A single mutation that causes phosphatidylglycerol deficiency impairs synthesis of photosystem II cores in Chlamydomonas reinhardtii

The oleaginous astaxanthin-producing alga Chromochloris zofingiensis: potential from production to an emerging model for studying lipid metabolism and carotenogenesis

The algal lipids-based biodiesel, albeit having advantages over plant oils, still remains high in the production cost. Co-production of value-added products with lipids has the potential to add benefits and is thus believed to be a promising strategy to improve the production economics of algal biodiesel. Chromochloris zofingiensis, a unicellular green alga, has been considered as a promising feedstock for biodiesel production because of its robust growth and ability of accumulating high levels of triacylglycerol under multiple trophic conditions. This alga is also able to synthesize high-value keto-carotenoids and has been cited as a candidate producer of astaxanthin, the strongest antioxidant found in nature. The concurrent accumulation of triacylglycerol and astaxanthin enables C. zofingiensis an ideal cell factory for integrated production of the two compounds and has potential to improve algae-based production economics. Furthermore, with the advent of chromosome-level whole genome sequence and genetic tools, C. zofingiensis becomes an emerging model for studying lipid metabolism and carotenogenesis. In this review, we summarize recent progress on the production of triacylglycerol and astaxanthin by C. zofingiensis. We also update our understanding in the distinctive molecular mechanisms underlying lipid metabolism and carotenogenesis, with an emphasis on triacylglycerol and astaxanthin biosynthesis and crosstalk between the two pathways. Furthermore, strategies for trait improvements are discussed regarding triacylglycerol and astaxanthin synthesis in C. zofingiensis.

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.

Sulfoquinovosyldiacylglycerol has an Essential Role in Thermosynechococcus elongatus BP-1 Under Phosphate-Deficient Conditions

Anionic lipids, sulfoquinovosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG), are major classes of the thylakoid membrane lipids in cyanobacteria and plant chloroplasts. PG is essential for growth and photosynthesis of cyanobacteria, algae and plants, but the requirement for SQDG differs even among cyanobacterial species. Although SQDG and PG can compensate each other in part presumably to maintain proper balance of anionic charge in lipid bilayers, the functional relationship of these lipids is largely unknown. In this study, we inactivated the sqdB gene, encoding a UDP-sulfoquinovose synthase and involved in SQDG biosynthesis, in Thermosynechococcus elongatus BP-1. In wild-type cells, PG accounted for only approximately 3.5 mol% of total membrane lipids, but its content was substantially increased along with complete loss of SQDG in the sqdB mutant. Under phosphate (Pi)-sufficient conditions, the growth rate and PSII activity were slightly lower in sqdB than in wild-type cells. In addition, the formation of PSI trimers and PSII dimers and energy transfer in phycobilisomes were perturbed in the mutant. Under Pi-deficient conditions, the growth of sqdB cells was severely impaired, with a decrease in PSII activity. PG supplementation could partially rescue the defective growth and PSII activity of Pi-deficient sqdB cells but fully recovered the impaired growth of the pgsA mutant of T. elongatus, which is deficient in PG biosynthesis. These data suggest that SQDG has a specific role in the growth and photosynthesis of T. elongatus, which cannot be complemented by PG, particularly under Pi-deficient conditions.

Role of membrane glycerolipids in photosynthesis, thylakoid biogenesis and chloroplast development

The lipid bilayer of the thylakoid membrane in plant chloroplasts and cyanobacterial cells is predominantly composed of four unique lipid classes; monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), sulfoquinovosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG). MGDG and DGDG are uncharged galactolipids that constitute the bulk of thylakoid membrane lipids and provide a lipid bilayer matrix for photosynthetic complexes as the main constituents. The glycolipid SQDG and phospholipid PG are anionic lipids with a negative charge on their head groups. SQDG and PG substitute for each other to maintain the amount of total anionic lipids in the thylakoid membrane, with PG having indispensable functions in photosynthesis. In addition to biochemical studies, extensive analyses of mutants deficient in thylakoid lipids have revealed important roles of these lipids in photosynthesis and thylakoid membrane biogenesis. Moreover, recent studies of Arabidopsis thaliana suggest that thylakoid lipid biosynthesis triggers the expression of photosynthesis-associated genes in both the nucleus and plastids and activates the formation of photosynthetic machineries and chloroplast development. Meanwhile, galactolipid biosynthesis is regulated in response to chloroplast functionality and lipid metabolism at transcriptional and post-translational levels. This review summarizes the roles of thylakoid lipids with their biosynthetic pathways in plants and discusses the coordinated regulation of thylakoid lipid biosynthesis with the development of photosynthetic machinery during chloroplast biogenesis.

Metabolism of acyl-lipids in Chlamydomonas reinhardtii

Microalgae 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.

Temperature dependence of photosynthesis and thylakoid lipid composition in the red snow alga Chlamydomonas cf. nivalis (Chlorophyceae)

Here, we report an effect of short acclimation to a wide span of temperatures on photosynthetic electron transfer, lipid and fatty acid composition in the snow alga Chlamydomonas cf. nivalis. The growth and oxygen evolution capacity were low at 2 °C yet progressively enhanced at 10 °C and were significantly higher at temperatures from 5 to 15 °C in comparison with the mesophilic control Chlamydomonas reinhardtii. In search of the molecular mechanisms responsible for the adaptation of photosynthesis to low temperatures, we have found unprecedented high rates of QA to QB electron transfer. The thermodynamics of the process revealed the existence of an increased structural flexibility that we explain with the amino acid changes in the D1 protein combined with the physico-chemical characteristics of the thylakoid membrane composed of > 80% negatively charged phosphatidylglycerol.

Remodeling of membrane lipids in iron-starved Chlamydomonas

Chlamydomonas reinhardtii cells exposed to abiotic stresses (e.g. nitrogen, zinc, or phosphorus deficiency) accumulate triacylglycerols (TAG), which are stored in lipid droplets. Here, we report that iron starvation leads to formation of lipid droplets and accumulation of TAGs. This occurs between 12 and 24 h after the switch to iron-starvation medium. C. reinhardtii cells deprived of iron have more saturated fatty acid (FA), possibly due to the loss of function of FA desaturases, which are iron-requiring enzymes with diiron centers. The abundance of a plastid acyl-ACP desaturase (FAB2) is decreased to the same degree as ferredoxin. Ferredoxin is a substrate of the desaturases and has been previously shown to be a major target of the iron deficiency response. The increase in saturated FA (C16:0 and C18:0) is concomitant with the decrease in unsaturated FA (C16:4, C18:3, or C18:4). This change was gradual for diacylglyceryl-N,N,N-trimethylhomoserine (DGTS) and digalactosyldiacylglycerol (DGDG), whereas the monogalactosyldiacylglycerol (MGDG) FA profile remained stable during the first 12 h, whereas MGDG levels were decreasing over the same period of time. These changes were detectable after only 2 h of iron starvation. On the other hand, DGTS and DGDG contents gradually decreased until a minimum was reached after 24-48 h. RNA-Seq analysis of iron-starved C. reinhardtii cells revealed notable changes in many transcripts coding for enzymes involved in FA metabolism. The mRNA abundances of genes coding for components involved in TAG accumulation (diacylglycerol acyltransferases or major lipid droplet protein) were increased. A more dramatic increase at the transcript level has been observed for many lipases, suggesting that major remodeling of lipid membranes occurs during iron starvation in C. reinhardtii.

Phospholipid metabolism in an industry microalga Chlorella sorokiniana: the impact of inoculum sizes

Chlorella sorokiniana is an important industry microalga potential for biofuel production. Inoculum size is one of the important factors in algal large-scale culture, and has great effects on the growth, lipid accumulation and metabolism of microalgae. As the first barrier of cell contents, membrane plays a vital role in algal inoculum-related metabolism. The knowledge of phospholipids, the main membrane component and high accumulation of phospholipids as the major content of total lipids mass in some microalgae, is necessary to understand the role of membrane in cell growth and metabolism under different inoculum density. Profiling of C. sorokiniana phospholipids with LC-MS led to the identification of 119 phospholipid species. To discover the phospholipid molecules most related to change of inoculum sizes, Partial Least Squares Discriminant Analysis (PLS-DA) was employed and the results revealed that inoculum sizes significantly affected phospholipid profiling. Phosphatidylglycerol (PG), phosphatidyl- ethanolamine (PE) and several phosphatidylcholine (PC) species might play an important role under our experimental conditions. Further analysis of these biomarkers indicated that cell membrane status of C. sorokiniana might play an important role in the adaption to the inoculum sizes. And the culture with inoculum size of 1×10⁶ cells mL⁻¹ presented the best membrane status with the highest content of PC and PG, and the lowest content of PE. We discovered that the inoculum size of 1×10⁶ cells mL⁻¹ might provide the best growth condition for C. sorokiniana. Also we proposed that PG, PE and several PC may play an important role in inoculum-related metabolism in C. sorokiniana, which may work through thylakoid membrane and photosynthetic pathway. Thus this study would provide more potential targets for metabolic engineering to improve biofuel production and productivity in microalgae.

Multiple sites of retardation of electron transfer in Photosystem II after hydrolysis of phosphatidylglycerol

Phosphatidylglycerol (PG), containing the unique fatty acid Delta3, trans-16:1-hexadecenoic acid, is a minor but ubiquitous lipid component of thylakoid membranes of chloroplasts and cyanobacteria. We investigated its role in electron transfers and structural organization of Photosystem II (PSII) by treating Arabidopsis thaliana thylakoids with phospholipase A(2) to decrease the PG content. Phospholipase A(2) treatment of thylakoids (a) inhibited electron transfer from the primary quinone acceptor Q(A) to the secondary quinone acceptor Q(B), (b) retarded electron transfer from the manganese cluster to the redox-active tyrosine Z, (c) decreased the extent of flash-induced oxidation of tyrosine Z and dark-stable tyrosine D in parallel, and (d) inhibited PSII reaction centres such that electron flow to silicomolybdate in continuous light was inhibited. In addition, phospholipase A(2) treatment of thylakoids caused the partial dissociation of (a) PSII supercomplexes into PSII dimers that do not have the complete light-harvesting complex of PSII (LHCII); (b) PSII dimers into monomers; and (c) trimers of LHCII into monomers. Thus, removal of PG by phospholipase A(2) brings about profound structural changes in PSII, leading to inhibition/retardation of electron transfer on the donor side, in the reaction centre, and on the acceptor side. Our results broaden the simple view of the predominant effect being on the Q(B)-binding site.

Electrospray ionization multiple stage quadrupole ion-trap and tandem quadrupole mass spectrometric studies on phosphatidylglycerol from Arabidopsis leaves

Phosphatidylglycerol (PG) is the major phospholipid of plant chloroplasts. PG from Arabidopsis thaliana has an unusual fatty acyl chain, 3-trans-hexadecenoyl (Delta(3)16:1) in the sn-2 position of the major 18:3/Delta(3)16:1-PG species, as well as in 18:2/Delta(3)16:1-PG and 16:0/Delta(3)16:1-PG. Upon low-energy collisionally activated dissociation (CAD) in a tandem quadrupole or in an ion-trap mass spectrometer, the [M - H]- ions of the PG molecules containing Delta(3)16:1 give product-ion spectra that are readily distinguishable from those arising from PGs without the Delta(3)16:1 species. The Delta(3)16:1-fatty acyl-containing PGs are characterized by MS(2) product-ion mass spectra that contain predominant [M - H - 236]- ions arising from loss of the Delta(3)16:1-fatty acyl substituent as a ketene. This is attributable to the fact that the alpha-hydrogen of the Delta(3)16:1-fatty acid substituent involved in the ketene loss is an allylic hydrogen, which is very labile. This leads to preferential neutral loss of 236 and drastic decline in the neutral loss of 254 (i.e., loss as a fatty acid), the unique features that signify the presence of Delta(3)16:1-fatty acyl containing PGs. The neutral loss scan of 236, thus, provides a sensitive tandem quadrupole mass spectrometric means to identify Delta(3)16:1-containing PG species in lipid mixtures. This low-energy tandem mass spectrometric approach also permits the structures of the Arabidopsis PGs that consist of two isomeric structures to be unveiled.

Impaired photosynthesis in phosphatidylglycerol-deficient mutant of cyanobacterium Anabaena sp. PCC7120 with a disrupted gene encoding a putative phosphatidylglycerophosphatase

Phosphatidylglycerol (PG) is a ubiquitous phospholipid in thylakoid membranes of cyanobacteria and chloroplasts and plays an important role in the structure and function of photosynthetic membranes. The last step of the PG biosynthesis is dephosphorylation of phosphatidylglycerophosphate (PGP) catalyzed by PGP phosphatase. However, the gene-encoding PGP phosphatase has not been identified and cloned from cyanobacteria or higher plants. In this study, we constructed a PG-deficient mutant from cyanobacterium Anabaena sp. PCC7120 with a disrupted gene (alr1715, a gene for Alr1715 protein, GenBank accession no. BAB78081) encoding a putative PGP phosphatase. The obtained mutant showed an approximately 30% reduction in the cellular content of PG. Following the reduction in the PG content, the photoautotrophical growth of the mutant was restrained, and the cellular content of chlorophyll was decreased. The decreases in net photosynthetic and photosystem II (PSII) activities on a cell basis also occurred in this mutant. Simultaneously, the photochemical efficiency of PSII was considerably declined, and less excitation energy was transferred toward PSII. These findings demonstrate that the alr1715 gene of Anabaena sp. PCC7120 is involved in the biosynthesis of PG and essential for photosynthesis.

Detection and analysis of protein–protein interactions in organellar and prokaryotic proteomes by native gel electrophoresis: (Membrane) protein complexes and supercomplexes

It is an essential and challenging task to unravel protein-protein interactions in their actual in vivo context. Native gel systems provide a separation platform allowing the analysis of protein complexes on a rather proteome-wide scale in a single experiment. This review focus on blue-native (BN)-PAGE as the most versatile and successful gel-based approach to separate soluble and membrane protein complexes of intricate protein mixtures derived from all biological sources. BN-PAGE is a charge-shift method with a running pH of 7.5 relying on the gentle binding of anionic CBB dye to all membrane and many soluble protein complexes, leading to separation of protein species essentially according to their size and superior resolution than other fractionation techniques can offer. The closely related colorless-native (CN)-PAGE, whose applicability is restricted to protein species with intrinsic negative net charge, proved to provide an especially mild separation capable of preserving weak protein-protein interactions better than BN-PAGE. The essential conditions determining the success of detecting protein-protein interactions are the sample preparations, e.g. the efficiency/mildness of the detergent solubilization of membrane protein complexes. A broad overview about the achievements of BN- and CN-PAGE studies to elucidate protein-protein interactions in organelles and prokaryotes is presented, e.g. the mitochondrial protein import machinery and oxidative phosphorylation supercomplexes. In many cases, solubilization with digitonin was demonstrated to facilitate an efficient and particularly gentle extraction of membrane protein complexes prone to dissociation by treatment with other detergents. In general, analyses of protein interactomes should be carried out by both BN- and CN-PAGE.

Lipids and lipid metabolism in eukaryotic algae

Eukaryotic algae are a very diverse group of organisms which inhabit a huge range of ecosystems from the Antarctic to deserts. They account for over half the primary productivity at the base of the food chain. In recent years studies on the lipid biochemistry of algae has shifted from experiments with a few model organisms to encompass a much larger number of, often unusual, algae. This has led to the discovery of new compounds, including major membrane components, as well as the elucidation of lipid signalling pathways. A major drive in recent research have been attempts to discover genes that code for expression of the various proteins involved in the production of very long-chain polyunsaturated fatty acids such as arachidonic, eicosapentaenoic and docosahexaenoic acids. Such work is described here together with information about how environmental factors, such as light, temperature or minerals, can change algal lipid metabolism and how adaptation may take place.

Subcellular localization and light-regulated expression of protoporphyrinogen IX oxidase and ferrochelatase in Chlamydomonas reinhardtii

Protoporphyrinogen IX oxidase (PPO) catalyzes the last common step in chlorophyll and heme synthesis, and ferrochelatase (FeC) catalyzes the last step of the heme synthesis pathway. In plants, each of these two enzymes is encoded by two or more genes, and the enzymes have been reported to be located in the chloroplasts or in the mitochondria. We report that in the green alga Chlamydomonas reinhardtii, PPO and FeC are each encoded by a single gene. Phylogenetic analysis indicates that C. reinhardtii PPO and FeC are most closely related to plant counterparts that are located only in chloroplasts. Immunoblotting results suggest that C. reinhardtii PPO and FeC are targeted exclusively to the chloroplast, where they are associated with membranes. These results indicate that cellular needs for heme in this photosynthetic eukaryote can be met by heme that is synthesized in the chloroplast. It is proposed that the multiplicity of genes for PPO and FeC in higher plants could be related to differential expression in differently developing tissues rather than to targeting of different gene products to different organelles. The FeC content is higher in C. reinhardtii cells growing in continuous light than in cells growing in the dark, whereas the content of PPO does not significantly differ in light- and dark-grown cells. In cells synchronized to a light/dark cycle, the level of neither enzyme varied significantly with the phase of the cycle. These results indicate that heme synthesis is not directly regulated by the levels of PPO and FeC in C. reinhardtii.

Annotation of genes involved in glycerolipid biosynthesis in Chlamydomonas reinhardtii: discovery of the betaine lipid synthase BTA1Cr

Lipid metabolism in flowering plants has been intensely studied, and knowledge regarding the identities of genes encoding components of the major fatty acid and membrane lipid biosynthetic pathways is very extensive. We now present an in silico analysis of fatty acid and glycerolipid metabolism in an algal model, enabled by the recent availability of expressed sequence tag and genomic sequences of Chlamydomonas reinhardtii. Genes encoding proteins involved in membrane biogenesis were predicted on the basis of similarity to proteins with confirmed functions and were organized so as to reconstruct the major pathways of glycerolipid synthesis in Chlamydomonas. This analysis accounts for the majority of genes predicted to encode enzymes involved in anabolic reactions of membrane lipid biosynthesis and compares and contrasts these pathways in Chlamydomonas and flowering plants. As an important result of the bioinformatics analysis, we identified and isolated the C. reinhardtii BTA1 (BTA1Cr) gene and analyzed the bifunctional protein that it encodes; we predicted this protein to be sufficient for the synthesis of the betaine lipid diacylglyceryl-N,N,N-trimethylhomoserine (DGTS), a major membrane component in Chlamydomonas. Heterologous expression of BTA1Cr led to DGTS accumulation in Escherichia coli, which normally lacks this lipid, and allowed in vitro analysis of the enzymatic properties of BTA1Cr. In contrast, in the bacterium Rhodobacter sphaeroides, two separate proteins, BtaARs and BtaBRs, are required for the biosynthesis of DGTS. Site-directed mutagenesis of the active sites of the two domains of BTA1Cr allowed us to study their activities separately, demonstrating directly their functional homology to the bacterial orthologs BtaARs and BtaBRs.

Effect of phosphatidylglycerol on molecular organization of photosystem I

Phosphatidylglycerol (PG) is the only anionic phospholipid in photosynthetic membrane. In this study, photosystem I (PSI) particles obtained from plant spinach were reconstituted into PG liposomes at a relatively high concentration. The results from visible absorption, fluorescence emission, and circular dichroism (CD) spectra reveal an existence of the interactions of PSI with PG. PG effect causes blue-shift and intensity decrease of Chl a peak bands in the absorption and 77 K fluorescence emission. The visible CD spectra indicate that the excitonic interactions for Chl a and Chl b molecules were enhanced upon reconstitution. Furthermore, more or less blue- or red-shift of the peaks characterized by Chl a, Chl b, and carotenoid molecules are also occurred. Simultaneously, an increase in alpha-helix and a decrease particularly in the disordered conformations of protein secondary structures are observed. In addition, the same effect also leads to somewhat more tryptophan (Trp) residues exposed to the polar environment. These results demonstrate that some alteration of molecular organization occurs within both the external antenna LHCI and PSI core complex after PSI reconstitution.

Photochemical activities of plant photosystem I particles reconstituted into phosphatidylglycerol liposomes

Phosphatidylglycerol (PG) is the only anionic phospholipid in photosynthetic membrane and the important component of photosystem I (PSI). In this study, the interaction of PG with PSI particle from spinach was investigated by using reconstitution method. The results from the properties of electron transport, fluorescence emission, turbidity, and protein secondary structures in PSI complex incorporated into PG liposomes revealed the existence of PSI-PG interactions. A stimulation and an inhibition of oxygen uptake in PSI particle at a low and higher PG/chlorophyll mass ratio, respectively, were observed. Moreover, an additional enhancement and depression of electron flow in the PSI-PG complexes were occurred in the reaction medium containing CaCl2 at concentrations below and above 5 mM, the aggregation threshold of the reconstituted membranes, respectively. The results demonstrated that the maintenance of the structural optimization was needed for a stimulation of electron transport at a low PG/PSI mass ratio, while a decay of this PSI activity at high PG/PSI ratio was the result of inhibition of the energy transfer from LHCI to PSI reaction center induced by the dissociation of LHCI-680.

Phosphatidylglycerol and sulfoquinovosyldiacylglycerol: anionic membrane lipids and phosphate regulation

Photosynthetic membranes of organisms from cyanobacteria to seed plants are characterized by the neutral galactolipids and the anionic glycerolipids sulfoquinovosyldiacylglycerol and phosphatidylglycerol. Recent findings have brought new insights into the biosynthesis of the anionic membrane lipids, the evolutionary origin of the enzymes involved in this process, and the importance of phosphatidylglycerol and sulfoquinovosyldiacylgycerol in photosynthesis. Photosynthetic membranes require a defined level of anionic membrane lipids for proper function, and phosphatidylglycerol and sulfoquinovosyldiacylglycerol can substitute for each other to a certain extent. A defined level of phosphatidylglycerol is, however, indispensable for photoautotrophic growth. On the other hand, sulfoquinovosyldiacylglycerol plays a conditionally important role in enabling photosynthetic organisms to survive the phosphate-limiting conditions frequently encountered in natural habitats.