The influence of phosphate depletion on lipid metabolism of microalgae
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- Institutt for biologi 
Photosynthetic algae play an essential role in the ocean ecosystem by capturing anthropogenic CO2 and are the primary producers in the food web. Important contribution in the ecosystem has been observed by the phylum of Ochrophyta belonging to Chromista, which evolved by secondary endosymbiosis, therefore gaining complex gene diversity. Understanding the biological response of such an evolutionarily diverse group to phosphorus (P) limitation is important ecologically and economically. In Ochrophyta, P limitation has been observed to influence the lipid class composition and leads to accumulation of triglycerides (TAGs), which can be used to produce alternative biofuels. Despite this, studies are limited on P stressed Ochrophyta and their complex lipid metabolism. In this thesis, Chromista lipid metabolism such as omega-3-fatty acid synthesis and supply of intermediates for TAG accumulation was reviewed. Transcriptome studies on the Eustigmatophyceae Nannochloropsis oceanica have given insights into the diversity of succession in phytoplankton communities regarding P limitation. A specific P stress response in N. oceanica is the ability to degrade dissolved organic P (DOP) using the enzyme purple acid phosphatase, as opposed to alkaline phosphatases that degrade phosphomonoesters in other Ochrophyta. Common with similar taxa was the overexpression of genes involved in P transportation across the cell membrane and an internal reshuffling of P. The functional study of the P stress-inducible MYB-related transcription factor (PtPSR) in diatom Phaeodactylum tricornutum elucidated the PtPSR regulation of genes involved in P acquisition and scavenging, and lipid remodelling. Furthermore, phylogenetic studies showed that the P stress-inducible transcription factor is conserved in stramenopiles. The complex interaction of lipid class shifts was identified through the phospholipid (PL)-recycling schema in the transcriptome and lipidome of N. oceanica. The PL-recycling schema includes not only up-regulated PL degradation pathways but also biosynthesis of PLs and P-free lipids that were obtained by intermediates of the Kennedy pathway. Of these pathways, some showed to be similar in P. tricornutum, such as the proposed PL degradation involving patatin-phospholipases and glycerolphosphodiesterases. However, the phospholipases D and C seemed to be regulated differently in N. oceanica and P. tricornutum. It was shown that PL degradation is a short-term and specific P stress response in N. oceanica and P. tricornutum to acquire P. In P. tricornutum the genes involved in PL degrading glycerolphosphodiesterases and phosphoethanolamine phosphorylase showed to be regulated by PtPSR. Furthermore, the single-copy genes S-adenosylmethionine (AdoMet):DAG 3-amino-3-carboxypropyltransferase and phospholipid/diacylglycerol acyltransferases are both conserved in stramenopiles, seemingly playing an important role in the synthesis of betaine lipid and TAG, respectively. The thesis contributes significantly to the understanding of the P limitation mechanisms and lipid class changes in the Ochrophyta, N. oceanica and P. tricornutum. The multiple evolutionary events and species specific adaptation to their environment lead to differences and plasticity in lipid metabolisms and P acquisition of Ochrophyta.