Glycerolipid remodeling of Phaeodactylum tricornutum in response to nitrogen stress

Abstract:

[Background] Phaeodactylum tricornutum has emerged as a potential producer for biofuel feedstocks. Under stress conditions, the glycerolipids of P. tricornutum could be reconstructed to adapt to alterations of external environmental factors, concomitant with accumulation of triacylglycerols (TAGs), which can be converted to biofuels. Thus, mechanistic studies of glycerolipid remodeling in P. tricornutum under nitrogen stress are essential for deciphering mechanism of TAG biosynthesis and accumulation. [Objective] In order to confirm the origin of the accumulated acyl groups of TAG and the fate of the decreased acyl groups of multiple polar lipids, the alterations of fatty acids and glycerolipid components were studied in detail for P. tricornutum under nitrogen repletion and depletion conditions, which could provide insights into the response mechanism of glycerolipid of this alga subjected to nitrogen stress. [Methods] The fatty acids and glycerolipid components were determined qualitatively and quantitatively for P. tricornutum using high performance thin layer chromatography coupled with gas chromatography. [Results] Although the total content of glycerolipids remained unaltered under both nitrogen repletion and depletion conditions in P. tricornutum, the content of the individual glycerolipid class and the respective fatty acyl compositions varied notably. A prominent decrease in content of each polar glycerolipid was observed, accompanied by an increase in TAG amount, up to 57.8 mg/g, in stress-induced P. tricornutum. Based on variations of the contents of the fatty acyl groups comprising those glycerolipids, it was concluded that the lipid remodeling took place in multiple pathways. First, the saturated and monounsaturated fatty acids of TAG were significantly produced. In particular, a majority of 16:0 was concentrated into TAG through the de novo synthesis pathway and most of 16:1n7 was assembled into TAG through turnover of polar glycerolipids. In addition, a portion of EPA derived from polar glycerolipids were recycled into TAG as an acyl donor. Second, the polyunsaturated fatty acids degraded at expense of notable reductions of polar lipid components, i.e., 16:2n4, 16:3n4 and EPA. [Conclusion] When the TAG of nitrogen-deprived P. tricornutum accumulated up to 57.8 mg/g, the accumulated acyl groups of TAG derived from both the de novo synthesis pathway and the polar lipid turnover pathway and the contributions of each pathway were 48% and 52%, respectively. In addition, there were 54% of the decreased acyl groups of polar lipids to be incorporated into TAG and the other 46% of those entered into degradation reactions in P. tricornutum under nitrogen stress.