Abstract
Many species of microalgae produce greatly enhanced amounts of triacylglycerides (TAGs), the key product for biodiesel production, in response to specific environmental stresses. Improvement of TAG production by microalgae through optimization of growth regimes is of great interest. This relies on understanding microalgal lipid metabolism in relation to stress response in particular the deprivation of nutrients that can induce enhanced TAG synthesis. In this study, a detailed investigation of changes in lipid composition in Chlorella sp. and Nannochloropsis sp. in response to nitrogen deprivation (N-deprivation) was performed to provide novel mechanistic insights into the lipidome during stress. As expected, an increase in TAGs and an overall decrease in polar lipids were observed. However, while most membrane lipid classes (phosphoglycerolipids and glycolipids) were found to decrease, the non-nitrogen containing phosphatidylglycerol levels increased considerably in both algae from initially low levels. Of particular significance, it was observed that the acyl composition of TAGs in Nannochloropsis sp. remain relatively constant, whereas Chlorella sp. showed greater variability following N-deprivation. In both algae the overall fatty acid profiles of the polar lipid classes were largely unaffected by N-deprivation, suggesting a specific FA profile for each compartment is maintained to enable continued function despite considerable reductions in the amount of these lipids. The changes observed in the overall fatty acid profile were due primarily to the decrease in proportion of polar lipids to TAGs. This study provides the most detailed lipidomic information on two different microalgae with utility in biodiesel production and nutraceutical industries and proposes the mechanisms for this rearrangement. This research also highlights the usefulness of the latest MS-based approaches for microalgae lipid research.
Highlights
Increasing and unstable oil prices, diminishing fossil fuel reserves, and increasing atmospheric CO2 levels have revitalized interest in microalgae as a renewable, lipid-rich feedstock for biofuel production [1,2,3,4,5]
As the physical properties and quality of the biodiesel are directly dependent on the nature and chemical structures of the recovered fatty acids (FA) [8,9], the relative amounts of each lipid class and their FA profiles largely determine the suitability of a given feedstock for biodiesel production
The assignment of fatty acid compositions for the glycolipids was ambiguous MS/MS of the high abundant MGDG and DGDG lipids was carried out to confirm the FA composition (Figure S1) For the other classes the LC-MS data was combined with previously obtained GC-MS data for these samples [37], allowing the most probable combination of individual fatty acids to each lipid species to be ascertained (Figures 4 and 5, and Table S3)
Summary
Increasing and unstable oil prices, diminishing fossil fuel reserves, and increasing atmospheric CO2 levels have revitalized interest in microalgae as a renewable, lipid-rich feedstock for biofuel production [1,2,3,4,5]. The TAGs are the preferred source of FAs for biodiesel production as they can be converted to biodiesel using conventional methods [7], these acyl chains (used here interchangeably with FA) are mainly present as building blocks for the algal membrane lipids. These include major plastid and photosynthesis-related glycolipids such as monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG) and sulfoquinovosyldiacylglycerol (SQDG); phosphoglycerolipids such as phosphatidylglycerol (PG), phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS) and phosphatidylinositol (PI), together with sphingolipids and other neutral lipids such as diacylglycerol (DAG). As the physical properties and quality of the biodiesel are directly dependent on the nature and chemical structures of the recovered FAs [8,9], the relative amounts of each lipid class and their FA profiles largely determine the suitability of a given feedstock for biodiesel production
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