三角褐指藻甘油酯对氮胁迫的响应

Glycerolipid remodeling of Phaeodactylum tricornutum in response to nitrogen stress

【背景】三角褐指藻作为生物燃料潜在的生产者,在胁迫条件下能通过改变其甘油酯组成来适应外部环境的变化,同时伴随着生物燃料原料甘油三酯(TAG)的积累,研究三角褐指藻甘油酯对氮胁迫的响应机制有利于深入认识TAG的积累过程。【目的】通过分析三角褐指藻在正常和氮胁迫条件下各类脂质含量及其脂肪酸成分的变化,揭示氮胁迫诱导积累的TAG酰基主要来源,以及在胁迫前生成的各极性甘油酯脂肪酸的去向,从而为进一步认识三角褐指藻对氮胁迫的响应机制提供新信息。【方法】利用高效薄层色谱结合气相色谱法分析三角褐指藻在正常和氮胁迫条件下的脂肪酸及甘油酯组分的变化。【结果】三角褐指藻在氮胁迫条件下TAG含量增加至57.8 mg/g时,总甘油酯含量几乎不变,但各甘油酯含量变化差异很大,表现为各极性脂含量显著降低。在此期间,各类甘油酯脂肪酸组成含量的变化表明,三角褐指藻TAG主要积累饱和及单不饱和脂肪酸,即16:0和16:1n7,分别以从头合成及原有极性脂转化为主,极性脂的部分二十碳五烯酸(EPA)作为酰基供体也向TAG发生了转化;此外组成极性脂的多不饱和脂肪酸16:2n4、16:3n4及EPA分解导致其含量显著下降。【结论】当氮胁迫诱导的三角褐指藻TAG含量为57.8 mg/g时,积累的TAG酰基中有48%来自从头合成,52%来自极性脂转化;而氮胁迫诱导所减少的极性脂酰基中有54%转化成TAG,46%发生了分解。

[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 16：1 n7 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., 16：2 n4, 16：3 n4 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.