Integrated analyses of nitrogen and phosphorus deprivation in the diatoms Phaeodactylum tricornutum and Seminavis robusta
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- Institutt for biologi 
Microalgae growth and production is highly dependent on nutrient availability. The availability of nutrients in the ocean is variable on temporal and spatial scales. Therefore, nutrient limitation is observed in many regions of the ocean. Nutrient limitation can strongly affect microalgal metabolism through extensive physiological and biochemical alterations. Diatoms, an ecologically important group of unicellular algae, have acquired several acclimation mechanisms to cope with nutrient limitation. In the present thesis, we utilized the diatoms Phaeodactylum tricornutum and Seminavis robusta as model organisms. The thesis has been divided into three parts: 1) Identification of acclimation mechanisms in P. tricornutum under nitrogen deprivation, 2) Uncovering molecular and metabolic modifications to phosphate deprivation in P. tricornutum, and 3) Analysis of mechanisms underlying S. robusta nitrogen stress response. In part one, we found that the photosynthetic capacity and chlorophyll content of the cells was reduced while neutral lipids increased in nitrogen-deprived cultures. We also observed reduced biosynthesis and increased recycling of nitrogenous compounds. Repression of the Calvin cycle and chrysolaminaran biosynthesis, and simultaneous induction of glycolysis, the tricarboxylic acid cycle and pyruvate metabolism, led to carbon reallocation. Neutral lipid accumulation was fed by funneling of carbon from breakdown of carbon stores along with remodeling of membrane lipids and induction of the de novo triacylglycerol biosynthesis. In the second part, we looked at acclimation mechanisms used by P. tricornutum under P deprivation. The strongest transcriptional responses were observed for genes encoding proteins involved in phosphate acquisition and scavenging. We could also observe changes in a number of processes involved in photosynthesis, nitrogen assimilation, and nucleic acid and ribosome biosynthesis. P deprivation resulted in carbon and lipid restructuring. Carbon metabolism was altered through induction of cytosolic glycolysis and the pentose phosphate pathway, and suppression of the Calvin cycle. Finally, we recognized several modifications in cellular lipids. Whereas phospholipid biosynthesis was repressed, neutral lipid and sulfolipid biosynthesis were induced. In the third part, we analyzed the acclimation of S. robusta to nitrogen deprivation. We detected significant drop in cell growth as well as strong increase in neutral lipids following nitrogen deprivation. While genes related to light absorption and photosynthetic electron transport were down-regulated, several genes involved in nitrogen uptake and assimilation were up-regulated. We suggested that the induction of several genes connected to glycolysis, the TCA cycle and mitochondrial pyruvate dehydrogenase complex could direct carbon skeleton and energy to lipid biosynthesis. Despite the suppression of de novo fatty acid biosynthetic pathway, a fatty acid synthase I gene was induced in N-deprived cells, which might lead to production of fatty acids for triacylglycerol biosynthesis. We also proposed that accumulation of triacylglycerol in nitrogen-replete cells mainly occurs via the up-regulation of phospholipases and phospholipid:diacylglycerol acyltransferase.