Lipid metabolism and potentials of biofuel and high added-value oil production in red algae

Enhanced growth and metabolite production from a novel strain of Porphyridium sp

Microalgae are capable of generating numerous metabolites that possess notable biological activities and hold substantial promise for various industrial applications. Nevertheless, the taxonomic diversity of these photosynthetic microorganisms has not received thorough investigation. Using the 18S rRNA encoding gene, a recently discovered strain originating from the Tunisian coast (the governorate of Mahdia) was identified as a member of the Porphyridium genus. The growth response as well as the metabolite accumulation of Porphyridium sp. to different culture media (Pm, F/2, and Hemerick) was investigated over a period of 52 days. The highest biomass production was recorded with Pm medium (2 × 107 cell/mL). The apparent growth rates (µ) and the doubling time (Dt) were about 0.081 day-1 and 12.34 days, respectively. The highest chlorophyll a (0.678 ± 0.005 pg/cell), total carotenoids (0.18 ± 0.003 pg/cell), phycoerythrin (3.88 ± 0.003 pg/cell), and proteins (14.58 ± 0.35 pg/cell) contents were observed with F/2 medium. Cultivating Porphyridium sp. in both F/2 and Hemerick media yielded similar levels of starch accumulation. The Hemerick medium has proven to be the most suitable for the production of lipids (2.23% DW) and exopolysaccharides (5.41 ± 0.56 pg/cell).

The carbon-concentrating mechanism of the extremophilic red microalga Cyanidioschyzon merolae

Cyanidioschyzon merolae is an extremophilic red microalga which grows in low-pH, high-temperature environments. The basis of C. merolae's environmental resilience is not fully characterized, including whether this alga uses a carbon-concentrating mechanism (CCM). To determine if C. merolae uses a CCM, we measured CO2 uptake parameters using an open-path infra-red gas analyzer and compared them to values expected in the absence of a CCM. These measurements and analysis indicated that C. merolae had the gas-exchange characteristics of a CCM-operating organism: low CO2 compensation point, high affinity for external CO2, and minimized rubisco oxygenation. The biomass δ13C of C. merolae was also consistent with a CCM. The apparent presence of a CCM in C. merolae suggests the use of an unusual mechanism for carbon concentration, as C. merolae is thought to lack a pyrenoid and gas-exchange measurements indicated that C. merolae primarily takes up inorganic carbon as carbon dioxide, rather than bicarbonate. We use homology to known CCM components to propose a model of a pH-gradient-based CCM, and we discuss how this CCM can be further investigated.

Flashing lights affect the photophysiology and expression of carotenoid and lipid synthesis genes in Nannochloropsis gaditana

Nannochloropsis gaditana is a promising microalga for biotechnology. One of the strategies to stimulate its full potential in metabolite production is exposure to flashing lights. Here, we report how N. gaditana adapts to different flashing light regimes (5, 50, and 500 Hz) by changing its cellular physiology and the relative expression of genes related to critical cellular functions. We analyzed the differential mRNA abundance of genes related to photosynthesis, nitrogen assimilation and biosynthesis of chlorophyll, carotenoids, lipids, fatty acids and starch. Analysis of photosynthetic efficiency and high mRNA abundance of photoprotection genes supported the inference that excess excitation energy provided by light absorbance during photosynthesis was produced under low frequency flashing lights and was dissipated by photopigments via the xanthophyll-cycle. Increased relative expression levels of genes related to the synthesis of carotenoids and chlorophyll confirmed the accumulation of photopigments previously observed at low frequency flashing lights. Higher differential mRNA abundance of genes related to the triacylglycerol biosynthesis were observed at lower frequency flashing lights, possibly triggered by a poor nitrogen assimilation caused by low mRNA abundance of a nitrate reductase gene. This study advances a new understanding of algal physiology and metabolism leading to improved cellular performance and metabolite production.

Bioactive Compounds from Red Microalgae with Therapeutic and Nutritional Value

Red microalgae represent a natural reservoir of beneficial substances with applications in different industrial sectors. They are rich in natural biomolecules known for their antihypertensive, antioxidant, antimicrobial, antiviral, anti-inflammatory, antitumor, and anticoagulant activities. Many red microalgae are a source of vitamins, minerals, photochemicals, polyunsaturated fatty acids, and a wide spectrum of polysaccharides. The content of their valuable compounds and their activities have turned red microalgae into cellular factories of special interest in food, nutraceutical, and pharmaceutical industries. Like all microalgae, the red ones are superior to traditional crops for the aims of biotechnology as they are renewable sources widely available in great quantities and are easy to culture. Moreover, some of the most studied red microalgae are generally recognized as safe. This review summarizes the valuable biochemicals from red microalgae and highlights their health and nutritional benefits.

Expression of Chlamydomonas reinhardtii chloroplast diacylglycerol acyltransferase 3 is induced by light in concert with triacylglycerol accumulation

Considerable progress has been made towards the understanding of triacylglycerol (TAG) accumulation in algae. One key aspect is finding conditions that trigger TAG production without reducing cell division. Previously, we identified a soluble diacylglycerol acyltransferase (DGAT), related to plant DGAT3, with heterologous DGAT activity. In this work, we demonstrate that Chlamydomonas reinhardtii DGAT3 localizes to the chloroplast and that its expression is induced by light, in correspondence with TAG accumulation. Dgat3 mRNAs and TAGs increase in both wild-type and starch-deficient cells grown with acetate upon transferring them from dark or low light to higher light levels, albeit affected by the particularities of each strain. The response of dgat3 mRNAs and TAGs to light depends on the pre-existing levels of TAGs, suggesting the existence of a negative regulatory loop in the synthesis pathway, although an effect of TAG turnover cannot be ruled out. Altogether, these results hint towards a possible role of DGAT3 in light-dependent TAG accumulation in C. reinhardtii.

Hyaluronic Acid Allows Enzyme Immobilization for Applications in Biomedicine

Enzymes are proteins that control the efficiency and effectiveness of biological reactions and systems, as well as of engineered biomimetic processes. This review highlights current applications of a diverse range of enzymes for biofuel production, plastics, and chemical waste management, as well as for detergent, textile, and food production and preservation industries respectively. Challenges regarding the transposition of enzymes from their natural purpose and environment into synthetic practice are discussed. For example, temperature and pH-induced enzyme fragilities, short shelf life, low-cost efficiency, poor user-controllability, and subsequently insufficient catalytic activity were shown to decrease pertinence and profitability in large-scale production considerations. Enzyme immobilization was shown to improve and expand upon enzyme usage within a profit and impact-oriented commercial world and through enzyme-material and interfaces integration. With particular focus on the growing biomedical market, examples of enzyme immobilization within or onto hyaluronic acid (HA)-based complexes are discussed as a definable way to improve upon and/or make possible the next generation of medical undertakings. As a polysaccharide formed in every living organism, HA has proven beneficial in biomedicine for its high biocompatibility and controllable biodegradability, viscoelasticity, and hydrophilicity. Complexes developed with this molecule have been utilized to selectively deliver drugs to a desired location and at a desired rate, improve the efficiency of tissue regeneration, and serve as a viable platform for biologically accepted sensors. In similar realms of enzyme immobilization, HA’s ease in crosslinking allows the molecule to user-controllably enhance the design of a given platform in terms of both chemical and physical characteristics to thus best support successful and sustained enzyme usage. Such examples do not only demonstrate the potential of enzyme-based applications but further, emphasize future market trends and accountability.

The Unicellular Red Alga Cyanidioschyzon merolae-The Simplest Model of a Photosynthetic Eukaryote

Several species of unicellular eukaryotic algae exhibit relatively simple genomic and cellular architecture. Laboratory cultures of these algae grow faster than plants and often provide homogeneous cellular populations exposed to an almost equal environment. These characteristics are ideal for conducting experiments at the cellular and subcellular levels. Many microalgal lineages have recently become genetically tractable, which have started to evoke new streams of studies. Among such algae, the unicellular red alga Cyanidioschyzon merolae is the simplest organism; it possesses the minimum number of membranous organelles, only 4,775 protein-coding genes in the nucleus, and its cell cycle progression can be highly synchronized with the diel cycle. These properties facilitate diverse omics analyses of cellular proliferation and structural analyses of the intracellular relationship among organelles. C. merolae cells lack a rigid cell wall and are thus relatively easily disrupted, facilitating biochemical analyses. Multiple chromosomal loci can be edited by highly efficient homologous recombination. The procedures for the inducible/repressive expression of a transgene or an endogenous gene in the nucleus and for chloroplast genome modification have also been developed. Here, we summarize the features and experimental techniques of C. merolae and provide examples of studies using this alga. From these studies, it is clear that C. merolae-either alone or in comparative and combinatory studies with other photosynthetic organisms-can provide significant insights into the biology of photosynthetic eukaryotes.

The regulatory activities of microRNAs in non-vascular plants: a mini review

MicroRNA-mediated gene regulation in non-vascular plants is potentially involved in several unique biological functions, including biosynthesis of several highly valuable exclusive bioactive compounds, and those small RNAs could be manipulated for the overproduction of essential bioactive compounds in the future. MicroRNAs (miRNAs) are a class of endogenous, small (20-24 nucleotides), non-coding RNA molecules that regulate gene expression through the miRNA-mediated mechanisms of either translational inhibition or messenger RNA (mRNA) cleavage. In the past years, studies have mainly focused on elucidating the roles of miRNAs in vascular plants as compared to non-vascular plants. However, non-vascular plant miRNAs have been predicted to be involved in a wide variety of specific biological mechanisms; nevertheless, some of them have been demonstrated explicitly, thus showing that the research field of this plant group owns a noteworthy potential to develop novel investigations oriented towards the functional characterization of these miRNAs. Furthermore, the insights into the roles of miRNAs in non-vascular plants might be of great importance for designing the miRNA-based genetically modified plants for valuable secondary metabolites, active compounds, and biofuels in the future. Therefore, in this current review, we provide an overview of the potential roles of miRNAs in different groups of non-vascular plants such as algae and bryophytes.

Photobiological Effects on Biochemical Composition in Porphyridium cruentum (Rhodophyta) with a Biotechnological Application

This study describes the relation of photosynthetic capacity, growth and biochemical compounds in the microalgae Porphyridium cruentum under saturated irradiance (200 μmol m^-2  s^-1 ) by white light (WL) and low-pressure sodium vapor lamps (SOX lamps-control) and supplemented by fluorescent lamps (FLs) with different light qualities (blue: λ_max = 440 nm; green: λ_max = 560 nm; and red: λ_max = 660 nm). The maximum photosynthetic efficiency (F_v / F_m ) showed a positive correlation with the light quality by saturating light SOX in mixture with stimulating blue light than the white light (WL) at the harvest day (10 days). The production, that is maximal electron transport rate (ETR_max ), and energy dissipation, that is maximal nonphotochemical quenching (NPQ_max ), had the same pattern throughout the time (3-6 days) being the values higher under white light (WL) compared with SOX and SOX plus supplemented different light qualities. Total protein levels increased significantly in the presence of SOX light, while phycoerythrin (B-PE) showed significant differences under SOX+ blue light. Arachidonic acid (ARA) was higher under SOX and SOX plus supplemented different light qualities than that under WL, whereas eicosapentaenoic acid (EPA) was the reverse. The high photomorphogenic potential by SOX light shows promising application for microalgal biotechnology.

Dynamism of Metabolic Carbon Flow of Starch and Lipids in Chlamydomonas debaryana

Microalgae have the potential to recycle CO2 as starch and triacylglycerol (TAG), which provide alternative source of biofuel and high added-value chemicals. Starch accumulates in the chloroplast, whereas TAG accumulates in the cytoplasmic lipid droplets (LD). Preferential accumulation of starch or TAG may be achieved by switching intracellular metabolic carbon flow, but our knowledge on this control remains limited. Are these two products mutually exclusive? Or, does starch act as a precursor to TAG synthesis, or vice versa? To answer these questions, we analyzed carbon flow in starch and lipids using a stable isotope 13C in Chlamydomonas debaryana NIES-2212, which accumulates, without nutrient limitation, starch in the exponential growth phase and TAG in the stationary phase. Pulse labeling experiments as well as pulse labeling and chase experiments were conducted, and then, gas chromatography-mass spectrometry (GC-MS) analysis was performed on starch-derived glucose and lipid-bound fatty acids. We exploited the previously developed method of isotopomer analysis to estimate the proportion of various pools with different isotopic abundance. Starch turned over rapidly to provide carbon for the synthesis of fatty acids in the exponential phase cells. Most fatty acids showed rapid and slow components of metabolism, whereas oleic acid decayed according to a single exponential curve. Highly labeled population of fatty acids that accumulated during the initial labeling decreased rapidly, and replaced by low abundance population during the chase time, indicating that highly labeled fatty acids were degraded and the resulting carbons were re-used in the re-synthesis with about 9-fold unlabeled, newly fixed carbons. Elongation of C16-C18 acids in vivo was indicated by partially labeled C18 acids. The accumulation of TAG in the stationary growth phase was accounted for by both de novo synthesis and remodeling of membrane lipids. These results suggest that de novo synthesis of starch and TAG was rapid and transient, and also almost independent to each other, but there is a pool of starch quickly turning over for the synthesis of fatty acids. Fatty acids were also subject to re-synthesis. Evidence was also provided for remodeling of lipids, namely, re-use of acyl groups in polar lipids for TAG synthesis.

Molecular mechanism of arachidonic acid biosynthesis in Porphyridium purpureum promoted by nitrogen limitation

The red alga Porphyridium purpureum has been known to produce polyunsaturated fatty acids, especially arachidonic acid (ARA), under stressful conditions. However, there is no consistent conclusion about the response of ARA in this alga to nitrogen (N) stress. Also, no research has been done to clearly elucidate the underlying molecular mechanisms of N stress. In this work, P. purpureum CoE1 was cultivated under nitrogen limitation conditions and the putative Δ5-desaturase related gene FADSD5 was isolated. The results showed that the fatty acids in P. purpureum CoE1 were significantly higher in the N limited cultures (54.3 mg g^-1) than in the N-replete cultures (45.3 mg g^-1) at the 18th day (t-test, p < 0.001), which was attributed to the upregulated abundance of the putative Δ5-desaturase related protein, Δ5-Des. The study also indicated that the expression of the putative Δ5-desaturase related gene, FADSD5, increased with cell growth, demonstrating considerable potentials for ARA biosynthesis in P. purpureum CoE1. These results might guide the direction in illuminating the biosynthetic pathway of fatty acids with molecular evidence and enable genetic modifications of P. purpureum CoE1 for enhancing the ARA accumulation.

Porphyridium cruentum Grown in Ultra-Filtered Swine Wastewater and Its Effects on Microalgae Growth Productivity and Fatty Acid Composition

Microalgae have been extensively tested for their ability to create bio-based fuels. Microalgae have also been explored as an alternative wastewater treatment solution due to their significant uptake of nitrogen and phosphorus, as well as their ability to grow in different water types. Recently, there has been significant interest in combining these two characteristics to create economic and environmentally friendly biofuel using wastewater. This study examined the growth and lipid production of the microalgae Porphyridium (P.) cruentum grown in swine wastewater (ultra-filtered and raw) as compared with control media (L−1, modified f/2) at two different salt concentrations (seawater and saltwater). The cultivation of P. cruentum in the treated swine wastewater media (seawater = 5.18 ± 2.3 mgL−1day−1, saltwater = 3.32 ± 1.93 mgL−1day−1) resulted in a statistically similar biomass productivity compared to the control medium (seawater = 2.61 ± 2.47 mgL−1day−1, saltwater = 6.53 ± 0.81 mgL−1day−1) at the corresponding salt concentration. Furthermore, no major differences between the fatty acid compositions of microalgae in the treated swine wastewater medium and the control medium were observed. For all conditions, saturated acids were present in the highest amounts (≥67%), followed by polyunsaturated (≤22%) and finally monounsaturated (≤12%). This is the first study to find that P. cruentum could be used to remediate wastewater and then be turned into fuel by using swine wastewater with a similar productivity to the microalgae grown in control media.

Characterization and Biosynthesis of Lipids in Paulinella micropora MYN1: Evidence for Efficient Integration of Chromatophores into Cellular Lipid Metabolism

The chromatophores found in the cells of photosynthetic Paulinella species, once believed to be endosymbiotic cyanobacteria, are photosynthetic organelles that are distinct from chloroplasts. The chromatophore genome is similar to the genomes of α-cyanobacteria and encodes about 1,000 genes. Therefore, the chromatophore is an intriguing model of organelle formation. In this study, we analyzed the lipids of Paulinella micropora MYN1 to verify that this organism is a composite of cyanobacterial descendants and a heterotrophic protist. We detected glycolipids and phospholipids, as well as a betaine lipid diacylglyceryl-3-O-carboxyhydroxymethylcholine, previously detected in many marine algae. Cholesterol was the only sterol component detected, suggesting that the host cell is similar to animal cells. The glycolipids, presumably present in the chromatophores, contained mainly C16 fatty acids, whereas other classes of lipids, presumably present in the other compartments, were abundant in C20 and C22 polyunsaturated fatty acids. This suggests that chromatophores are metabolically distinct from the rest of the cell. Metabolic studies using isotopically labeled substrates showed that different fatty acids are synthesized in the chromatophore and the cytosol, which is consistent with the presence of both type I and type II fatty acid synthases, supposedly present in the cytosol and the chromatophore, respectively. Nevertheless, rapid labeling of the fatty acids in triacylglycerol and phosphatidylcholine by photosynthetically fixed carbon suggested that the chromatophores efficiently provide metabolites to the host. The metabolic and ultrastructural evidence suggests that chromatophores are tightly integrated into the whole cellular metabolism.

Algae: Critical Sources of Very Long-Chain Polyunsaturated Fatty Acids

Polyunsaturated fatty acids (PUFAs), which are divided into n-3 and n-6 classes, are essential for good health in humans and many animals. They are metabolised to lipid mediators, such as eicosanoids, resolvins and protectins. Increasing interest has been paid to the 20 or 22 carbon very long chain PUFAs, since these compounds can be used to form lipid mediators and, thus, avoid inefficient formation of dietary plant PUFAs. The ultimate sources of very long chain PUFAs are algae, which are consumed by fish and then by humans. In this review, I describe the biosynthesis of very long chain PUFAs by algae and how this synthesis can be manipulated for commercial purposes. Ultimately, the production of algal oils is critical for ecosystems worldwide, as well as for human dietary lipids.

Stable nuclear transformation of rhodophyte species Porphyridium purpureum: advanced molecular tools and an optimized method

A mutated phytoene desaturase (pds) gene, pds-L504R, conferring resistance to the herbicide norflurazon has been reported as a dominant selectable marker for the genetic engineering of microalgae (Steinbrenner and Sandmann in Appl Environ Microbiol 72:7477-7484, 2006; Prasad et al. in Appl Microbiol Biotechnol 98(20):8629-8639, 2014). However, this mutated genomic clone harbors several introns and the entire expression cassette including its native promoter and terminator has a length > 5.6 kb, making it unsuitable as a standard selection marker. Therefore, we designed a synthetic, short pds gene (syn-pds-int) by removing introns and unwanted internal restriction sites, adding suitable restriction sites for cloning purposes, and introduced the first intron from the Chlamydomonas reinhardtii RbcS2 gene close to the 5'end without changing the amino acid sequence. The syn-pds-int gene (1872 bp) was cloned into pCAMBIA 1380 under the control of a short sequence (615 bp) of the promoter of pds (pCAMBIA 1380-syn-pds-int). This vector and the plasmid pCAMBIA1380-pds-L504R hosting the mutated genomic pds were used for transformation studies. To broaden the existing transformation portfolio, the rhodophyte Porphyridium purpureum was targeted. Agrobacterium-mediated transformation of P. purpureum with both the forms of pds gene, pds-L504R or syn-pds-int, yielded norflurazon-resistant (NR) cells. This is the first report of a successful nuclear transformation of P. purpureum. Transformation efficiency and lethal norflurazon dosage were determined to evaluate the usefulness of syn-pds-int gene and functionality of the short promoter of pds. PCR and Southern blot analysis confirmed transgene integration into the microalga. Both forms of pds gene expressed efficiently as evidenced by the stability, tolerance and the qRT-PCR analysis. The molecular toolkits and transformation method presented here could be used to genetically engineer P. purpureum for fundamental studies as well as for the production of high-value-added compounds.

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.

Isolation and Characterization of Chlamydomonas Autophagy-Related Mutants in Nutrient-Deficient Conditions

Autophagy is a recycling system for amino acids and carbon- and nitrogen (N)-containing compounds. To date, the functional importance of autophagy in microalgae in nutrient-deficient conditions has not been evaluated by using autophagy-defective mutants. Here, we provide evidence which supports the following notions by characterizing an insertional mutant of the autophagy-related gene ATG8, encoding a ubiquitin-like protein necessary for the formation of the autophagosome in the green alga, Chlamydomonas reinhardtii. First, ATG8 is required for maintenance of cell survival and Chl content in N-, sulfur- and phosphate-deficient conditions. Secondly, ATG8 supports the degradation of triacylglycerol and lipid droplets after the resupply of N to cells cultured in N-limiting conditions. Thirdly, ATG8 is also necessary for accumulation of starch in phosphate-deficient conditions. Additionally, autophagy is not essential for maternal inheritance of the organelle genomes in gametogenesis.

Uncommon properties of lipid biosynthesis of isolated plastids in the unicellular red alga Cyanidioschyzon merolae

Red algae are a large group of photosynthetic eukaryotes that diverged from green algae over one billion years ago, and have various traits distinct from those of both green algae and land plants. Although most red algae are marine species (both unicellular and macrophytic), the Cyanidiales class of red algae includes unicellular species which live in hot springs, such as Cyanidioschyzon merolae, which is a model species for biochemical and molecular biological studies. Lipid metabolism in red algae has previously been studied in intact cells. Here, we present the results of radiolabeling and stable isotope labeling experiments in intact plastids isolated from the unicellular red alga C. merolae. We focused on two uncommon features: First, the galactose moiety of monogalactosyldiacylglycerol was efficiently labeled with bicarbonate, indicating that an unknown pathway for providing UDP-galactose exists within the plastid. Second, saturated fatty acids, namely, palmitic and stearic acids, were the sole products of fatty acid synthesis in the plastid, and they were efficiently exported. This finding suggests that the endoplasmic reticulum is the sole site of desaturation. We present a general principle of red algal lipid biosynthesis, namely, 'indigenous C18 fatty acids are neither desaturated nor directly utilized within the plastid'. We believe that this is valid in both C. merolae lacking polyunsaturated fatty acids and marine red algae with a high content of arachidonic and eicosapentaenoic acids.

New horizons in culture and valorization of red microalgae

Research on marine microalgae has been abundantly published and patented these last years leading to the production and/or the characterization of some biomolecules such as pigments, proteins, enzymes, biofuels, polyunsaturated fatty acids, enzymes and hydrocolloids. This literature focusing on metabolic pathways, structural characterization of biomolecules, taxonomy, optimization of culture conditions, biorefinery and downstream process is often optimistic considering the valorization of these biocompounds. However, the accumulation of knowledge associated with the development of processes and technologies for biomass production and its treatment has sometimes led to success in the commercial arena. In the history of the microalgae market, red marine microalgae are well positioned particularly for applications in the field of high value pigment and hydrocolloid productions. This review aims to establish the state of the art of the diversity of red marine microalgae, the advances in characterization of their metabolites and the developments of bioprocesses to produce this biomass.

Properties and Biotechnological Applications of Acyl-CoA:diacylglycerol Acyltransferase and Phospholipid:diacylglycerol Acyltransferase from Terrestrial Plants and Microalgae

Triacylglycerol (TAG) is the major storage lipid in most terrestrial plants and microalgae, and has great nutritional and industrial value. Since the demand for vegetable oil is consistently increasing, numerous studies have been focused on improving the TAG content and modifying the fatty-acid compositions of plant seed oils. In addition, there is a strong research interest in establishing plant vegetative tissues and microalgae as platforms for lipid production. In higher plants and microalgae, TAG biosynthesis occurs via acyl-CoA-dependent or acyl-CoA-independent pathways. Diacylglycerol acyltransferase (DGAT) catalyzes the last and committed step in the acyl-CoA-dependent biosynthesis of TAG, which appears to represent a bottleneck in oil accumulation in some oilseed species. Membrane-bound and soluble forms of DGAT have been identified with very different amino-acid sequences and biochemical properties. Alternatively, TAG can be formed through acyl-CoA-independent pathways via the catalytic action of membrane-bound phospholipid:diacylglycerol acyltransferase (PDAT). As the enzymes catalyzing the terminal steps of TAG formation, DGAT and PDAT play crucial roles in determining the flux of carbon into seed TAG and thus have been considered as the key targets for engineering oil production. Here, we summarize the most recent knowledge on DGAT and PDAT in higher plants and microalgae, with the emphasis on their physiological roles, structural features, and regulation. The development of various metabolic engineering strategies to enhance the TAG content and alter the fatty-acid composition of TAG is also discussed.

"Prokaryotic Pathway" Is Not Prokaryotic: Noncyanobacterial Origin of the Chloroplast Lipid Biosynthetic Pathway Revealed by Comprehensive Phylogenomic Analysis

Lipid biosynthesis within the chloroplast, or more generally plastids, was conventionally called “prokaryotic pathway,” which produces glycerolipids bearing C18 acids at the sn-1 position and C16 acids at the sn-2 position, as in cyanobacteria such as Anabaena and Synechocystis. This positional specificity is determined during the synthesis of phosphatidate, which is a precursor to diacylglycerol, the acceptor of galactose for the synthesis of galactolipids. The first acylation at sn-1 is catalyzed by glycerol-3-phosphate acyltransferase (GPAT or GPT), whereas the second acylation at sn-2 is performed by lysophosphatidate acyltransferase (LPAAT, AGPAT, or PlsC). Here we present comprehensive phylogenomic analysis of the origins of various acyltransferases involved in the synthesis of phosphatidate, as well as phosphatidate phosphatases in the chloroplasts. The results showed that the enzymes involved in the two steps of acylation in cyanobacteria and chloroplasts are entirely phylogenetically unrelated despite a previous report stating that the chloroplast LPAAT (ATS2) and cyanobacterial PlsC were sister groups. Phosphatidate phosphatases were separated into eukaryotic and prokaryotic clades, and the chloroplast enzymes were not of cyanobacterial origin, in contrast with another previous report. These results indicate that the lipid biosynthetic pathway in the chloroplasts or plastids did not originate from the cyanobacterial endosymbiont and is not “prokaryotic” in the context of endosymbiotic theory of plastid origin. This is another line of evidence for the discontinuity of plastids and cyanobacteria, which has been suggested in the glycolipid biosynthesis.

Characterization of an easy-to-use method for the routine analysis of the central metabolism using an affordable low-resolution GC-MS system: application to Arthrospira platensis

We developed an easy-to-use method for the routine analysis of the central metabolism using an affordable low-resolution GC-MS system run in SIM mode. The profiling approach was optimized for the derivatization protocol of some 60 targeted metabolites. The performance of two silylation reagents (MSTFA and BSTFA) that allowed the comprehensive derivatization of 42 key intermediary metabolites of the 60 initially targeted (organic acids, phosphate derivatives, monosaccharides and amino acids) was measured. The experimental results unequivocally showed that the MSTFA reagent met mandatory criteria including ease of handling (a very simple one-step protocol was developed), comprehensiveness of derivatization (the 42 compounds covered the extended metabolic pathways of the central carbon metabolism, with a coverage percentage ranging from 17% for the worst to 90% for the best result), optimized response coefficient of the whole derivatives (median value greater than the others by one order of magnitude) and repeatability of the protocol (RSD value below 25% for the whole procedure). When tested in real conditions (cyanobacteria polar extract), the experimental results showed that the profiling methodology was adequately repeatable (RSD = 35%) to ensure quantification results comparable with much more sensitive analytical techniques (capillary electrophoresis/mass spectrometry and liquid chromatography/triple quadrupole mass spectrometry system), while needing only about twice the quantity of biomass. Graphical abstract Schematic overview of an easy-to-use profiling method for the routine analysis of the central metabolism using a low-resolution GC-MS system.

Scale-up cultivation enhanced arachidonic acid accumulation by red microalgae Porphyridium purpureum

The present study attempts to cultivate Porphyridium purpureum under different scale-up conditions for further development and commercialization of microalgae-derived PUFAs such as ARA and EPA. Different temperatures (25, 30, and 35 °C) and light intensities (70, 165, and 280 μmol/m²s) were applied to the 50 L pilot-scale cultivation of P. purpureum in ASW. The cultivation under the light intensity of 280 μmol/m²s at 35 °C obtained biomass concentration up to 9.52 g/L, total fatty acid content to 56.82 mg/g, and ARA content to 22.29 mg/g. While the maximum EPA content of 7.00 mg/g was achieved under the light intensity of 280 μmol/m²s at 25 °C and the highest ratio of UFAs to TFAs of 74.66% was also obtained in this trial. Both biomass concentration and TFAs content were improved by increasing light intensity and temperature. Moreover, the ratio of ARA to EPA was enhanced by increasing cultivation temperature under the light intensity of 280 μmol/m²s. In contrast with flask culture, the conversion of linoleic acid (C18:2) to ARA was enhanced in scale-up culture, leading to more ARA content. Phosphate limitation enhanced the synthesis of lipid and LPUFAs. Moreover, the biomass concentration and biosynthesis of palmitic acid were preferred by sufficient C (NaHCO₃).