Storage lipid metabolism during nitrogen assimilation in a marine diatom

Four major experiments were conducted to investigate the patterns of storage carbon accumulation and degradation in the marine diatom, Phaeodactylum tricolymtum.; When the cultures were N-starved in the light for more than 24 h, there was an accumulation of storage triacylglycerols containing primarily hexadecanoic (16:0) and cis-9-hexadecenoic (16:1) fatty acids. After cultures that had been N-starved for 3 d were recovered with an addition of nitrate to the medium, carbohydrates were degraded before triacylglycerols, irrespective of the size of the intracellular triacylglycerol reserve. There was no difference in the timing or magnitude of triacylglycerol degradation during the assimilacion of either nitrate or ammonium in the light. Triacylglycerol and carbohydrate degradation appeared to be metabolically linked. The activity of the glyoxylate cycle enzyme, isocitrate lyase, was 4-fold higher during nitrate assimilation compared to ammonium assimilation 24 h after adding either N source to N-starved cultures. Triacylglycerol degradation appeared to be targeted towards replacing, via gluconeogenesis, the carbohydrates that were respired during nitrate assimilation. This was confirmed with radiolabeling experiments. In dark N-recovered cultures, there was a net degradation of triacylglycerols and a net synthesis of carbohydrates irrespective of the added N source. During nitrate or ammonium assimilation in the light, some of the triacylglycerol carbon was directed towards polar lipid synthesis. However, during nitrate assimilation, more triacylglycerol carbon was directed towards low molecular weight metabolite and carbohydrate synthesis than during ammonium assimilation. Carbon stable isotope data showed that triacylglycerol carbon accumulation during N-starvation, or loss during recovery with nitrate, had little effect on whole cell {dollar}deltasp{lcub}13{rcub}C{dollar} values. However, the triacylglycerol pool had a stable {dollar}deltasp{lcub}13{rcub}{dollar}C signature irrespective of the growth phase of the cells.