CHANGES IN CARBON AND NITROGEN PHYSIOLOGY DURING AMMONIUM AND NITRATE NUTRITION 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 medium. During exponential growth ammonium-growing cells contained half the fatty acid content, higher concentrations of intracellular free glutamine and asparagine, and little nitrate reductase activity in comparison with nitrate-growing cells. Differences in the glutamine/glutamate 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 to levels similar to those in nitrate-grown cells and the proportion of oleic acid increased in both cultures. Possible reasons for the increased synthesis of fatty acids, with its additional demand for reductant, in cells growing on nitrate-N are discussed. Nitrate reductase activity remained low in the ammonium-grown cells and decreased rapidly in the nitrate-grown cells during N-starvation. Net chlorophyll synthesis ceased within 36 h of the start of N-deprivation but synthesis of some carotenoids continued, with proportions of echinenone increasing and 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 during N-starvation and the cells reacted to fixation by swelling, possibly because of decreased protein content in membranes.