Triacylglycerol and phytyl ester synthesis in Synechocystis sp. PCC6803

Microalgae biofuels: illuminating the path to a sustainable future amidst challenges and opportunities

The development of microalgal biofuels is of significant importance in advancing the energy transition, alleviating food pressure, preserving the natural environment, and addressing climate change. Numerous countries and regions across the globe have conducted extensive research and strategic planning on microalgal bioenergy, investing significant funds and manpower into this field. However, the microalgae biofuel industry has faced a downturn due to the constraints of high costs. In the past decade, with the development of new strains, technologies, and equipment, the feasibility of large-scale production of microalgae biofuel should be re-evaluated. Here, we have gathered research results from the past decade regarding microalgae biofuel production, providing insights into the opportunities and challenges faced by this industry from the perspectives of microalgae selection, modification, and cultivation. In this review, we suggest that highly adaptable microalgae are the preferred choice for large-scale biofuel production, especially strains that can utilize high concentrations of inorganic carbon sources and possess stress resistance. The use of omics technologies and genetic editing has greatly enhanced lipid accumulation in microalgae. However, the associated risks have constrained the feasibility of large-scale outdoor cultivation. Therefore, the relatively controllable cultivation method of photobioreactors (PBRs) has made it the mainstream approach for microalgae biofuel production. Moreover, adjusting the performance and parameters of PBRs can also enhance lipid accumulation in microalgae. In the future, given the relentless escalation in demand for sustainable energy sources, microalgae biofuels should be deemed a pivotal constituent of national energy planning, particularly in the case of China. The advancement of synthetic biology helps reduce the risks associated with genetically modified (GM) microalgae and enhances the economic viability of their biofuel production.

Acylated plastoquinone is a novel neutral lipid accumulated in cyanobacteria

Although cyanobacteria do not possess bacterial triacylglycerol (TAG)-synthesizing enzymes, the accumulation of TAGs and/or lipid droplets has been repeatedly reported in a wide range of species. In most cases, the identification of TAG has been based on the detection of the spot showing the mobility similar to the TAG standard in thin-layer chromatography (TLC) of neutral lipids. In this study, we identified monoacyl plastoquinol (acyl PQH) as the predominant molecular species in the TAG-like spot from the unicellular Synechocystis sp. PCC 6803 (S.6803) as well as the filamentous Nostocales sp., Nostoc punctiforme PCC 73102, and Anabaena sp. PCC 7120. In S.6803, the accumulation level of acyl PQH but not TAG was affected by deletion or overexpression of slr2103, indicating that acyl PQH is the physiological product of Slr2103 having homology with the eukaryotic diacylglycerol acyltransferase-2 (DGAT2). Electron microscopy revealed that cyanobacterial strains used in this study do not accumulate lipid droplet structures such as those observed in oleaginous microorganisms. Instead, they accumulate polyhydroxybutyrate (PHB) granules and/or aggregates of alkane, free C16 and C18 saturated fatty acids, and low amounts of TAG in the cytoplasmic area, which can be detected by staining with a fluorescent dye specific to neutral lipids. Unlike these lipophilic materials, acyl PQH is exclusively localized in the membrane fraction. There must be DGAT2-like enzymatic activity esterifying de novo-synthesized C16 and C18 fatty acids to PQH₂ in the thylakoid membranes.

Plastoquinone Lipids: Their Synthesis via a Bifunctional Gene and Physiological Function in a Euryhaline Cyanobacterium, Synechococcus sp. PCC 7002

Eukaryotic photosynthetic organisms synthesize triacylglycerols, which are crucial physiologically as major carbon and energy storage compounds and commercially as food oils and raw materials for carbon-neutral biofuel production. TLC analysis has revealed triacylglycerols are present in several cyanobacteria. However, mass spectrometric analysis has shown that freshwater cyanobacterium, Synechocystis sp. PCC 6803, contains plastoquinone-B and acyl plastoquinol with triacylglycerol-like TLC mobility, concomitantly with the absence of triacylglycerol. Synechocystis contains slr2103, which is responsible for the bifunctional synthesis of plastoquinone-B and acyl plastoquinol and also for NaCl-stress acclimatizing cell growth. However, information is limited on the taxonomical distribution of these plastoquinone lipids, and their synthesis genes and physiological roles in cyanobacteria. In this study, a euryhaline cyanobacterium, Synechococcus sp. PCC 7002, shows the same plastoquinone lipids as those in Synechocystis, although the levels are much lower than in Synechocystis, triacylglycerol being absent. Furthermore, through an analysis of a disruptant to the homolog of slr2103 in Synechococcus, it is found that the slr2103 homolog in Synechococcus, similar to slr2103 in Synechocystis, contributes bifunctionally to the synthesis of plastoquinone-B and acyl plastoquinol; however, the extent of the contribution of the homolog gene to NaCl acclimatization is smaller than that of slr2103 in Synechocystis. These observations suggest strain- or ecoregion-dependent development of the physiological roles of plastoquinone lipids in cyanobacteria and show the necessity to re-evaluate previously identified cyanobacterial triacylglycerol through TLC analysis with mass spectrometric techniques.

Functions and interaction of plant lipid signalling under abiotic stresses

Lipids are the primary form of energy storage and a major component of plasma membranes, which form the interface between the cell and the extracellular environment. Several lipids - including phosphoinositide, phosphatidic acid, sphingolipids, lysophospholipids, oxylipins, and free fatty acids - also serve as substrates for the generation of signalling molecules. Abiotic stresses, such as drought and temperature stress, are known to affect plant growth. In addition, abiotic stresses can activate certain lipid-dependent signalling pathways that control the expression of stress-responsive genes and contribute to plant stress adaptation. Many studies have focused either on the enzymatic production and metabolism of lipids, or on the mechanisms of abiotic stress response. However, there is little information regarding the roles of plant lipids in plant responses to abiotic stress. In this review, we describe the metabolism of plant lipids and discuss their involvement in plant responses to abiotic stress. As such, this review provides crucial background for further research on the interactions between plant lipids and abiotic stress.

Acyl plastoquinol is a major cyanobacterial substance that co-migrates with triacylglycerol in thin-layer chromatography

Various studies have suggested the presence of triacylglycerol in cyanobacteria, but no convincing evidence exists. We purified a substance co-migrating with triacylglycerol in thin-layer chromatography and determined its structure using mass spectrometry, gas chromatography, and ¹H and ¹³C NMR. The major components were palmitoyl and stearoyl plastoquinols (acyl plastoquinol). Acyl plastoquinol has never been described before, although acyloxy derivative of plastoquione has been described as plastoquinone B. The level of acyl plastoquinol was 0.4% of the total lipids. We still do not have clear evidence for the presence of triacylglycerol. If present, the maximum triacylglycerol level must be at most 10% of acyl plastoquinol. The Synechocystis Slr2103 protein was suggested to synthesize triacylglycerol, but the product could be acyl plastoquinol. The possible roles of this novel compound in photosynthesis should be a new focus of research.

Deacylation of galactolipids decomposes photosystem II dimers to enhance degradation of damaged D1 protein

Photosystem II (PSII) contains many lipid molecules that are essential for the function and maintenance of PSII. Under strong light conditions, PSII complexes are dynamically modified during the repair process; however, the molecular mechanism of the dynamic changes in the PSII structure is still unclear. In the present study, we investigated the role of a lipase in the repair of PSII in Synechocystis sp. PCC 6803. We identified a protein encoded by the sll1969 gene, previously named lipase A (lipA), in the Synechocystis sp. PCC 6803 genome as a candidate for the lipase involved in PSII repair. Recombinant protein expressed in Escherichia coli cells hydrolyzed fatty acids at the sn-1 position of monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol as well as triacylglycerol esterified with stearic acids. PSII repair in a disrupted mutant of the lipA gene was suppressed by the slow degradation of damaged D1 protein under strong light. The level of the PSII dimer remained higher in lipA mutant cells than wild-type (WT) cells under strong light. LipA protein was associated with the PSII dimer in vivo, and recombinant LipA protein decomposed PSII dimers purified from WT cells to monomers by reducing MGDG content in the PSII complex. These results indicate that LipA reacts with PSII dimers, dissociates them into monomers by digesting MGDG, and enhances D1 degradation during PSII repair.

slr2103, a homolog of type-2 diacylglycerol acyltransferase genes, for plastoquinone-related neutral lipid synthesis and NaCl-stress acclimatization in a cyanobacterium, Synechocystis sp. PCC 6803

A cyanobacterium, Synechocystis sp. PCC 6803, contains a lipid with triacylglycerol-like TLC mobility but its identity and physiological roles remain unknown. Here, on ESI-positive LC-MS² analysis, it is shown that the triacylglycerol-like lipid (lipid X) is related to plastoquinone and can be grouped into two subclasses, X_a and X_b, the latter of which is esterified by 16:0 and 18:0. This study further shows that a Synechocystis homolog of type-2 diacylglycerol acyltransferase genes, slr2103, is essential for lipid X synthesis: lipid X disappears in a Synechocystis slr2103-disruptant whereas it appears in an slr2103-overexpressing transformant (OE) of Synechococcus elongatus PCC 7942 that intrinsically lacks lipid X. The slr2103 disruption causes Synechocystis cells to accumulate plastoquinone-C at an abnormally high level whereas slr2103 overexpression in Synechococcus causes the cells to almost completely lose it. It is thus deduced that slr2103 encodes a novel acyltransferase that esterifies 16:0 or 18:0 with plastoquinone-C for the synthesis of lipid X_b. Characterization of the slr2103-disruptant in Synechocystis shows that slr2103 contributes to sedimented-cell growth in a static culture, and to bloom-like structure formation and its expansion by promoting cell aggregation and floatation upon imposition of saline stress (0.3-0.6 M NaCl). These observations provide a basis for elucidation of the molecular mechanism of a novel cyanobacterial strategy to acclimatize to saline stress, and one for development of a system of seawater-utilization and economical harvesting of cyanobacterial cells with high-value added compounds, or blooming control of toxic cyanobacteria.

Regulation of Heat Stress in Physcomitrium (Physcomitrella) patens Provides Novel Insight into the Functions of Plant RNase H1s

RNase H1s are associated with growth and development in both plants and animals, while the roles of RNase H1s in bryophytes have been rarely reported. Our previous data found that PpRNH1A, a member of the RNase H1 family, could regulate the development of Physcomitrium (Physcomitrella) patens by regulating the auxin. In this study, we further investigated the biological functions of PpRNH1A and found PpRNH1A may participate in response to heat stress by affecting the numbers and the mobilization of lipid droplets and regulating the expression of heat-related genes. The expression level of PpRNH1A was induced by heat stress (HS), and we found that the PpRNH1A overexpression plants (A-OE) were more sensitive to HS. At the same time, A-OE plants have a higher number of lipid droplets but with less mobility in cells. Consistent with the HS sensitivity phenotype in A-OE plants, transcriptomic analysis results indicated that PpRNH1A is involved in the regulation of expression of heat-related genes such as DNAJ and DNAJC. Taken together, these results provide novel insight into the functions of RNase H1s.

Acyl-CoA:diacylglycerol acyltransferase: Properties, physiological roles, metabolic engineering and intentional control

Acyl-CoA:diacylglycerol acyltransferase (DGAT, EC 2.3.1.20) catalyzes the last reaction in the acyl-CoA-dependent biosynthesis of triacylglycerol (TAG). DGAT activity resides mainly in membrane-bound DGAT1 and DGAT2 in eukaryotes and bifunctional wax ester synthase-diacylglycerol acyltransferase (WSD) in bacteria, which are all membrane-bound proteins but exhibit no sequence homology to each other. Recent studies also identified other DGAT enzymes such as the soluble DGAT3 and diacylglycerol acetyltransferase (EaDAcT), as well as enzymes with DGAT activities including defective in cuticular ridges (DCR) and steryl and phytyl ester synthases (PESs). This review comprehensively discusses research advances on DGATs in prokaryotes and eukaryotes with a focus on their biochemical properties, physiological roles, and biotechnological and therapeutic applications. The review begins with a discussion of DGAT assay methods, followed by a systematic discussion of TAG biosynthesis and the properties and physiological role of DGATs. Thereafter, the review discusses the three-dimensional structure and insights into mechanism of action of human DGAT1, and the modeled DGAT1 from Brassica napus. The review then examines metabolic engineering strategies involving manipulation of DGAT, followed by a discussion of its therapeutic applications. DGAT in relation to improvement of livestock traits is also discussed along with DGATs in various other eukaryotic organisms.

Metabolic transformation of cyanobacteria for biofuel production

World-wide, an emerging demand is moving towards the biofuels to replace the fossil fuels. In alternative biofuel production strategies, cyanobacteria have unique characteristic of accumulating glycogen, lipid, and fuel molecules through natural mechanisms. Moreover, the cyanobacteria can be easily engineered to synthesis a plenty of fuel molecules from CO₂. To obtain the fuel molecule from cyanobacteria, various techniques were invented in which the metabolic engineering is found to be a prerequisite to develop an economically feasible process. The expression of indigenous or heterologous pathways plays an important role in developing successful production process. In addition, the engineering of photosynthetic apparatus, destruction of competitive pathways and improvement of tolerance were also proven to improve the product specific synthesis. Although various metabolic engineering approaches have been developed, there are certain obstacles when it comes to implementation for the production. In this review, the important biosynthetic pathways for biofuels, alteration of other genes to improve the actual pathway and possibilities of developing cyanobacterial fuel production have been elaborated.

Nordic cyanobacterial and algal lipids: Triacylglycerol accumulation, chemotaxonomy and bioindustrial potential

The ability to capture and convert sunlight, water and nutrients into useful compounds make photosynthetic microbes ideal candidates for the bio-industrial factories of the future. However, the suitability of isolates from temperate regions to grow under Nordic conditions is questionable. In this work, we explore the chemotaxonomy of Nordic strains of cyanobacteria and one green alga and evaluate their potential as raw materials for the production of lipid-based bio-industrial compounds. Thin-layer chromatography was used to identify the presence of triacylglycerol, which were detected in the majority of strains. Fatty acid methyl ester profiles were analysed to determine the suitability of strains for the production of biodiesel or the production of polyunsaturated fatty acids for the nutraceutical industry. The Nordic Synechococcus strains were unique in demonstrating fatty acid profiles comprised mostly C14:0, C16:0 and C16:1 and lacking polyunsaturated fatty acids. These properties translated to superior predicted biodiesel qualities, including cetane number, cold filter plugging point and oxidative stability compared to the other evaluated strains. Polyunsaturated fatty acids were detected at high levels (38-53%), with Calothrix sp. 336/3 being abundant in two essential fatty acids, linoleic and alpha-linolenic acid (21 and 17%, respectively). Gamma-linoleic acid was the predominant polyunsaturated fatty acid for the remaining strains (13-21%). In addition to assessing the potential of Nordic strains for bio-industrial production, this work also discusses issues such as taxonomy and predictive modelling, which can affect the identification of prospective high-performing strains.

Evolutionary and biochemical characterization of a Chromochloris zofingiensis MBOAT with wax synthase and diacylglycerol acyltransferase activity

Wax synthase (WS) catalyzes the last step in wax ester biosynthesis in green plants. Two unrelated sub-families of WS, including the bifunctional acyltransferase and plant-like WS have been reported, but the latter is largely uncharacterized in microalgae. Here, we functionally characterized a putative plant-like WS (CzWS1) from the emerging model green microalga Chromochloris zofingiensis. Our results showed that plant-like WS evolved under different selection constraints in plants and microalgae, with positive selection likely contributing to functional divergence. Unlike jojoba with high amounts of wax ester in seeds and a highly active WS enzyme, C. zofingiensis has no detectable wax ester but a high abundance of WS transcripts. Co-expression analysis showed that C. zofingiensis WS has different expression correlation with lipid biosynthetic genes from jojoba, and may have a divergent function. In vitro characterization indicated that CzWS1 had diacylglycerol acyltransferase activity along with WS activity, and overexpression of CzWS1 in yeast and Chlamydomonas reinhardtii affected triacylglycerol accumulation. Moreover, biochemical and bioinformatic analyses revealed the relevance of the C-terminal region of CzWS1 in enzyme function. Taken together, our results indicated a functional divergence of plant-like WS in plants and microalgae, and the importance of its C-terminal region in specialization of enzyme function.

Fatty acid isoprenoid alcohol ester synthesis in fruits of the African Oil Palm (Elaeis guineensis)

The African Oil Palm (Elaeis guineensis; family Arecaceae) represents the most important oil crop for food and feed production and for biotechnological applications. Two types of oil can be extracted from palm fruits, the mesocarp oil which is rich in palmitic acid and in carotenoids (provitamin A) and tocochromanols (vitamin E), and the kernel oil with high amounts of lauric and myristic acid. We identified fatty acid phytyl esters (FAPEs) in the mesocarp and kernel tissues of mature fruits, mostly esterified with oleic acid and very long chain fatty acids. In addition, fatty acid geranylgeranyl esters (FAGGEs) accumulated in mesocarp and kernels to even larger amounts. In contrast, FAPEs and FAGGEs amounts and fatty acid composition in leaves were very similar. Analysis of wild accessions of African Oil Palm from Cameroon revealed a considerable variation in the amounts and composition of FAPEs and FAGGEs in mesocarp and kernel tissues. Exogenous supplementation of phytol or geranylgeraniol to mesocarp slices resulted in the incorporation of these alcohols into FAPEs and FAGGEs, respectively, indicating that they are synthesized via enzymatic reactions. Three candidate genes of the esterase/lipase/thioesterase (ELT) family were identified in the Oil Palm genome. The genes are differentially expressed in mesocarp tissue with EgELT1 showing the highest expression. Geranylgeraniol from FAGGE might be recycled and used as a substrate for the synthesis of carotenoids and tocotrienols during fruit development. Thus, FAPEs and FAGGEs in the mesocarp and kernel of Oil Palm provide an additional metabolic source for fatty acids and phytol or geranylgeraniol, respectively.

Ties between Stress and Lipid Droplets Pre-date Seeds

Seeds were a key evolutionary innovation. These durable structures provide a concerted solution to two challenges on land: dispersal and stress. Lipid droplets (LDs) that act as nutrient storage reservoirs are one of the main cell-biological reasons for seed endurance. Although LDs are key structures in spermatophytes and are especially abundant in seeds, they are found across plants and algae, and increase during stress. Further, the proteins that underpin their form and function often have deep homologs. We propose an evolutionary scenario in which (i) the generation of LDs arose as a mechanism to mediate general drought and desiccation resilience, and (ii) the required protein framework was co-opted by spermatophytes for a seed-specific program.

Identification and culturing of cyanobacteria isolated from freshwater bodies of Sri Lanka for biodiesel production

The present study was carried out to investigate cyanobacteria as a potential source for biodiesel production isolated from fresh water bodies of Sri Lanka. Semi mass culturing and mass culturing were carried out to obtain biomass for extracting total lipids. Fatty acid methyl ester (FAME) or biodiesel was produced from extracted lipid by trans-esterification reaction. FAME component was identified using gas chromatography (GC). Atotal of 74 uni-algal cultures were obtained from Biofuel and Bioenergy laboratory of the National Institute of Fundamental Studies (NIFS), Kandy, Sri Lanka. The total lipid content was recorded highest in Oscillatoria sp. (31.9 ± 2.01% of dry biomass) followed by Synechococcus sp. (30.6 ± 2.87%), Croococcidiopsis sp. (22.7 ± 1.36%), Leptolyngbya sp. (21.15 ± 1.99%), Limnothrixsp. (20.73 ± 3.26%), Calothrix sp. (18.15 ± 4.11%) and Nostoc sp. (15.43 ± 3.89%), Cephalothrixsp. (13.95 ± 4.27%), Cephalothrix Komarekiana (13.8 ± 3.56%) and Westiellopsisprolifica (12.80 ± 1.97%). FAME analysis showed cyanobacteria contain Methyl palmitoleate, Linolelaidic acid methyl ester, Cis-8,11,14-eicosatrienoic acid methyl ester, Cis-10-heptadecanoic acid methyl ester, Methyl myristate, Methyl pentadecanoate, Methyl octanoate, Methyl decanoate, Methyl laurate, Methyl tridecanoate, Methyl palmitoleate, Methyl pentadeconoate, Methyl heptadeconoate, Linolaidic acid methyl ester, Methyl erucate, Methyl myristate, Myristoloeic acid, Methyl palmitate, Cis-9-oleic acid methyl ester, Methyl arachidate and Cis-8,11,14-ecosatrieconoic acid methyl ester. The present study revealed that cyanobacteria isolated from Sri Lanka are potential source for biodiesel industry because of their high fatty acid content. Further studies are required to optimize the mass culture conditions to increase thelipid content from cyanobacterial biomass along with the research in the value addition to the remaining biomass.

Quantitative and Qualitative Analyses of Triacylglycerol Production in the Wild-Type Cyanobacterium Synechocystis sp. PCC 6803 and the Strain Expressing AtfA from Acinetobacter baylyi ADP1

Although cyanobacteria do not possess wax ester synthase/acyl-CoA:diacylglycerol acyltransferase (WS/DGAT), the bacterial enzyme for triacylglycerol (TAG) production, there have been several studies reporting the accumulation of TAG-like compounds in cyanobacteria. In this study, we aimed to evaluate TAG productivity of the ΔrecJ::atfA strain of Synechocystis sp. PCC 6803 generated by inserting atfA encoding WS/DGAT from Acinetobacter baylyi ADP1 into recJ (sll1354), together with the wild type (WT) and the gene-disrupted strain of slr2103 having homology with eukaryotic DGAT2 gene family (Δ2103). Thin-layer chromatography (TLC) of neutral lipids or isolation of the neutral lipid-enriched fraction followed by gas chromatography or liquid chromatography–tandem mass spectrometry was employed for analyses. The ΔrecJ::atfA strain accumulated 0.508 nmol ml−1OD730−1 of TAG after a week of incubation at 100 μmol photons m−2 s−1. The saturated fatty acids C16:0 and C18:0 accounted for about 50% and 20% of the TAG fatty acids, respectively, suggesting that de novo-synthesized fatty acids were preferentially incorporated into TAG molecules. When the neutral lipid profile of the lipid extracts was examined by TLC, a spot located in a slightly lower position compared with the TAG standard was detected in WT but not in the Δ2103 strain. TAG accumulation levels of both strains was only 0.01–0.03 nmol ml−1OD730−1, but the fatty acid composition was substantially different from that of the background. These results suggest that trace amounts of TAG can be produced in Synechocystis cells by enzymes other than Slr2103, and major constituents of the TAG-like spot are unknown lipid species produced by Slr2103.