Whole cell response to nitrogen deprivation in the diatom Phaeodactylum tricornutum

We combined transcriptional and metabolite analyses to monitor the effect of N deficiency at different molecular levels in order to get better insight on the acclimation strategies P. tricornutum employs under nitrogen limitation. Physiological data like neutral lipid measurement, cell growth and other cell chemistry measurements further complemented our molecular data. We showed that P. tricornutum is able to remobilize nitrogen through catabolism of various internal nitrogen-containing resources such as amino acids and proteins. N starvation was also accompanied by reduction in pigment pools as well as photosynthetic capacity. The global expression data showed that majority of metabolic changes were related to carbon and lipid metabolism. Decreased levels of carbon skeletons due to suppression of the Calvin cycle was compensated by breakdown of chrysolaminaran leading to up-regulation of OPPP, glycolysis, pyruvate metabolism and TCA cycle. These pathways provide precursors for fatty acid biosynthesis. In addition, remodeling of membrane lipids and up-regulation of the de novo TAG biosynthetic pathway was further supported by physiological measurements of neutral lipids, indicating TAG accumulation under N limitation. Our study gives a detailed image of adaptation of P. tricornutum to N starvation, and can be used for future metabolic manipulations to increase TAG production. Axenic cultures of Phaeodactylum tricornutum were grown in f/2 medium and kept in exponential growth at 15ºC under continuous white fluorescent light (60 µmol photons m-2 s-1) inside the shaking incubator for three weeks. Three or four replicates and parallels of the start culture were transferred to media supplemented with complete f/2 nutrients (replete) or f/2 without nitrate (deplete). Cells were incubated in batch cultures with a start cell density of 5×10^4 ml-1 in 75 cm2 sterile culture flasks and volume of 220 ml. Cell counting and variable in vivo Chl a fluorescence (Fv/Fm) was measured daily. For the other experiments, samples were harvested 48 h and 72 h after the beginning of the treatment.