Metabolism of acyl‐lipids in Chlamydomonas reinhardtii

Abstract

SUMMARYMicroalgae are emerging platforms for production of a suite of compounds targeting several markets, including food, nutraceuticals, green chemicals, and biofuels. Many of these products, such as biodiesel or polyunsaturated fatty acids (PUFAs), derive from lipid metabolism. A general picture of lipid metabolism in microalgae has been deduced from well characterized pathways of fungi and land plants, but recent advances in molecular and genetic analyses of microalgae have uncovered unique features, pointing out the necessity to study lipid metabolism in microalgae themselves. In the past 10 years, in addition to its traditional role as a model for photosynthetic and flagellar motility processes, Chlamydomonas reinhardtii has emerged as a model organism to study lipid metabolism in green microalgae. Here, after summarizing data on total fatty acid composition, distribution of acyl-lipid classes, and major acyl-lipid molecular species found in C. reinhardtii, we review the current knowledge on the known or putative steps for fatty acid synthesis, glycerolipid desaturation and assembly, membrane lipid turnover, and oil remobilization. A list of characterized or putative enzymes for the major steps of acyl-lipid metabolism in C. reinhardtii is included, and subcellular localizations and phenotypes of associated mutants are discussed. Biogenesis and composition of Chlamydomonas lipid droplets and the potential importance of lipolytic processes in increasing cellular oil content are also highlighted.

Two Glycerol-3-Phosphate Dehydrogenases from Chlamydomonas Have Distinct Roles in Lipid Metabolism

“…PG is a major lipid of chloroplast membranes, alongside the nonphosphorous glycoglycerolipids monogalactosyldiacylglycerol, diagalactosyldiacylglycerol, and sulfoquinovosyldiacylglycerol. PG is thought to be mostly synthesized in the chloroplast from prokaryotic pathway PA, although additional synthesis in the ER cannot be discounted (Boudière et al, 2014;Li-Beisson et al, 2015). Indeed the localization of either PGPS isoform is unknown, with one predicted to be chloroplastic and the other nonchloroplastic (Hung et al, 2015;Li-Beisson et al, 2015).…”

“…PG is thought to be mostly synthesized in the chloroplast from prokaryotic pathway PA, although additional synthesis in the ER cannot be discounted (Boudière et al, 2014;Li-Beisson et al, 2015). Indeed the localization of either PGPS isoform is unknown, with one predicted to be chloroplastic and the other nonchloroplastic (Hung et al, 2015;Li-Beisson et al, 2015). It is unclear whether the PG inhibition within the GPD3-OE lines is due to reduced chloroplastderived PG, perhaps due to up-regulated and overcompensated eukaryotic pathway activity or reduced ER-derived PG.…”

“…The conversion of PA to DAG and then TAG in the Kennedy pathway (Fig. 1) is mediated by a PA phosphatase (PAP) enzyme and then subsequent conversion of DAG to TAG by a DAG acyltransferase (DGAT) or by a phospholipid DAG acyltransferase Li-Beisson et al, 2015). To date, relatively little is known about C. reinhardtii PAP genes, and only one of the three known isoforms, PAP2, has been functionally characterized, whereby it was shown to be induced by N starvation and can increase lipid content when overexpressed (Deng et al, 2013).…”

Microalgae Synthesize Hydrocarbons from Long-Chain Fatty Acids via a Light-Dependent Pathway

“…for algal physiology, including lipid metabolism (Merchant et al, 2012;Li-Beisson et al, 2015). Initial experiments showed no significant differences in heptadecene content between the vegetative cells of the strains CC124 (mating type: minus) and CC125 (mating type: plus) (Supplemental Fig.…”

Revisiting the Algal “Chloroplast Lipid Droplet”: The Absence of an Entity That Is Unlikely to Exist

“…Three studies on the presence of lipid droplets in the chloroplast have reported two lines of evidence: one based on starchless mutants under nitrogen-deficient conditions (Fan et al, 2011;Goodson et al, 2011) and the other based on wildtype cells under high-light stress (Goold et al, 2016). Localization of the lipid droplets is crucial to engineering microalgae for biofuel production, which relies on the biochemical and cytological understanding of TAG synthesis and accumulation (Merchant et al, 2012;Johnson and Alric, 2013;Li-Beisson et al, 2015). Thus, we sought to resolve the controversy by carefully repeating the previous electron microscopy experiments and using confocal 3D image reconstruction.…”