CHANGES IN CARBON AND NITROGEN PHYSIOLOGY DURING AMMONIUM AND NITRATENUTRITION AND NITROGEN STARVATION IN ISOCHRYSIS-GALBANA

Isochrysis galbana was grown in a 12/12 h light/dark cycle with either nitrate or ammonium as the N-source, and then resuspended in N-free m edium. During exponential growth ammonium-growing cells contained half the fatty acid content, higher concentrations of intracellular free g lutamine and asparagine, and little nitrate reductase activity in comp arison with nitrate-growing cells. Differences in the glutamine/glutam ate and asparagine/aspartate ratios suggest that nitrate-growing cells were relatively more N-stressed than ammonium-growing cells. During N -starvation the fatty acid content of ammonium-grown cells increased t o levels similar to those in nitrate-grown cells and the proportion of oleic acid increased in both cultures. Possible reasons for the incre ased synthesis of fatty acids, with its additional demand for reductan t, in cells growing on nitrate-N are discussed. Nitrate reductase acti vity remained low in the ammonium-grown cells and decreased rapidly in the nitrate-grown cells during N-starvation. Net chlorophyll synthesi s ceased within 36 h of the start of N-deprivation but synthesis of so me carotenoids continued, with proportions of echinenone increasing an d fucoxanthin decreasing. The ratio of chlorophyll a to c was the same in both cultures and remained constant during N-deprivation. Electron microscopy showed increasing numbers of lipid bodies in the cells dur ing N-starvation and the cells reacted to fixation by swelling, possib ly because of decreased protein content in membranes.