Dynamics of Lipid Biosynthesis and Redistribution in the Marine Diatom Phaeodactylum tricornutum Under Nitrate Deprivation

One approach to achieve continuous overproduction of lipids in microalgal "cell factories" relies upon depletion or removal of nutrients that act as competing electron sinks (e. g., nitrate and sulfate). However, this strategy can only be effective for bioenergy applications if lipid is synthesized primarily de novo (from CO ₂ fixation) rather than from the breakdown and interconversion of essential cellular components. In the marine diatom, Phaeodactylum tricornutum, it was determined, using ¹³C-bicarbonate, that cell growth in nitrate (NO ₃ ^-)-deprived cultures resulted predominantly in de novo lipid synthesis (60 % over 3 days), and this new lipid consisted primarily of triacylglycerides (TAGs). Nearly complete preservation of ¹²C occurred in all previously existing TAGs in NO ₃ ^--deprived cultures and thus, further TAG accumulation would not be expected from inhibition of TAG lipolysis. In contrast, both high turnover and depletion of membrane lipids, phosphatidylcholines (PCs), were observed in NO ₃ ^--deprived cultures (both the headgroups and fatty acid chains), while less turnover was observed in NO ₃ ^- replete cultures. Liquid chromatography-tandem mass spectrometry mass spectra and ¹³C labeling patterns of PC headgroups provided insight into lipid synthesis in marine diatoms, including suggestion of an internal pool of glycine betaine that feeds choline synthesis. It was also observed that 16C fatty acid chains incorporated into TAGs and PCs contained an average of 14 ¹³C carbons, indicating substantial incorporation of ¹³C-bicarbonate into fatty acid chains under both nutrient states.