Triglyceride quantification by catalytic saturation and LC–MS/MS reveals an evolutionary divergence in regioisometry among green microalgae

Comparative lipidomic studies of Scenedesmus sp. (Chlorophyceae) and Cylindrotheca closterium (Bacillariophyceae) reveal their differences in lipid production under nitrogen starvation

Microalgae are a promising resource for the highly sustainable production of various biomaterials (food and feed), high-value biochemicals, or biofuels. However, factors influencing the valued lipid production from oleaginous algae require a more detailed investigation. This study elucidates the variations in lipid metabolites between a marine diatom (Cylindrotheca closterium) and a freshwater green alga (Scenedesmus sp.) under nitrogen starvation at the molecular species level, with emphasis on triacylglycerols using liquid chromatography-electrospray ionization mass spectrometry techniques. A comprehensive analysis was carried out by comparing the changes in total lipids, growth kinetics, fatty acid compositions, and glycerolipid profiles at the molecular species level at different time points of nitrogen starvation. A total of 60 and 72 triacylglycerol molecular species, along with numerous other polar lipids, were identified in Scenedesmus sp. and C. closterium, respectively, providing the most abundant triacylglycerol profiles for these two species. During nitrogen starvation, more triacylglycerol of Scenedesmus sp. was synthesized via the "eukaryotic pathway" in the endoplasmic reticulum, whereas the increase in triacylglycerol in C. closterium was mainly a result of the "prokaryotic pathway" in the chloroplasts after 96 h of nitrogen starvation. The distinct responses of lipid synthesis to nitrogen starvation exhibited by the two species indicate different strategies of lipid accumulation, notably triacylglycerols, in green algae and diatoms. Scenedesmus sp. and Cylindrotheca closterium could serve as excellent candidates for the mass production of biofuels or polyunsaturated fatty acids for nutraceutical purposes.

The lipid biochemistry of eukaryotic algae

Algal lipid metabolism fascinates both scientists and entrepreneurs due to the large diversity of fatty acyl structures that algae produce. Algae have therefore long been studied as sources of genes for novel fatty acids; and, due to their superior biomass productivity, algae are also considered a potential feedstock for biofuels. However, a major issue in a commercially viable "algal oil-to-biofuel" industry is the high production cost, because most algal species only produce large amounts of oils after being exposed to stress conditions. Recent studies have therefore focused on the identification of factors involved in TAG metabolism, on the subcellular organization of lipid pathways, and on interactions between organelles. This has been accompanied by the development of genetic/genomic and synthetic biological tools not only for the reference green alga Chlamydomonas reinhardtii but also for Nannochloropsis spp. and Phaeodactylum tricornutum. Advances in our understanding of enzymes and regulatory proteins of acyl lipid biosynthesis and turnover are described herein with a focus on carbon and energetic aspects. We also summarize how changes in environmental factors can impact lipid metabolism and describe present and potential industrial uses of algal lipids.

Endoplasmic reticulum acyltransferase with prokaryotic substrate preference contributes to triacylglycerol assembly in Chlamydomonas

Understanding the unique features of triacylglycerol (TAG) metabolism in microalgae may be necessary to realize the full potential of these organisms for biofuel and biomaterial production. In the unicellular green alga Chlamydomonas reinhardtii a chloroplastic (prokaryotic) pathway has been proposed to play a major role in TAG precursor biosynthesis. However, as reported here, C. reinhardtii contains a chlorophyte-specific lysophosphatidic acid acyltransferase, CrLPAAT2, that localizes to endoplasmic reticulum (ER) membranes. Unlike canonical, ER-located LPAATs, CrLPAAT2 prefers palmitoyl-CoA over oleoyl-CoA as the acyl donor substrate. RNA-mediated suppression of CrLPAAT2 indicated that the enzyme is required for TAG accumulation under nitrogen deprivation. Our findings suggest that Chlamydomonas has a distinct glycerolipid assembly pathway that relies on CrLPAAT2 to generate prokaryotic-like TAG precursors in the ER.

An Extended Approach to Quantify Triacylglycerol in Microalgae by Characteristic Fatty Acids

Microalgae represent a third generation biofuel feedstock due to their high triacylglycerol (TAG) content under adverse environmental conditions. Microalgal TAG resides in a single cell and serves as a lipid class mixed with complicated compositions. We previously showed that TAG possessed characteristic fatty acids (CFAs) for quantification and was linearly correlated with the relative abundance of CFA within certain limits in microalgae. Here, we defined the application range of the linear correlation between TAG and CFA in the oleaginous microalgae Chlamydomonas reinhardtii and Phaeodactylum tricornutum. In addition, TAG quantification was further expanded to a wide range of levels and the absolute amounts of saturated or monounsaturated CFAs, 16:0 and 18:1n9 of C. reinhardtii and 16:0 and 16:1n7 of P. tricornutum, instead of polyunsaturated CFAs, were verified to be linearly correlated to TAG levels throughout the entire period of nitrogen stress. This approach utilizes a single fatty acid to quantify TAG mixtures, and is rapid, simple and precise, which provides a useful tool for monitoring TAG accumulation of distinct microalgal species and facilitating high-throughput mutant screening for microalgae.

Carbon and Acyl Chain Flux during Stress-induced Triglyceride Accumulation by Stable Isotopic Labeling of the Polar Microalga Coccomyxa subellipsoidea C169

Deriving biofuels and other lipoid products from algae is a promising future technology directly addressing global issues of atmospheric CO₂ balance. To better understand the metabolism of triglyceride synthesis in algae, we examined their metabolic origins in the model species, Coccomyxa subellipsoidea C169, using stable isotopic labeling. Labeling patterns arising from [U-¹³C]glucose, ¹³CO₂, or D₂O supplementation were analyzed by GC-MS and/or LC-MS over time courses during nitrogen starvation to address the roles of catabolic carbon recycling, acyl chain redistribution, and de novo fatty acid (FA) synthesis during the expansion of the lipid bodies. The metabolic origin of stress-induced triglyceride was found to be a continuous 8:2 ratio between de novo synthesized FA and acyl chain transfer from pre-stressed membrane lipids with little input from lipid remodeling. Membrane lipids were continually synthesized with associated acyl chain editing during nitrogen stress, in contrast to an overall decrease in total membrane lipid. The incorporation rates of de novo synthesized FA into lipid classes were measured over a time course of nitrogen starvation. The synthesis of triglycerides, phospholipids, and galactolipids followed a two-stage pattern where nitrogen starvation resulted in a 2.5-fold increase followed by a gradual decline. Acyl chain flux into membrane lipids was dominant in the first stage followed by triglycerides. These data indicate that the level of metabolic control that determines acyl chain flux between membrane lipids and triglycerides during nitrogen stress relies primarily on the Kennedy pathway and de novo FA synthesis with limited, defined input from acyl editing reactions.

Regioisomeric and enantiomeric analysis of triacylglycerols

A survey of useful methods for separation and identification of regioisomers and enantiomers of triacylglycerols. Gas chromatography, gas chromatography-mass spectrometry, ¹³C NMR determination of regioisomers by enzymatic methods, and supercritical fluid chromatography are briefly surveyed, whereas a detailed description is given of the analysis of triacylglycerols by liquid chromatography, especially with silver ion (Ag⁺; argentation), and nonaqueous reversed phase liquid chromatography. Special attention is paid to chiral chromatography. Details of mass spectrometry of triacylglycerols are also described, especially the identification of important triacylglycerol ions such as [M + H-RCOOH]⁺ in atmospheric pressure chemical ionization mass spectra.

Modeling of rhythmic behavior in neutral lipid production due to continuous supply of limited nitrogen: mutual growth and lipid accumulation in microalgae

The relative effects of three precise nitrogen limitation regimes on green micro-algae were assessed using the Trebouxiophycean alga Coccomyxa subellipsoidea grown in a chemostatic bioreactor system. The data provides further evidence that growth and triglyceride (TAG) accumulation are concurrent and independently proportional to the degree of nitrogen limitation in algae. Additionally, TAG accumulation was observed to proceed via oscillations with respect to time and percent dry weight quantity. The predator-prey model was applied to fit the experimental data and to obtain the physiological significance of these oscillations. The results determine the conditions of maximum neutral lipid productivity with respect to nitrate stress and highlight an area of potential future research.