiration did not decrease, but rite changes in the photosynthetic rate and the lipid content per cell were similar to cultures that were nitrogen-starved in the presence of sodium. The ca rbon-to-nitrogen ratio increased in both cultures, Nitrogen, in the fo rm of nitrate, and sodium were resupplied to cultures that had been pr econditioned in nitrogen- and sodium-deficient medium for 5 d. Control cultures to which neither nitrate or sodium were added remained in a static state with respect to cell number, volume, and carbohydrate but showed slight increases in lipid. Cells in cultures to which 10 mM ni trate alone was added showed a similar response to cultures where no a dditions were made. Cells in cultures to which 50 mM sodium alone was added divided for 2 d, with concomitant small decreases in all measure d constituents. Cell division resumed in cultures to which both sodium and nitrate were added. The lipid content fell dramatically in these cells and was correlated to metabolic oxidation via measured increases in the activity of the glyoxylate cycle enzyme, isocitrate lyase. We conclude that lipids are stored as a function of decreased growth rate and are metabolized to a small extent when cell division resumes. How ever, much higher rates of metabolism occur if cell division resumes i n the presence of a nitrogen source.