Identification and functional characterization of genes encoding omega-3 polyunsaturated fatty acid biosynthetic activities from unicellular microalgae

Towards Lipid from Microalgae: Products, Biosynthesis, and Genetic Engineering

Microalgae can convert carbon dioxide into organic matter through photosynthesis. Thus, they are considered as an environment-friendly and efficient cell chassis for biologically active metabolites. Microalgal lipids are a class of organic compounds that can be used as raw materials for food, feed, cosmetics, healthcare products, bioenergy, etc., with tremendous potential for commercialization. In this review, we summarized the commercial lipid products from eukaryotic microalgae, and updated the mechanisms of lipid synthesis in microalgae. Moreover, we reviewed the enhancement of lipids, triglycerides, polyunsaturated fatty acids, pigments, and terpenes in microalgae via environmental induction and/or metabolic engineering in the past five years. Collectively, we provided a comprehensive overview of the products, biosynthesis, induced strategies and genetic engineering in microalgal lipids. Meanwhile, the outlook has been presented for the development of microalgal lipids industries, emphasizing the significance of the accurate analysis of lipid bioactivity, as well as the high-throughput screening of microalgae with specific lipids.

The Clinical Promise of Microalgae in Rheumatoid Arthritis: From Natural Compounds to Recombinant Therapeutics

Rheumatoid arthritis (RA) is an invalidating chronic autoimmune disorder characterized by joint inflammation and progressive bone damage. Dietary intervention is an important component in the treatment of RA to mitigate oxidative stress, a major pathogenic driver of the disease. Alongside traditional sources of antioxidants, microalgae-a diverse group of photosynthetic prokaryotes and eukaryotes-are emerging as anti-inflammatory and immunomodulatory food supplements. Several species accumulate therapeutic metabolites-mainly lipids and pigments-which interfere in the pro-inflammatory pathways involved in RA and other chronic inflammatory conditions. The advancement of the clinical uses of microalgae requires the continuous exploration of phytoplankton biodiversity and chemodiversity, followed by the domestication of wild strains into reliable producers of said metabolites. In addition, the tractability of microalgal genomes offers unprecedented possibilities to establish photosynthetic microbes as light-driven biofactories of heterologous immunotherapeutics. Here, we review the evidence-based anti-inflammatory mechanisms of microalgal metabolites and provide a detailed coverage of the genetic engineering strategies to enhance the yields of endogenous compounds and to develop innovative bioproducts.

Using field evaluation and systematic iteration to rationalize the accumulation of omega-3 long-chain polyunsaturated fatty acids in transgenic Camelina sativa

The Brassicaceae Camelina sativa (gold of pleasure) is now an established niche crop and being used as a transgenic host for a range of novel seed traits. Most notable of these is the accumulation of omega-3 long-chain polyunsaturates such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), fatty acids normally only found in marine organisms. As part of continued efforts to optimize the accumulation of these non-native fatty acids via seed-specific expression of algal genes, a new series of iterative constructs was built and introduced into Camelina. Seed fatty acid composition was determined, and the presence of EPA and DHA was confirmed. To provide an additional level of evaluation, full environmental release was carried out on selected events, providing a real-world gauntlet against which to assess the performance of these novel lines. Composition of the seed oil triacylglycerol was determined by mass spectrometry, allowing for conclusions as to the contribution of different activities to the final accumulation of EPA and DHA. Since these data were derived from field-grown material, they also represent a robust demonstration of the stability of the omega-3 LC-PUFA trait in Camelina. We propose that field trialling should be routinely incorporated in the plant synthetic biology 'design-build-test-learn' cycle.

Media engineering in marine diatom Phaeodactylum tricornutum employing cost-effective substrates for sustainable production of high-value renewables

Phaeodactylum tricornutum is a marine diatom, rich in omega-3 polyunsaturated fatty acids especially eicosapentaenoic acid (EPA) and brown pigment, that is, fucoxanthin. These high-value renewables (HVRs) have a high commercial and nutritional relevance. In this study, our focus was to enhance the productivities of such renewables by employing media engineering strategy via., photoautotrophic (P1, P2, P3) and mixotrophic (M1, M2, M3, M4) modes of cultivation with varying substrate combinations of carbon (glycerol: 0.1 m) and nitrogen (urea: 441 mm and/or sodium nitrate: 882 mm). Our results demonstrate that mixotrophic [M4] condition supplemented with glycerol (0.1 m) and urea (441 mm) feed enhanced productivities (mg L^-1  day^-1 ) as follows: biomass (770.0), total proteins (36.0), total lipids (22.0), total carbohydrates (23.0) with fatty acid methyl esters (9.6), EPA (2.7), and fucoxanthin (1.1), respectively. The overall yield of EPA represents 28% of total fatty acids in the mixotrophic [M4] condition. In conclusion, our improved strategy of feeding urea to a glycerol-supplemented medium defines a new efficient biomass valorization paradigm with cost-effective substrates for the production of HVRs in oleaginous diatoms P. tricornutum.

Synthesis of C20-38 Fatty Acids in Plant Tissues

Very-long-chain fatty acids (VLCFA) are involved in a number of important plant physiological functions. Disorders in the expression of genes involved in the synthesis of VLCFA lead to a number of phenotypic consequences, ranging from growth retardation to the death of embryos. The elongation of VLCFA in the endoplasmic reticulum (ER) is carried out by multiple elongase complexes with different substrate specificities and adapted to the synthesis of a number of products required for a number of metabolic pathways. The information about the enzymes involved in the synthesis of VLCFA with more than 26 atoms of Carbon is rather poor. Recently, genes encoding enzymes involved in the synthesis of both regular-length fatty acids and VLCFA have been discovered and investigated. Polyunsaturated VLCFA in plants are formed mainly by 20:1 elongation into new monounsaturated acids, which are then imported into chloroplasts, where they are further desaturated. The formation of saturated VLCFA and their further transformation into a number of aliphatic compounds included in cuticular waxes and suberin require the coordinated activity of a large number of different enzymes.

A 13CO2 Enrichment Experiment to Study the Synthesis Pathways of Polyunsaturated Fatty Acids of the Haptophyte Tisochrysis lutea

The production of polyunsaturated fatty acids (PUFA) in Tisochrysis lutea was studied using the gradual incorporation of a ¹³C-enriched isotopic marker, ¹³CO₂, for 24 h during the exponential growth of the algae. The ¹³C enrichment of eleven fatty acids was followed to understand the synthetic pathways the most likely to form the essential polyunsaturated fatty acids 20:5n-3 (EPA) and 22:6n-3 (DHA) in T. lutea. The fatty acids 16:0, 18:1n-9 + 18:3n-3, 18:2n-6, and 22:5n-6 were the most enriched in ¹³C. On the contrary, 18:4n-3 and 18:5n-3 were the least enriched in ¹³C after long chain polyunsaturated fatty acids such as 20:5n-3 or 22:5n-3. The algae appeared to use different routes in parallel to form its polyunsaturated fatty acids. The use of the PKS pathway was hypothesized for polyunsaturated fatty acids with n-6 configuration (such as 22:5n-6) but might also exist for n-3 PUFA (especially 20:5n-3). With regard to the conventional n-3 PUFA pathway, Δ6 desaturation of 18:3n-3 appeared to be the most limiting step for T. lutea, “stopping” at the synthesis of 18:4n-3 and 18:5n-3. These two fatty acids were hypothesized to not undergo any further reaction of elongation and desaturation after being formed and were therefore considered “end-products”. To circumvent this limiting synthetic route, Tisochrysis lutea seemed to have developed an alternative route via Δ8 desaturation to produce longer chain fatty acids such as 20:5n-3 and 22:5n-3. 22:6n-3 presented a lower enrichment and appeared to be produced by a combination of different pathways: the conventional n-3 PUFA pathway by desaturation of 22:5n-3, the alternative route of ω-3 desaturase using 22:5n-6 as precursor, and possibly the PKS pathway. In this study, PKS synthesis looked particularly effective for producing long chain polyunsaturated fatty acids. The rate of enrichment of these compounds hypothetically synthesized by PKS is remarkably fast, making undetectable the ¹³C incorporation into their precursors. Finally, we identified a protein cluster gathering PKS sequences of proteins that are hypothesized allowing n-3 PUFA synthesis.

Diatoms and Plants Acyl-CoA:lysophosphatidylcholine Acyltransferases (LPCATs) Exhibit Diverse Substrate Specificity and Biochemical Properties

The search of the Phaeodactylum tricornutum genome database revealed the existence of six genes potentially encoding lysophospholipid acyltransferases. One of these genes, Phatr3_J20460, after introduction to yeast ale1 mutant disrupted in the LPCAT gene, produced a very active acyl-CoA:lysophosphatidylcholine (LPCAT) enzyme. Using in vitro assays applying different radioactive and non-radioactive substrates and microsomal fractions from such yeast, we have characterized the biochemical properties and substrate specificities of this PtLPCAT1. We have found that the substrate specificity of this enzyme indicates that it can completely supply phosphatidylcholine (PC) with all fatty acids connected with a biosynthetic pathway of very long-chain polyunsaturated fatty acids (VLC-PUFAs) used further for the desaturation process. Additionally, we have shown that biochemical properties of the PtLPCAT1 in comparison to plant LPCATs are in some cases similar (such as the dependency of its activity on pH value), differ moderately (such as in response to temperature changes), or express completely different properties (such as in reaction to calcium and magnesium ions or toward some acyl-CoA with 20C polyunsaturated fatty acids). Moreover, the obtained results suggest that cloned “Phatr3_J20460” gene can be useful in oilseeds plant engineering toward efficient production of VLC-PUFA as LPCAT it encodes can (contrary to plant LPCATs) introduce 20:4-CoA (n-3) to PC for further desaturation to 20:5 (EPA, eicosapentaenoic acid).

High level accumulation of EPA and DHA in field-grown transgenic Camelina - a multi-territory evaluation of TAG accumulation and heterogeneity

The transgene-directed accumulation of non-native omega-3 long chain polyunsaturated fatty acids in the seed oil of Camelina sativa (Camelina) was evaluated in the field, in distinct geographical and regulatory locations. A construct, DHA2015.1, containing an optimal combination of biosynthetic genes, was selected for experimental field release in the UK, USA and Canada, and the accumulation of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) determined. The occurrence of these fatty acids in different triacylglycerol species was monitored and found to follow a broad trend irrespective of the agricultural environment. This is a clear demonstration of the stability and robust nature of the transgenic trait for omega-3 long chain polyunsaturated fatty acids in Camelina. Examination of non-seed tissues for the unintended accumulation of EPA and DHA failed to identify their presence in leaf, stem, flower, anther or capsule shell material, confirming the seed-specific accumulation of these novel fatty acids. Collectively, these data confirm the promise of GM plant-based sources of so-called omega-3 fish oils as a sustainable replacement for oceanically derived oils.

Δ6 fatty acid desaturases in polyunsaturated fatty acid biosynthesis: insights into the evolution, function with substrate specificities and biotechnological use

Δ6 fatty acid desaturases (FADS6) have different substrate specificities that impact the ratio of omega-6/omega-3 polyunsaturated fatty acids, which are involved in regulating multiple signalling pathways associated with various diseases. For decades, FADS6 with different substrate specificities have been characterized and the functions of these crucial enzymes have been investigated, while it remains enigmatic that the substrate specificities of FADS6 from various species have a huge difference. This review summarizes the substrate specificities of FADS6 in different species and reveals the underlying relationship. Further evaluation of biochemical properties has revealed that the FADS6 prefer linoleic acid that is more hydrophilic and stable. Domain-swapping and site-directed mutagenesis have been employed to delineate the regions and sites that affect the substrate specificities of FADS6. These analyses improve our understanding of the functions of FADS6 and offer information for the discovery of novel biological resources. KEY POINTS: • Outline of the excavation and identification of Δ6 fatty acid desaturases. • Overview of methods used to determine the pivotal resides of desaturases. • Application of substrate properties to generate specific fatty acids.

Effect of inorganic carbon limitation on the conversion of organic carbon to total fatty acids by Monodus subterraneus

Traditional autotrophic microalgae exhibit low rates of organic carbon assimilation and conversion to useful compounds when switching to mixotrophic or heterotrophic growth. The goal of this study was to investigate the effect of inorganic carbon limitation on the efficiency of organic carbon (glycerol) assimilation and conversion to total fatty acids (TFAs) or the long-chain polyunsaturated fatty acid eicosapentaenoic acid (EPA). An oleaginous Monodus subterraneus was selected and six cultivation conditions were set, including Autotrophy-no aeration, Autotrophy-aeration, Mixotrophy-no aeration, Mixotrophy-no aeration & no Na₂CO₃, Mixotrophy-aeration, and Heterotrophy. The results showed M. subterraneus could utilize glycerol and grow under mixotrophic condition, while it was not occurred under heterotrophy. Superiority of mixotrophy to autotrophy on biomass productivity was more obvious under inorganic carbon limitation (no aeration or no Na₂CO₃) than inorganic carbon supply (aeration and existing Na₂CO₃ in the medium). CO₂ limitation (no aeration) decreased content (of dry weight) and production (in medium) of TFAs, which was not evident in mixotrophy. CO₂ limitation and inorganic carbon substrate stress largely improved the COD yield of TFAs and EPA under mixotrophic condition. TFA yield (%COD) under Mixotrophy-no aeration & no Na₂CO₃ was maximum (22.82%), and was almost two-fold higher than that under Mixotrophy-no aeration and nearly three-fold higher than that with Mixotrophy-aeration. EPA yield (% COD) under mixotrophy-no aeration & no Na₂CO₃ was maximum (6.58%). These results suggested that inorganic carbon limitation is a potentially useful method to enhance conversion of organic carbon to TFAs. Furthermore, the results suggest an application to obtain high value compounds (TFAs or EPA) combined with a high assimilation rate of waste glycerol from biodiesel and epichlorohydrin production by microalgae.

Plastidic Δ6 Fatty-Acid Desaturases with Distinctive Substrate Specificity Regulate the Pool of C18-PUFAs in the Ancestral Picoalga Ostreococcus tauri

Eukaryotic Δ6-desaturases are microsomal enzymes that balance the synthesis of ω-3 and ω-6 C18-polyunsaturated fatty acids (C18-PUFAs) according to their specificity. In several microalgae, including Ostreococcus tauri, plastidic C18-PUFAs are strictly regulated by environmental cues suggesting an autonomous control of Δ6-desaturation of plastidic PUFAs. Here, we identified two putative front-end Δ6/Δ8-desaturases from O tauri that, together with putative homologs, cluster apart from other characterized Δ6-desaturases. Both were plastid-located and unambiguously displayed a Δ6-desaturation activity when overexpressed in the heterologous hosts Nicotiana benthamiana and Synechocystis sp. PCC6803, as in the native host. Detailed lipid analyses of overexpressing lines unveiled distinctive ω-class specificities, and most interestingly pointed to the importance of the lipid head-group and the nonsubstrate acyl-chain for the desaturase efficiency. One desaturase displayed a broad specificity for plastidic lipids and a preference for ω-3 substrates, while the other was more selective for ω-6 substrates and for lipid classes including phosphatidylglycerol as well as the peculiar 16:4-galactolipid species occurring in the native host. Overexpression of both Δ6-desaturases in O tauri prevented the regulation of C18-PUFA under phosphate deprivation and triggered glycerolipid fatty-acid remodeling, without causing any obvious alteration in growth or photosynthesis. Tracking fatty-acid modifications in eukaryotic hosts further suggested the export of plastidic lipids to extraplastidic compartments.

Study of Synthesis Pathways of the Essential Polyunsaturated Fatty Acid 20:5n-3 in the Diatom Chaetoceros Muelleri Using 13C-Isotope Labeling

The present study sought to characterize the synthesis pathways producing the essential polyunsaturated fatty acid (PUFA) 20:5n-3 (EPA). For this, the incorporation of ¹³C was experimentally monitored into 10 fatty acids (FA) during the growth of the diatom Chaetoceros muelleri for 24 h. Chaetoceros muelleri preferentially and quickly incorporated ¹³C into C₁₈ PUFAs such as 18:2n-6 and 18:3n-6 as well as 16:0 and 16:1n-7, which were thus highly ¹³C-enriched. During the experiment, 20:5n-3 and 16:3n-4 were among the least-enriched fatty acids. The calculation of the enrichment percentage ratio of a fatty acid B over its suspected precursor A allowed us to suggest that the diatom produced 20:5n-3 (EPA) by a combination between the n-3 (via 18:4n-3) and n-6 (via 18:3n-6 and 20:4n-6) synthesis pathways as well as the alternative ω-3 desaturase pathway (via 20:4n-6). In addition, as FA from polar lipids were generally more enriched in ¹³C than FA from neutral lipids, particularly for 18:1n-9, 18:2n-6 and 18:3n-6, the existence of acyl-editing mechanisms and connectivity between polar and neutral lipid fatty acid pools were also hypothesized. Because 16:3n-4 and 20:5n-3 presented the same concentration and enrichment dynamics, a structural and metabolic link was proposed between these two PUFAs in C. muelleri.

Protective Action of Ostreococcus tauri and Phaeodactylum tricornutum Extracts towards Benzo[a]Pyrene-Induced Cytotoxicity in Endothelial Cells

Marine microalgae are known to be a source of bioactive molecules of interest to human health, such as n-3 polyunsaturated fatty acids (n-3 PUFAs) and carotenoids. The fact that some of these natural compounds are known to exhibit anti-inflammatory, antioxidant, anti-proliferative, and apoptosis-inducing effects, demonstrates their potential use in preventing cancers and cardiovascular diseases (CVDs). Benzo[a]pyrene (B[a]P), a polycyclic aromatic hydrocarbon (PAH), is an ubiquitous environmental pollutant known to contribute to the development or aggravation of human diseases, such as cancer, CVDs, and immune dysfunction. Most of these deleterious effects are related to the activation of the polycyclic aromatic hydrocarbon receptor (AhR). In this context, two ethanolic microalgal extracts with concentrations of 0.1 to 5 µg/mL are tested, Ostreoccoccus tauri (OT) and Phaeodactylum tricornutum (PT), in order to evaluate and compare their potential effects towards B[a]P-induced toxicity in endothelial HMEC-1 cells. Our results indicate that the OT extract can influence the toxicity of B[a]P. Indeed, apoptosis and the production of extracellular vesicles were decreased, likely through the reduction of the expression of CYP1A1, a B[a]P bioactivation enzyme. Furthermore, the B[a]P-induced expression of the inflammatory cytokines IL-8 and IL1-β was reduced. The PT extract only inhibited the expression of the B[a]P-induced cytokine IL-8 expression. The OT extract therefore seems to be a good candidate for counteracting the B[a]P toxicity.

Marine unsaturated fatty acids: structures, bioactivities, biosynthesis and benefits

Unsaturated fatty acids (UFAs) are an important category of monounsaturated and polyunsaturated fatty acids with nutritional properties. These secondary metabolites have been obtained from multitudinous natural resources, including marine organisms. Because of the increasing numerous biological importance of these marine derived molecules, this review covers 147 marine originated UFAs reported from 1978 to 2018. The review will focus on the structural characterizations, biological properties, proposed biosynthetic processes, and healthy benefits mediated by gut microbiota of these marine naturally originated UFAs.

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.

Cnidarian peptide neurotoxins: a new source of various ion channel modulators or blockers against central nervous systems disease

Cnidaria provide the largest source of bioactive peptides for new drug development. The venoms contain enzymes, potent pore-forming toxins and neurotoxins. The neurotoxins can immobilize predators rapidly when discharged via modifying sodium-channel-gating or blocking the potassium channel during the repolarization stage. Most cnidarian neurotoxins remain conserved under the strong influence of negative selection. Neuroactive peptides targeting the central nervous system through affinity with ion channels could provide insight leading to drug treatment of neurological diseases, which arise from ion channel dysfunctions. Although marine resources offer thousands of possible peptides, only one peptide derived from Cnidaria: ShK-186, also named dalazatide, has reached the pharmaceutical market. This review focuses on neuroprotective agents derived from cnidarian neurotoxic peptides.

Identification and characterization of candidates involved in production of OMEGAs in microalgae: a gene mining and phylogenomic approach

Optimizing the production of the high-value renewables such as OMEGAs through pathway engineering requires an in-depth understanding of the structure-function relationship of genes involved in the OMEGA biosynthetic pathways. In this preliminary study, our rationale is to identify and characterize the ∼221 putative genes involved in production of OMEGAs using bioinformatic analysis from the Streptophyte (plants), Chlorophyte (green algae), Rhodophyta (red algae), and Bacillariophyta (diatoms) lineages based on their phylogenomic profiling, conserved motif/domain organization and physico-chemical properties. The MEME suite predicted 12 distinct protein domains, which are conserved among these putative genes. The phylogenomic analysis of the putative candidate genes [such as FAD2 (delta-12 desaturase); ECR (enoyl-CoA reductase); FAD2 (delta-12 desaturase); ACOT (acyl CoA thioesterase); ECH (enoyl-CoA hydratase); and ACAT (acetyl-CoA acyltransferase)] with similar domains and motif patterns were remarkably well conserved. Furthermore, the subcellular network prediction of OMEGA biosynthetic pathway genes revealed a unique interaction between the light-dependent chlorophyll biosynthesis and glycerol-3-phosphate dehydrogenase, which predicts a major cross-talk between the key essential pathways. Such bioinformatic analysis will provide insights in finding the key regulatory genes to optimize the productivity of OMEGAs in microalgal cell factories.

Effects of nitrogen and phosphorous stress on the formation of high value LC-PUFAs in Porphyridium cruentum

This study systematically examined the effect of nitrogen and phosphorous stress on the formation of linoleic acid (LA), arachidonic acid (ARA), and eicosapentaenoic acid (EPA) in Porphyridium cruentum gy-h56. P. cruentum was cultivated in six different media conferring different conditions of nitrogen (N) sufficiency/deprivation and phosphorous (P) sufficiency/limitation/deprivation. Over a 16-day cultivation process, the dry-weight content, proportion of total fatty acids (TFAs), and the concentration in the medium of linoleic acid (LA) were greatly improved by a maximum of 2.5-, 1.6-, and 1.1-fold, respectively, under conditions of N or P deprivation compared with N and P sufficiency. In contrast, levels of EPA or ARA were not enhanced under N or P stress conditions. Additionally, the results showed that N deprivation weakened the impact of P deficiency on the content and proportions of LA and EPA, while P deprivation enhanced the impact of N starvation on the content and proportions of LA and EPA. The conditions of N sufficiency and P deprivation (N+P-) were the optimal conditions for the production of LA, while the optimal conditions for EPA, ARA, and TFAs production were N sufficiency and P limitation (N+P-lim). This study suggests the potential application of combining N removal from saline wastewater with the production of LA, ARA, EPA, and biodiesel.

Effect of cultivation mode on the production of docosahexaenoic acid by Tisochrysis lutea

In this study, Tisochrysis lutea was cultivated in mixotrophic and heterotrophic cultures with glycerol as a carbon source and with glucose and acetate for comparison; autotrophic cultivation was the control group without a carbon source. It was found that T. lutea used glycerol and did not use glucose and acetate under mixotrophy. Mixotrophy slightly elevated the docosahexaenoic acid (DHA) and total fatty acids (TFA) content in the dry-weight and enhanced the DHA and TFA production in medium (41.3 and 31.9% respectively) at the end of a 16-day cultivation, while heterotrophy reduced the DHA content and TFA production. Under the mixotrophy, the glycerol contribution to the DHA production (16.19 mg/L) and the TFA production (97.8 mg/L) was not very high and the DHA yield [2.63% chemical oxygen demand (COD)] and TFA yield (13.1% COD) were also very low. Furthermore, T. lutea using glycerol had a period of adaptation, indicating that T. lutea was not an ideal microalga for organic carbon utilization.

Modulation of lipid biosynthesis by stress in diatoms

Diatoms are responsible for up to 40% of the carbon fixation in our oceans. The fixed carbon is moved through carbon metabolism towards the synthesis of organic molecules that are consumed through interlocking foodwebs, and this process is strongly impacted by the abiotic environment. However, it has become evident that diatoms can be used as 'platform' organisms for the production of high valuable bio-products such as lipids, pigments and carbohydrates where stress conditions can be used to direct carbon metabolism towards the commercial production of these compounds. In the first section of this review, some aspects of carbon metabolism in diatoms and how it is impacted by environmental factors are briefly described. The second section is focused on the biosynthesis of lipids and in particular omega-3 long-chain polyunsaturated fatty acids and how low temperature stress impacts on the production of these compounds. In a third section, we review the recent advances in bioengineering for lipid production. Finally, we discuss new perspectives for designing strains for the sustainable production of high-value lipids.This article is part of the themed issue 'The peculiar carbon metabolism in diatoms'.

Glycerolipid Characterization and Nutrient Deprivation-Associated Changes in the Green Picoalga Ostreococcus tauri

The picoalga Ostreococcus tauri is a minimal photosynthetic eukaryote that has been used as a model system. O. tauri is known to efficiently produce docosahexaenoic acid (DHA). We provide a comprehensive study of the glycerolipidome of O. tauri and validate this species as model for related picoeukaryotes. O. tauri lipids displayed unique features that combined traits from the green and the chromalveolate lineages. The betaine lipid diacylglyceryl-hydroxymethyl-trimethyl-β-alanine and phosphatidyldimethylpropanethiol, both hallmarks of chromalveolates, were identified as presumed extraplastidial lipids. DHA was confined to these lipids, while plastidial lipids of prokaryotic type were characterized by the overwhelming presence of ω-3 C18 polyunsaturated fatty acids (FAs), 18:5 being restricted to galactolipids. C16:4, an FA typical of green microalgae galactolipids, also was a major component of O. tauri extraplastidial lipids, while the 16:4-coenzyme A (CoA) species was not detected. Triacylglycerols (TAGs) displayed the complete panel of FAs, and many species exhibited combinations of FAs diagnostic for plastidial and extraplastidial lipids. Importantly, under nutrient deprivation, 16:4 and ω-3 C18 polyunsaturated FAs accumulated into de novo synthesized TAGs while DHA-TAG species remained rather stable, indicating an increased contribution of FAs of plastidial origin to TAG synthesis. Nutrient deprivation further severely down-regulated the conversion of 18:3 to 18:4, resulting in obvious inversion of the 18:3/18:4 ratio in plastidial lipids, TAGs, as well as acyl-CoAs. The fine-tuned and dynamic regulation of the 18:3/18:4 ratio suggested an important physiological role of these FAs in photosynthetic membranes. Acyl position in structural and storage lipids together with acyl-CoA analysis further help to determine mechanisms possibly involved in glycerolipid synthesis.

Metabolic profiling identifies trehalose as an abundant and diurnally fluctuating metabolite in the microalga Ostreococcus tauri

INTRODUCTION

The picoeukaryotic alga Ostreococcus tauri (Chlorophyta) belongs to the widespread group of marine prasinophytes. Despite its ecological importance, little is known about the metabolism of this alga.

OBJECTIVES

In this work, changes in the metabolome were quantified when O. tauri was grown under alternating cycles of 12 h light and 12 h darkness.

METHODS

Algal metabolism was analyzed by gas chromatography-mass spectrometry. Using fluorescence-activated cell sorting, the bacteria associated with O. tauri were depleted to below 0.1% of total cells at the time of metabolic profiling.

RESULTS

Of 111 metabolites quantified over light-dark cycles, 20 (18%) showed clear diurnal variations. The strongest fluctuations were found for trehalose. With an intracellular concentration of 1.6 mM in the dark, this disaccharide was six times more abundant at night than during the day. This fluctuation pattern of trehalose may be a consequence of starch degradation or of the synchronized cell cycle. On the other hand, maltose (and also sucrose) was below the detection limit (~10 μM). Accumulation of glycine in the light is in agreement with the presence of a classical glycolate pathway of photorespiration. We also provide evidence for the presence of fatty acid methyl and ethyl esters in O. tauri.

CONCLUSIONS

This study shows how the metabolism of O. tauri adapts to day and night and gives new insights into the configuration of the carbon metabolism. In addition, several less common metabolites were identified.

Activity, Expression, and Substrate Preference of the Δ(6)-Desaturase in Slow- or Fast-Growing Rabbit Genotypes

In the present paper liver fatty acid Δ(6) desaturation (fads2) activity was analyzed in two rabbit strains with slow- (S, 27.5 g/day) or fast-growing (F, 48.5 g/day) rate. The fatty acid profile of the liver showed a different PUFA profile in the two strains with a lower n-6/n-3 ratio in the S rabbits. The expression of fads2 was 2-fold higher in S than in F rabbits, whereas enzyme activity was higher in F and more oriented toward the desaturation of linoleic acid (90%). In contrast, S showed a higher preference for linolenic acid (38.9 vs 10%). This study identified a single difference in the fads2 amino acid sequence between these two strains. Such a difference consists in the substitution of Gly104 to Ser104 in the sequence of F fads2. These results indicate for the first time that genetic selection for performance may affect the preference for PUFA toward desaturation of linoleic/linolenic acid.

Microalgae as a Source for VLC-PUFA Production

Microalgae present a huge and still insufficiently tapped resource of very long-chain omega-3 and omega-6 polyunsaturated fatty acids (VLC-PUFA) for human nutrition and medicinal applications. This chapter describes the diversity of unicellular eukaryotic microalgae in respect to VLC-PUFA biosynthesis. Then, we outline the major biosynthetic pathways mediating the formation of VLC-PUFA by sequential desaturation and elongation of C18-PUFA acyl groups. We address the aspects of spatial localization of those pathways and elaborate on the role for VLC-PUFA in microalgal cells. Recent progress in microalgal genetic transformation and molecular engineering has opened the way to increased production efficiencies for VLC-PUFA. The perspectives of photobiotechnology and metabolic engineering of microalgae for altered or enhanced VLC-PUFA production are also discussed.

Microalgal lipids biochemistry and biotechnological perspectives

In the last few years, there has been an intense interest in using microalgal lipids in food, chemical and pharmaceutical industries and cosmetology, while a noteworthy research has been performed focusing on all aspects of microalgal lipid production. This includes basic research on the pathways of solar energy conversion and on lipid biosynthesis and catabolism, and applied research dealing with the various biological and technical bottlenecks of the lipid production process. In here, we review the current knowledge in microalgal lipids with respect to their metabolism and various biotechnological applications, and we discuss potential future perspectives. The committing step in fatty acid biosynthesis is the carboxylation of acetyl-CoA to form malonyl-CoA that is then introduced in the fatty acid synthesis cycle leading to the formation of palmitic and stearic acids. Oleic acid may also be synthesized after stearic acid desaturation while further conversions of the fatty acids (i.e. desaturations, elongations) occur after their esterification with structural lipids of both plastids and the endoplasmic reticulum. The aliphatic chains are also used as building blocks for structuring storage acylglycerols via the Kennedy pathway. Current research, aiming to enhance lipogenesis in the microalgal cell, is focusing on over-expressing key-enzymes involved in the earlier steps of the pathway of fatty acid synthesis. A complementary plan would be the repression of lipid catabolism by down-regulating acylglycerol hydrolysis and/or β-oxidation. The tendency of oleaginous microalgae to synthesize, apart from lipids, significant amounts of other energy-rich compounds such as sugars, in processes competitive to lipogenesis, deserves attention since the lipid yield may be considerably increased by blocking competitive metabolic pathways. The majority of microalgal production occurs in outdoor cultivation and for this reason biotechnological applications face some difficulties. Therefore, algal production systems need to be improved and harvesting systems need to be more effective in order for their industrial applications to become more competitive and economically viable. Besides, a reduction of the production cost of microalgal lipids can be achieved by combining lipid production with other commercial applications. The combined production of bioactive products and lipids, when possible, can support the commercial viability of both processes. Hydrophobic compounds can be extracted simultaneously with lipids and then purified, while hydrophilic compounds such as proteins and sugars may be extracted from the defatted biomass. The microalgae also have applications in environmental biotechnology since they can be used for bioremediation of wastewater and to monitor environmental toxicants. Algal biomass produced during wastewater treatment may be further valorized in the biofuel manufacture. It is anticipated that the high microalgal lipid potential will force research towards finding effective ways to manipulate biochemical pathways involved in lipid biosynthesis and towards cost effective algal cultivation and harvesting systems, as well.