Animal-like prostaglandins in marine microalgae

Prostaglandin pathway activation in the diatom Skeletonema marinoi under grazer pressure

Prostaglandins (Pgs) are eicosanoid lipid mediators detected in all vertebrates, in some marine invertebrates, macroalgae and in diatoms, a class of eukaryotic microalgae composing the phytoplankton. The enzymes involved in the Pgs pathway were found to be differentially expressed in two strains of the diatom Skeletonema marinoi, named FE7 and FE60, already known to produce different levels of oxylipins, a class of secondary metabolites involved in the defence of diatoms against copepod predation, with FE7 being higher producer than FE60. In the present study we investigated the response of genes involved in the production of oxylipins and Pgs, evaluating their expression after the exposure to the copepod Temora stylifera. Our results highlighted a grazer feeding preference for FE60, the strain having low oxylipins content and reduced expression of Pgs enzymes, and an impact on the gene expression of the enzymes involved in oxylipins (i.e. lipoxygenase) and Pgs (i.e. cyclooxygenase) biosynthesis, especially in FE7. A time course evaluation of the gene expression over 24 h showed an upregulation of the essential enzyme in the Pgs pathway, the cyclooxygenase, in FE60 after 6 h of exposure to the grazer, differently from FE7 where no upregulation of gene expression in the presence of copepods was revealed. These results provide preliminary indications regarding the existence of a complex involvement of the Pgs pathway in the prey-predator interaction that requires further investigations.

Diatom-Bacteria Interactions in the Marine Environment: Complexity, Heterogeneity, and Potential for Biotechnological Applications

Diatom-bacteria interactions evolved during more than 200 million years of coexistence in the same environment. In this time frame, they established complex and heterogeneous cohorts and consortia, creating networks of multiple cell-to-cell mutualistic or antagonistic interactions for nutrient exchanges, communication, and defence. The most diffused type of interaction between diatoms and bacteria is based on a win-win relationship in which bacteria benefit from the organic matter and nutrients released by diatoms, while these last rely on bacteria for the supply of nutrients they are not able to produce, such as vitamins and nitrogen. Despite the importance of diatom-bacteria interactions in the evolutionary history of diatoms, especially in structuring the marine food web and controlling algal blooms, the molecular mechanisms underlying them remain poorly studied. This review aims to present a comprehensive report on diatom-bacteria interactions, illustrating the different interplays described until now and the chemical cues involved in the communication and exchange between the two groups of organisms. We also discuss the potential biotechnological applications of molecules and processes involved in those fascinating marine microbial networks and provide information on novel approaches to unveiling the molecular mechanisms underlying diatom-bacteria interactions.

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.

Untargeted metabolomics to expand the chemical space of the marine diatom Skeletonema marinoi

Diatoms (Bacillariophyceae) are aquatic photosynthetic microalgae with an ecological role as primary producers in the aquatic food web. They account substantially for global carbon, nitrogen, and silicon cycling. Elucidating the chemical space of diatoms is crucial to understanding their physiology and ecology. To expand the known chemical space of a cosmopolitan marine diatom, Skeletonema marinoi, we performed High-Resolution Liquid Chromatography-Tandem Mass Spectrometry (LC-MS2) for untargeted metabolomics data acquisition. The spectral data from LC-MS2 was used as input for the Metabolome Annotation Workflow (MAW) to obtain putative annotations for all measured features. A suspect list of metabolites previously identified in the Skeletonema spp. was generated to verify the results. These known metabolites were then added to the putative candidate list from LC-MS2 data to represent an expanded catalog of 1970 metabolites estimated to be produced by S. marinoi. The most prevalent chemical superclasses, based on the ChemONT ontology in this expanded dataset, were organic acids and derivatives, organoheterocyclic compounds, lipids and lipid-like molecules, and organic oxygen compounds. The metabolic profile from this study can aid the bioprospecting of marine microalgae for medicine, biofuel production, agriculture, and environmental conservation. The proposed analysis can be applicable for assessing the chemical space of other microalgae, which can also provide molecular insights into the interaction between marine organisms and their role in the functioning of ecosystems.

Microbial Interactions between Marine Microalgae and Fungi: From Chemical Ecology to Biotechnological Possible Applications

Chemical interactions have been shown to regulate several marine life processes, including selection of food sources, defense, behavior, predation, and mate recognition. These chemical communication signals have effects not only at the individual scale, but also at population and community levels. This review focuses on chemical interactions between marine fungi and microalgae, summarizing studies on compounds synthetized when they are cultured together. In the current study, we also highlight possible biotechnological outcomes of the synthetized metabolites, mainly for human health applications. In addition, we discuss applications for bio-flocculation and bioremediation. Finally, we point out the necessity of further investigating microalgae-fungi chemical interactions because it is a field still less explored compared to microalga-bacteria communication and, considering the promising results obtained until now, it is worthy of further research for scientific advancement in both ecology and biotechnology fields.

Transcriptome Sequencing of the Diatom Asterionellopsis thurstonii and In Silico Identification of Enzymes Potentially Involved in the Synthesis of Bioactive Molecules

Microalgae produce a plethora of primary and secondary metabolites with possible applications in several market sectors, including cosmetics, human nutrition, aquaculture, biodiesel production and treatment/prevention of human diseases. Diatoms, in particular, are the most diversified microalgal group, many species of which are known to have anti-cancer, anti-oxidant, anti-diabetes, anti-inflammatory and immunomodulatory properties. Compounds responsible for these activities are often still unknown. The aim of this study was to de novo sequence the full transcriptome of two strains of the diatom Asterionellopsis thurstonii, sampled from two different locations and cultured in both control and phosphate starvation conditions. We used an RNA-sequencing approach to in silico identify transcripts potentially involved in the synthesis/degradation of compounds with anti-cancer and immunomodulatory properties. We identified transcript coding for L-asparaginase I, polyketide cyclase/dehydrase, bifunctional polyketide phosphatase/kinase, 1-deoxy-D-xylulose-5-phosphate synthase (fragment), inositol polyphosphate 5-phosphatase INPP5B/F, catechol O-Methyltransferase, digalactosyldiacylglycerol synthase (DGD1), 1,2-diacylglycerol-3-beta-galactosyltransferase and glycerolphosphodiester phosphodiesterase. Differential expression analysis also allowed to identify in which culturing condition these enzymes are more expressed. Overall, these data give new insights on the annotation of diatom genes, enzymatic pathways involved in the generation of bioactive molecules and possible exploitation of Asterionellopsis thurstonii.

Identification of Effector Metabolites Using Exometabolite Profiling of Diverse Microalgae

Dissolved exometabolites mediate algal interactions in aquatic ecosystems, but microalgal exometabolomes remain understudied. We conducted an untargeted metabolomic analysis of nonpolar exometabolites exuded from four phylogenetically and ecologically diverse eukaryotic microalgal strains grown in the laboratory, freshwater Chlamydomonas reinhardtii, brackish Desmodesmus sp., marine Phaeodactylum tricornutum, and marine Microchloropsis salina, to identify released metabolites based on relative enrichment in the exometabolomes compared to cell pellet metabolomes. Exudates from the different taxa were distinct, but we did not observe clear phylogenetic patterns. We used feature-based molecular networking to explore the identities of these metabolites, revealing several distinct di- and tripeptides secreted by each of the algae, lumichrome, a compound that is known to be involved in plant growth and bacterial quorum sensing, and novel prostaglandin-like compounds. We further investigated the impacts of exogenous additions of eight compounds selected based on exometabolome enrichment on algal growth. Of these compounds, five (lumichrome, 5′-S-methyl-5′-thioadenosine, 17-phenyl trinor prostaglandin A2, dodecanedioic acid, and aleuritic acid) impacted growth in at least one of the algal cultures. Two of these compounds (dodecanedioic acid and aleuritic acid) produced contrasting results, increasing growth in some algae and decreasing growth in others. Together, our results reveal new groups of microalgal exometabolites, some of which could alter algal growth when provided exogenously, suggesting potential roles in allelopathy and algal interactions.

IMPORTANCE Microalgae are responsible for nearly half of primary production on earth and play an important role in global biogeochemical cycling as well as in a range of industrial applications. Algal exometabolites are important mediators of algal-algal and algal-bacterial interactions that ultimately affect algal growth and physiology. In this study, we characterize exometabolomes across marine and freshwater algae to gain insights into the diverse metabolites they release into their environments (“exudates”). We observe that while phylogeny can play a role in exometabolome content, environmental conditions or habitat origin (freshwater versus marine) are also important. We also find that several of these compounds can influence algal growth (as measured by chlorophyll production) when provided exogenously, highlighting the importance of characterization of these novel compounds and their role in microalgal ecophysiology.

The role of methyl jasmonate in enhancing biomass yields and bioactive metabolites in Stauroneis sp. (Bacillariophyceae) revealed by proteome and biochemical profiling

The diatom Stauroneis sp. was previously identified as a promising source of fucoxanthin and omega-3 oils. Methyl jasmonate (MJ) supplementation is known to enhance metabolite yields in this species without impacting on growth or photosynthesis. Therefore, a label-free proteomics approach was undertaken to further evaluate the functional role of MJ on the diatom's physiology. Of the twenty cultivation regimes were screened, Uf/2 medium with green+white LED's induced the greatest metabolic response when exposed to 10 μM MJ treatment. These conditions significantly enhanced the pigment and total cellular lipids contents. The increase in fucoxanthin correlating with a 20% increase in Trolox reducing equivalent in the total antioxidant assay, indicating a non-enzymatic antioxidant role of fucoxanthin to mitigate the detrimental effects of a redox imbalance within chloroplasts. The proteomics identified 197 proteins up-regulated 48 h after MJ exposure including cell signalling cascades, photosynthetic processes, carbohydrate metabolism, lipid biosynthesis and chloroplast biogenesis. MJ strengthened the dark reactions of photosynthesis to support growth and metabolite fluxes. The MJ-induced ER stress protein triggered lipid body production, facilitating metabolite turnover and trafficking between cellular organelles. Plastid terminal oxidase and glutamate 1-semialdehyde 2,1-aminomutase may act as MJ-induced ROS responsive regulatory switch to support chloroplast biosynthesis. SIGNIFICANCE STATEMENT: Phytohormones represents a promising tool to enhance the high-value metabolite yields in plants and algae, however little is known of the role of methyl jasmonate in diatoms at a molecular level. A shotgun proteomics approach was undertaken to determine the influence of MJ on the diatom's cellular physiology in the marine diatom Stauroneis sp., revealing a signal transduction cascade leading to increased lipid and pigment content and identified promising targets for genetic engineering.

Cell wall proteomic analysis of the cnidarian photosymbionts Breviolum minutum and Cladocopium goreaui

The algal cell wall is an important cellular component that functions in defense, nutrient utilization, signaling, adhesion, and cell–cell recognition—processes important in the cnidarian–dinoflagellate symbiosis. The cell wall of symbiodiniacean dinoflagellates is not well characterized. Here, we present a method to isolate cell walls of Symbiodiniaceae and prepare cell‐wall‐enriched samples for proteomic analysis. Label‐free liquid chromatography–electrospray ionization tandem mass spectrometry was used to explore the surface proteome of two Symbiodiniaceae species from the Great Barrier Reef: Breviolum minutum and Cladocopium goreaui. Transporters, hydrolases, translocases, and proteins involved in cell‐adhesion and protein–protein interactions were identified, but the majority of cell wall proteins had no homologues in public databases. We propose roles for some of these proteins in the cnidarian–dinoflagellate symbiosis. This work provides the first proteomics investigation of cell wall proteins in the Symbiodiniaceae and represents a basis for future explorations of the roles of cell wall proteins in Symbiodiniaceae and other dinoflagellates.

Prostaglandin production by the microalga with heterologous expression of cyclooxygenase

Prostaglandins (PGs) are the physiologically active compounds synthesized from C20 polyunsaturated fatty acids (PUFAs) by cyclooxygenase (COX) and a series of PG synthases, and are utilized as pharmaceuticals. Currently, commercialized PGs are mainly produced by chemical synthesis under harsh conditions. By contrast, bioproduction of PGs can be an alternative, environmental-friendly, and inexpensive process with genetic engineering of model plants, although these conventional host organisms contain a limited quantity of PG precursors. In this study, we established an efficient PG production process using the genetically engineered microalga Fistulifera solaris which is rich in C20 PUFAs. A cox gene derived from the red alga Agarophyton vermiculophyllum was introduced into F. solaris. As a result, a transformant clone with high cox expression produced PGs (i.e., PGD₂ , PGE₂ , PGF_2α , and 15-ketoPGF_2α derived from arachidonic acid, and PGD₃ , PGE₃ , and PGF_3α derived from eicosapentaenoic acid) as revealed by liquid chromatography/mass spectrometry. The total content of PGs was 1290.4 ng/g of dry cell weight, which was higher than that produced in the transgenic plant reported previously. The results obtained in this study indicate that the C20 PUFA-rich microalga functionally expressing COX is a promising host for PG bioproduction.

Does diclofenac act like a photosynthetic herbicide on green algae? Chlamydomonas reinhardtii synchronous culture-based study with atrazine as reference

The non-steroidal anti-inflammatory drug diclofenac (DCF) is one of the commonly used and frequently detected drugs in water bodies, and several studies indicate its toxic effect on plants and algae. Studies performed with asynchronous Chlamydomonas reinhardtii cultures indicated that DCF inhibit the growth of population of the algae. Here, a synchronous population of C. reinhardtii, in which all cells are in the same developmental phase, is used. Following changes in cells size, photosynthetic activity and gene expression, we could compare, at the level of single cell, DCF-mediated effects with the effects caused by atrazine, a triazine herbicide that inhibits photosynthesis and triggers oxidative stress. Application of DCF and atrazine at the beginning of the cell cycle allowed us to follow the changes occurring in the cells in the subsequent stages of their development. Synchronized Chlamydomonas reinhardtii cultures (strain CC-1690, wild type) were exposed to diclofenac sodium salt (135 mg/L) or atrazine (77.6 µg/L). The cell suspension was sampled hourly (0-10 h) in the light period of the cell cycle to determine cell number and volume, photosynthetic pigment content, chlorophyll a fluorescence (OJIP test) in vivo, and selected gene expression (real-time qPCR), namely psbA, psaA, FSD1, MSD3 and APX1. The two toxicants differently influenced C. reinhardtii cells. Both substances decreased photosynthetic "vitality" (PI - performance index) of the cells, albeit for different reasons. While atrazine significantly disrupted the photosynthetic electron transport, resulting in excessive production of reactive oxygen species (ROS) and limited cell growth, DCF caused silencing of photosystem II (PSII) reaction centers, transforming them into "heat sinks", thus preventing significant ROS overproduction. Oxidative stress caused by atrazine was the probable reason for the rapid appearance of phytotoxic action soon after entering the cells, while the effects of DCF could only be seen several hours after treatment. A comparison of DCF-caused effects with the effects caused by atrazine led us to conclude that, although DCF cannot be regarded as typical photosynthetic herbicide, it exhibits an algicidal activity and can be potentially dangerous for aquatic plants and algae.

Mammalian-Like Inflammatory and Pro-Resolving Oxylipins in Marine Algae

Oxylipins constitute a family of oxidized fatty acids, that are well known as tissue hormones in mammals. They contribute to inflammation and its resolution. The major classes of these lipid mediators are inflammatory prostaglandins (PGs) and leukotrienes (LTs) as well as pro-resolving resolvins (Rvs). Understanding their biosynthetic pathways and modes of action is important for anti-inflammatory interventions. Besides mammals, marine algae also biosynthesize mammalian-like oxylipins and thus offer new opportunities for oxylipin research. They provide prolific sources for these compounds and offer unique opportunities to study alternative biosynthetic pathways to the well-known lipid mediators. Herein, we discuss recent findings on the biosynthesis of oxylipins in mammals and algae including an alternative pathway to prostaglandin E2 , a novel pathway to a precursor of leukotriene B4 , and the production of resolvins in algae. We evaluate the pharmacological potential of the algal metabolites with implications in health and disease.

Isoprostanoid Profiling of Marine Microalgae

Algae result from a complex evolutionary history that shapes their metabolic network. For example, these organisms can synthesize different polyunsaturated fatty acids, such as those found in land plants and oily fish. Due to the presence of numerous double-bonds, such molecules can be oxidized nonenzymatically, and this results in the biosynthesis of high-value bioactive metabolites named isoprostanoids. So far, there have been only a few studies reporting isoprostanoid productions in algae. To fill this gap, the current investigation aimed at profiling isoprostanoids by liquid chromatography -mass spectrometry/mass spectrometry (LC-MS/MS) in four marine microalgae. A good correlation was observed between the most abundant polyunsaturated fatty acids (PUFAs) produced by the investigated microalgal species and their isoprostanoid profiles. No significant variations in the content of oxidized derivatives were observed for Rhodomonas salina and Chaetoceros gracilis under copper stress, whereas increases in the production of C18-, C20- and C22-derived isoprostanoids were monitored in Tisochrysis lutea and Phaeodactylum tricornutum. In the presence of hydrogen peroxide, no significant changes were observed for C. gracilis and for T. lutea, while variations were monitored for the other two algae. This study paves the way to further studying the physiological roles of isoprostanoids in marine microalgae and exploring these organisms as bioresources for isoprostanoid production.

Human health benefits of very-long-chain polyunsaturated fatty acids from microalgae

Microalgae are single-cell, photosynthetic organisms whose biodiversity places them at the forefront of biological producers of high-value molecules including lipids and pigments. Some of these organisms particular are capable of synthesizing n-3 very long chain polyunsaturated fatty acids (VLC-PUFAs), known to have beneficial effects on human health. Indeed, VLC-PUFAs are the precursors of many signaling molecules in humans involved in the complexities of inflammatory processes. This mini-review provides an inventory of knowledge on the synthesis of VLC-PUFAs in microalgae and on the diversity of signaling molecules (prostanoids, leukotrienes, SPMs, EFOX, isoprostanoids) that arise in humans from VLC-PUFAs.

Carbon fixation gene expression in Skeletonema marinoi in nitrogen-, phosphate-, silicate-starvation, and low-temperature stress exposure

Diatoms are unicellular algae with a set of extraordinary genes, metabolic pathways, and physiological functions acquired by secondary endosymbiosis, especially for their efficient photosynthetic carbon fixation mechanisms, which can be a reason for their successful environmental adaptation and great contribution to primary production. Based on the available genomic information, the expression patterns of carbon fixation genes were analyzed using transcriptomic sequencing and reverse transcription-quantitative polymerase chain reaction (RT-qPCR) in Skeletonema marinoi. Meanwhile, suitable reference genes applying to specific experimental treatments were selected. In our results, carbon fixation genes were standardized by actin and TATA box-binding protein-coding genes in growth phase samples and stress conditions, respectively. It was found that a series of carbon fixation genes, such as the pyruvate orthophosphate dikinase (PPDK)-coding gene, had significantly up-regulated expression in nitrogen-starvation, phosphate-starvation, and low-temperature conditions, but consistently down-regulated in silicate-starvation treatment. These carbon fixation genes exhibited variable expression levels in different conditions and will be useful for investigating gene expression mechanisms in S. marinoi and improve our understanding of diatom carbon fixation pathways.

Variation in prostaglandin metabolism during growth of the diatom Thalassiosira rotula

Prostaglandins (PGs) are hormone-like mediators in many physiological and pathological processes that are present in all vertebrates, in some terrestrial and aquatic invertebrates, and have also been identified in some macroalgae. They have recently been reported also in marine microalgae but their role as chemical mediators is largely unknown. Here we studied the expression pattern of the PG biosynthetic pathway during different growth phases of the centric diatom Thalassiosira rotula and assessed the release of PGs in the surrounding environment for the first time. We show that enzymes responsible for PGs formation such as cyclooxygenase, prostaglandin E synthase 2-like and prostaglandin F synthase are mainly expressed at the end of the exponential phase and that PGs are released especially during the stationary and senescent phases, suggesting a possible signaling function for these compounds. Phylogenetic analysis of the limiting enzyme, COX, indicate the presence in diatoms of more than one enzyme related to the oxidative metabolism of fatty acids belonging to the peroxidase-cyclooxygenase superfamily. These findings suggest a more complex evolution and diversity of metabolic pathways leading to the synthesis of lipid mediators in diatoms.

De novo transcriptome of the diatom Cylindrotheca closterium identifies genes involved in the metabolism of anti-inflammatory compounds

Diatoms are the most diverse and abundant group of phytoplankton species and represent a huge reservoir of marine natural products with possible application for human health. Several diatoms are known to have anticancer, anti-inflammatory, antioxidant and anti-microbial properties, but the compounds responsible of these activities are often still unknown. The diatom Cylindrotheca closterium showed anti-inflammatory properties inhibiting TNFα release in human monocytic leukemia cells. In this study, we present the full transcriptome of C. closterium, and used an -omic approach to identify transcripts coding enzymes that can be involved in the synthesis/degradation of anti-inflammatory compounds. This approach allowed to identify phosphatidylinositol-3-phosphatase, phosphatidylinositol 3-kinase catalytic subunit type 3, phosphatidylinositol N-acetylglucosaminyltransferase subunit A, monogalactosyldiacylglycerol synthase and violaxanthin de-epoxidase, which are known to be involved in anti-inflammatory compound metabolism. When C. closterium was cultured in silica-starvation conditions, selected as stress condition to potentially trigger the synthesis of bioactive metabolites, anti-inflammatory activity was lost and expression levels of the analyzed transcripts were reduced. These data suggested that the control culturing condition was the most active. This study used for the first time a transcriptomic-guided approach to identify enzymes involved in anti-inflammatory compound metabolism, directing future discoveries of marine natural products in microalgae.

Identification of Prostaglandin Pathway in Dinoflagellates by Transcriptome Data Mining

Dinoflagellates, a major class of marine eukaryote microalgae composing the phytoplankton, are widely recognised as producers of a large variety of toxic molecules, particularly neurotoxins, which can also act as potent bioactive pharmacological mediators. In addition, similarly to other microalgae, they are also good producers of polyunsaturated fatty acids (PUFAs), important precursors of key molecules involved in cell physiology. Among PUFA derivatives are the prostaglandins (Pgs), important physiological mediators in several physiological and pathological processes in humans, also used as “biological” drugs. Their synthesis is very expensive because of the elevated number of reaction steps required, thus the search for new Pgs production methods is of great relevance. One possibility is their extraction from microorganisms (e.g., diatoms), which have been proved to produce the same Pgs as humans. In the present study, we took advantage of the available transcriptomes for dinoflagellates in the iMicrobe database to search for the Pgs biosynthetic pathway using a bioinformatic approach. Here we show that dinoflagellates express nine Pg-metabolism related enzymes involved in both Pgs synthesis and reduction. Not all of the enzymes were expressed simultaneously in all the species analysed and their expression was influenced by culturing conditions, especially salinity of the growth medium. These results confirm the existence of a biosynthetic pathway for these important molecules in unicellular microalgae other than diatoms, suggesting a broad diffusion and conservation of the Pgs pathway, which further strengthen their importance in living organisms.

15-Hydroperoxy-PGE2 : Intermediate in Mammalian and Algal Prostaglandin Biosynthesis

Arachidonic‐acid‐derived prostaglandins (PGs), specifically PGE₂, play a central role in inflammation and numerous immunological reactions. The enzymes of PGE₂ biosynthesis are important pharmacological targets for anti‐inflammatory drugs. Besides mammals, certain edible marine algae possess a comprehensive repertoire of bioactive arachidonic‐acid‐derived oxylipins including PGs that may account for food poisoning. Described here is the analysis of PGE₂ biosynthesis in the red macroalga Gracilaria vermiculophylla that led to the identification of 15‐hydroperoxy‐PGE₂, a novel precursor of PGE₂ and 15‐keto‐PGE₂. Interestingly, this novel precursor is also produced in human macrophages where it represents a key metabolite in an alternative biosynthetic PGE₂ pathway in addition to the well‐established arachidonic acid‐PGG₂‐PGH₂‐PGE₂ route. This alternative pathway of mammalian PGE₂ biosynthesis may open novel opportunities to intervene with inflammation‐related diseases.

Sex-dependent and -independent transcriptional changes during haploid phase gametogenesis in the sugar kelp Saccharina latissima

In haplodiplontic lineages, sexual reproduction occurs in haploid parents without meiosis. Although widespread in multicellular lineages such as brown algae (Phaeophyceae), haplodiplontic gametogenesis has been little studied at the molecular level. We addressed this by generating an annotated reference transcriptome for the gametophytic phase of the sugar kelp, Saccharina latissima. Transcriptional profiles of microscopic male and female gametophytes were analysed at four time points during the transition from vegetative growth to gametogenesis. Gametogenic signals resulting from a switch in culture irradiance from red to white light activated a core set of genes in a sex-independent manner, involving rapid activation of ribosome biogenesis, transcription and translation related pathways, with several acting at the post-transcriptional or post-translational level. Additional genes regulating nutrient acquisition and key carbohydrate-energy pathways were also identified. Candidate sex-biased genes under gametogenic conditions had potentially key roles in controlling female- and male-specific gametogenesis. Among these were several sex-biased or -specific E3 ubiquitin-protein ligases that may have important regulatory roles. Females specifically expressed several genes that coordinate gene expression and/or protein degradation, and the synthesis of inositol-containing compounds. Other female-biased genes supported parallels with oogenesis in divergent multicellular lineages, in particular reactive oxygen signalling via an NADPH-oxidase. Males specifically expressed the hypothesised brown algal sex-determining factor. Male-biased expression mainly involved upregulation of genes that control mitotic cell proliferation and spermatogenesis in other systems, as well as multiple flagella-related genes. Our data and results enhance genome-level understanding of gametogenesis in this ecologically and economically important multicellular lineage.

Prostaglandins in Marine Organisms: A Review

Prostaglandins (PGs) are lipid mediators belonging to the eicosanoid family. PGs were first discovered in mammals where they are key players in a great variety of physiological and pathological processes, for instance muscle and blood vessel tone regulation, inflammation, signaling, hemostasis, reproduction, and sleep-wake regulation. These molecules have successively been discovered in lower organisms, including marine invertebrates in which they play similar roles to those in mammals, being involved in the control of oogenesis and spermatogenesis, ion transport, and defense. Prostaglandins have also been found in some marine macroalgae of the genera Gracilaria and Laminaria and very recently the PGs pathway has been identified for the first time in some species of marine microalgae. In this review we report on the occurrence of prostaglandins in the marine environment and discuss the anti-inflammatory role of these molecules.

Unveiling the presence of biosynthetic pathways for bioactive compounds in the Thalassiosira rotula transcriptome

Diatoms are phytoplankton eukaryotic microalgae that are widely distributed in the world’s oceans and are responsible for 20–25% of total carbon fixation on the planet. Using transcriptome sequencing here we show for the first time that the ubiquitous diatom Thalassiosira rotula expresses biosynthetic pathways that potentially lead to the synthesis of interesting secondary metabolites with pharmaceutical applications such as polyketides, prostaglandins and secologanin. We also show that these pathways are differentially expressed in conditions of silica depletion in comparison with standard growth conditions.

Non-Enzymatic Synthesis of Bioactive Isoprostanoids in the Diatom Phaeodactylum following Oxidative Stress

The ecological success of diatoms requires a remarkable ability to survive many types of stress, including variations in temperature, light, salinity, and nutrient availability. On exposure to these stresses, diatoms exhibit common responses, including growth arrest, impairment of photosynthesis, production of reactive oxygen species, and accumulation of triacylglycerol (TAG). We studied the production of cyclopentane oxylipins derived from fatty acids in the diatom Phaeodactylum tricornutum in response to oxidative stress. P. tricornutum lacks the enzymatic pathway for producing cyclopentane-oxylipins, such as jasmonate, prostaglandins, or thromboxanes. In cells subjected to increasing doses of hydrogen peroxide (H₂O₂), we detected nonenzymatic production of isoprostanoids, including six phytoprostanes, three F_2t-isoprostanes, two F_3t-isoprostanes, and three F_4t-neuroprostanes, by radical peroxidation of α-linolenic, arachidonic, eicosapentaenoic, and docosahexanoic acids, respectively. H₂O₂ also triggered photosynthesis impairment and TAG accumulation. F_1t-phytoprostanes constitute the major class detected (300 pmol per 1 million cells; intracellular concentration, ∼4 µm). Only two glycerolipids, phosphatidylcholine and diacylglycerylhydroxymethyl-trimethyl-alanine, could provide all substrates for these isoprostanoids. Treatment of P. tricornutum with nine synthetic isoprostanoids produced an effect in the micromolar range, marked by the accumulation of TAG and reduced growth, without affecting photosynthesis. Therefore, the emission of H₂O₂ and free radicals upon exposure to stresses can lead to glycerolipid peroxidation and nonenzymatic synthesis of isoprostanoids, inhibiting growth and contributing to the induction of TAG accumulation via unknown processes. This characterization of nonenzymatic oxylipins in P. tricornutum opens a field of research on the study of processes controlled by isoprostanoid signaling in various physiological and environmental contexts in diatoms.

Grazer-induced transcriptomic and metabolomic response of the chain-forming diatom Skeletonema marinoi

Diatoms and copepods are main actors in marine food webs. The prey–predator interactions between them affect bloom dynamics, shape marine ecosystems and impact the energy transfer to higher trophic levels. Recently it has been demonstrated that the presence of grazers may affect the diatom prey beyond the direct effect of grazing. Here, we investigated the response of the chain-forming centric diatom Skeletonema marinoi to grazer cues, including changes in morphology, gene expression and metabolic profile. S. marinoi cells were incubated with Calanus finmarchicus or with Centropages typicus and in both cases responded by reducing the chain length, whereas changes in gene expression indicated an activation of stress response, changes in the lipid and nitrogen metabolism, in cell cycle regulation and in frustule formation. Transcripts linked to G protein-coupled receptors and to nitric oxide synthesis were differentially expressed suggesting involvement of these signalling transduction pathways in the response. Downregulation of a lipoxygenase in the transcriptomic data and of its products in the metabolomic data also indicate an involvement of oxylipins. Our data contribute to a better understanding of the gene function in diatoms, providing information on the nature of genes implicated in the interaction with grazers, a crucial process in marine ecosystems.