Phospholipase A2 activity triggers the wound-activated chemical defense in the diatom Thalassiosira rotula

Stress-Induced Production of Bioactive Oxylipins in Marine Microalgae

Microalgae, stemming from a complex evolutionary lineage, possess a metabolic composition influenced by their evolutionary journey. They have the capacity to generate diverse polyunsaturated fatty acids (PUFAs), akin to those found in terrestrial plants and oily fish. Also, because of their numerous double bonds, these metabolic compounds are prone to oxidation processes, leading to the creation of valuable bioactive molecules called oxylipins. Moreover, owing to their adaptability across various environments, microalgae offer an intriguing avenue for biosynthesizing these compounds. Thus, modifying the culture conditions could potentially impact the profiles of oxylipins. Indeed, the accumulation of oxylipins in microalgae is subject to the influence of growth conditions, nutrient availability, and stressors, and adjusting these factors can enhance their production in microalgae culture. Consequently, the present study scrutinized the LC-MS/MS profiles of oxylipins from three marine microalgae species (two Haptagophytes and one Chlorophyte) cultivated in 1 L of photobioreactors under varying stress-inducing conditions, such as the introduction of H2O2, EtOAc, and NaCl, during their exponential growth phase. Approximately 50 oxylipins were identified, exhibiting different concentrations depending on the species and growth circumstances. This research suggests that microalgae metabolisms can be steered toward the production of bioactive oxylipins through modifications in the culture conditions. In this instance, the application of a low dose of hydrogen peroxide to Mi 124 appears to stimulate the production of nonenzymatic oxylipins. For Mi136, it is the application of salt stress that seems to increase the overall production of oxylipins. In the case of Mi 168, either a low concentration of H2O2 or a high concentration of AcOEt appears to have this effect.

Unraveling the source-water fishy odor occurrence during low-temperature periods: Odorants identification, typical algae species and odor-producing potential

In this study, odor characteristics and phytoplankton composition were systematically investigated in two winter periods in a reservoir with fishy odor in north China. Ten potential fishy odor-producing algae were isolated and odorant-producing potentials were evaluated. Olfactometry profile and odorant composition of water samples were analyzed using GC-Olfactometry combined with GC × GC-TOFMS. The results showed that 2,4-heptadienal and hexanal were major fishy odor contributors. The abundance of Uroglena sp., Synura sp. and Peridinium sp. was negatively correlated with total dissolved organic carbon, ammonia nitrogen, and nitrate, illustrating nutrient level might be major drivers for the succession of fishy odor-producing algae. Dinobryon sp. and Uroglena sp. made the greatest contribution to fishy odor, followed by Peridinium sp., Synura sp., and Ochromonas sp. Fishy odor in 2016 winter and the early of 2017 winter was mainly caused by Dinobryon sp., while Uroglena sp. contributes mostly in March in 2017 winter. This study demonstrates the main odorants and algae causing fishy odor in reservoir, which will provide a scientific basis for the management of seasonal fishy odor problems in water source.

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.

Density-Dependent Prophylaxis in Freshwater Snails Driven by Oxylipin Chemical Cues

While animal aggregations can benefit the fitness of group members, the behaviour may also lead to higher risks of parasite infection as group density increases. Some animals are known to moderate their investment in immunity relative to the risk of infection. These animals exhibit density-dependent prophylaxis (DDP) by increasing their immune investment as group density increases. Despite being documented in many taxa, the mechanisms of DDP remain largely unexplored. Snails are known to aggregate and experience large fluctuations in density and serve as required hosts for many parasites. Further, they are known to use chemical cues to aggregate. To test whether freshwater snails exhibit DDP and investigate the role that chemical signaling compounds may play in triggering this phenomenon, we performed four experiments on the freshwater snail Stagnicola elodes, which is a common host for many trematode parasite species. First, we tested if DDP occurred in snails in laboratory-controlled conditions (control vs snail-conditioned water) and whether differences in exposure to chemical cues affected immune function. Second, we used gas chromatography to characterize fatty acids expressed in snail-conditioned water to determine if precursors for particular signaling molecules, such as oxylipins, were being produced by snails. Third, we characterized the oxylipins released by infected and uninfected field-collected snails, to better understand how differences in oxylipin cocktails may play a role in inducing DDP. Finally, we tested the immune response of snails exposed to four oxylipins to test the ability of specific oxylipins to affect DDP. We found that snails exposed to water with higher densities of snails and raised in snail-conditioned water had higher counts of haemocytes. Additionally, lipid analysis demonstrated that fatty acid molecules that are also precursors for oxylipins were present in snail-conditioned water. Trematode-infected snails emitted 50 oxylipins in higher amounts, with 24 of these oxylipins only detected in this group. Finally, oxylipins that were higher in infected snails induced naïve snails to increase their immune responses compared to sham-exposed snails. Our results provide evidence that snails exhibit DDP, and the changes in oxylipins emitted by infected hosts may be one of the molecular mechanisms driving this phenomenon.

Resolving the microalgal gene landscape at the strain level: A novel hybrid transcriptome of Emiliania huxleyi CCMP3266

Microalgae are key ecological players with a complex evolutionary history. Genomic diversity, in addition to limited availability of high-quality genomes, challenge studies that aim to elucidate molecular mechanisms underlying microalgal ecophysiology. Here, we present a novel and comprehensive transcriptomic hybrid approach to generate a reference for genetic analyses, and resolve the microalgal gene landscape at the strain level. The approach is demonstrated for a strain of the coccolithophore microalga Emiliania huxleyi, which is a species complex with considerable genome variability. The investigated strain is commonly studied as a model for algal-bacterial interactions, and was therefore sequenced in the presence of bacteria to elicit the expression of interaction-relevant genes. We applied complementary PacBio Iso-Seq full-length cDNA, and poly(A)-independent Illumina total RNA sequencing, which resulted in a de novo assembled, near complete hybrid transcriptome. In particular, hybrid sequencing improved the reconstruction of long transcripts and increased the recovery of full-length transcript isoforms. To use the resulting hybrid transcriptome as a reference for genetic analyses, we demonstrate a method that collapses the transcriptome into a genome-like dataset, termed "synthetic genome" (sGenome). We used the sGenome as a reference to visually confirm the robustness of the CCMP3266 gene assembly, to conduct differential gene expression analysis, and to characterize novel E. huxleyi genes. The newly-identified genes contribute to our understanding of E. huxleyi genome diversification, and are predicted to play a role in microbial interactions. Our transcriptomic toolkit can be implemented in various microalgae to facilitate mechanistic studies on microalgal diversity and ecology. Importance Microalgae are key players in the ecology and biogeochemistry of our oceans. Efforts to implement genomic and transcriptomic tools in laboratory studies involving microalgae suffer from the lack of published genomes. In the case of coccolithophore microalgae, the problem has long been recognized; the model species Emiliania huxleyi is a species complex with genomes composed of a core, and a large variable portion. To study the role of the variable portion in niche adaptation, and specifically in microbial interactions, strain-specific genetic information is required. Here we present a novel transcriptomic hybrid approach, and generated strain-specific genome-like information. We demonstrate our approach on an E. huxleyi strain that is co-cultivated with bacteria. By constructing a "synthetic genome", we generated comprehensive gene annotations that enabled accurate analyses of gene expression patterns. Importantly, we unveiled novel genes in the variable portion of E. huxleyi that play putative roles in microbial interactions.

Inclusion of photoautotrophic cultivated diatom biomass in salmon feed can deter lice

The aim of this study was to evaluate the potential of diatom (microalgae) biomass as a lice-reducing ingredient in salmon feed. The original hypothesis was based on the fact that polyunsaturated aldehydes (PUAs), e.g. 2-trans, 4-trans decadenial (A3) produced by diatoms can function as grazing deterrents and harm copepod development. Salmon lice (Lepeophtheirus salmonis) is a copepod, and we intended to test if inclusion of diatom biomass in the feed could reduce the infestation of lice on salmon. We performed experiments where salmon kept in tanks were offered four different diets, i.e. basic feed with diatoms, fish oil, Calanus sp. oil or rapeseed oil added. After a feeding period of 67 days a statistically representative group of fishes, tagged with diet group origin, were pooled in a 4000L tank and exposed to salmon lice copepodites whereafter lice infestation was enumerated. Salmon from all four diet groups had good growth with SGR values from 1.29 to 1.44% day-1 (increase from ca. 130 g to 350 g). At the termination of the experiment the number of lice on salmon offered diatom feed were statistically significantly lower than on salmon fed the other diets. Mean lice infestation values increased from diatom feed through Calanus and fish oil to standard feed with terrestrial plant ingredients. Analysis of the chemical composition of the different diets (fatty acids, amino acids) failed to explain the differences in lice infestation. The only notable result was that diatom and Calanus feed contained more FFA (free fatty acids) than feed with fish oil and the control feed. None of the potential deleterious targeted polyunsaturated aldehydes could be detected in skin samples of the salmon. What was exclusive for salmon that experienced reduced lice was diatom inclusion in the feed. This therefore still indicates the presence of some lice deterring ingredient, either in the feed, or an ingredient can have triggered production of an deterrent in the fish. An obvious follow up of this will be to perform experiments with different degrees of diatom inclusion in the feeds, i.e. dose response experiments combined with targeted PUA analyses, as well as to perform large scale experiments under natural conditions in aquaculture pens.

Parasite-Modified Chemical Communication: Implications for Aquatic Community Dynamics

Chemical communication within an aquatic environment creates an intricate signaling web that provides species with information about their surroundings. Signaling molecules, like oxylipins, mediate a multitude of interactions between free-living members of a community including non-consumptive effects by predators. Parasites are another source of signaling molecules in aquatic communities and contribute directly by synthesizing them or indirectly by manipulating host chemical cues. If chemical cues of infected hosts are altered, then non-consumptive interactions between other members of the community may also be affected. Different cues from infected hosts may alter behaviors in other individuals related to foraging, competition, and defense priming. Here, we discuss how parasites could modify host chemical cues, which may have far reaching consequences for other community members and the ecosystem. We discuss how the modification of signaling molecules by parasites may also represent a mechanism for parasite-modified behavior within some systems and provide a mechanism for non-consumptive effects of parasites. Further, we propose a host-parasite system that could be used to investigate some key, unanswered questions regarding the relationship between chemical cues, parasite-modified behavior, and non-consumptive effects. We explain how trematode-gastropod systems can be used to test whether there are alterations in the diversity and amounts of signaling molecules available, and if habitat use, immune function, and behavior of other individuals and species are affected. Finally, we argue that changes to pathway crosstalk by parasites within communities may have broad ecological implications.

Lipid yield from the diatom Porosira glacialis is determined by solvent choice and number of extractions, independent of cell disruption

Cell wall disruption is necessary to maximize lipid extraction yields in conventional species of mass-cultivated microalgae. This study investigated the effect of sonication, solvent choice and number of extractions on the lipid yield, lipid class composition and fatty acid composition of the diatom Porosira glacialis. For comparison, the diatom Odontella aurita and green alga Chlorella vulgaris were included in the study. Sonication effectively disrupted P. glacialis cells, but did not increase the total lipid yield compared to physical stirring (mixing). In all three microalgae, the content of membrane-associated glyco- and phosopholipids in the extracted lipids was strongly dependent on the solvent polarity. A second extraction resulted in higher yields from the microalgae only when polar solvents were used. In conclusion, choice of solvent and number of extractions were the main factors that determined lipid yield and lipid class composition in P. glacialis.

Distinctive Growth and Transcriptional Changes of the Diatom Seminavis robusta in Response to Quorum Sensing Related Compounds

In aquatic habitats, diatoms are frequently found in association with Proteobacteria, many members of which employ cell-to-cell communication via N-acyl homoserine lactones (AHLs). It has been suggested that diatoms could distinguish between beneficial and algicidal bacteria in their surroundings by sensing AHLs. Although some microalgae can interfere with AHL signaling, e.g., by releasing AHL mimics or degrading them, molecular responses to AHLs in microalgae are still unclear. Therefore, we tested the effects of short-chained AHLs, i.e., N-hexanoyl homoserine lactone (C6-HSL), N-3-hydroxyhexanoyl homoserine lactone (OH-C6-HSL), and N-3-oxohexanoyl homoserine lactone (oxo-C6-HSL) and long-chained AHLs, i.e., N-tetradecanoyl homoserine lactone (C14-HSL), N-3-hydroxytetradecanoyl homoserine lactone (OH-C14-HSL), and N-3-oxotetradecanoyl homoserine lactone (oxo-C14-HSL), on growth of the benthic diatom Seminavis robusta. All tested short-chained AHLs did not affect diatom growth, while long-chained AHLs promoted (C14-HSL) or inhibited (OH-C14-HSL and oxo-C14-HSL) growth. To investigate the physiological effects of these long-chained AHLs in more detail, an RNA-seq experiment was performed during which S. robusta was treated with the growth-promoting C14-HSL and the growth-inhibiting oxo-C14-HSL. One tetramic acid was also tested (TA14), a structural rearrangement product of oxo-C14-HSL, which also induced growth inhibition in S. robusta. After 3 days of treatment, analysis revealed that 3,410 genes were differentially expressed in response to at least one of the compounds. In the treatment with the growth-promoting C14-HSL many genes involved in intracellular signaling were upregulated. On the other hand, exposure to growth-inhibiting oxo-C14-HSL and TA14 triggered a switch in lipid metabolism towards increased fatty acid degradation. In addition, oxo-C14-HSL led to downregulation of cell cycle genes, which is in agreement with the stagnation of cell growth in this treatment. Combined, our results indicate that bacterial signaling molecules with high structural similarity induce contrasting physiological responses in S. robusta.

Combined Effects of Diatom-Derived Oxylipins on the Sea Urchin Paracentrotus lividus

Oxylipins are diatom-derived secondary metabolites, deriving from the oxidation of polyunsatured fatty acids that are released from cell membranes after cell damage or senescence of these single-celled algae. Previous results revealed harmful toxic effects of polyunsaturated aldehydes (PUAs) and hydroxyacids (HEPEs) on sea urchin Paracentrotus lividus embryonic development by testing individual compounds and mixtures of the same chemical group. Here, we investigated the combined effects of these compounds on sea urchin development at the morphological and molecular level for the first time. Our results demonstrated that oxylipin mixtures had stronger effects on sea urchin embryos compared with individual compounds, confirming that PUAs induce malformations and HEPEs cause developmental delay. This harmful effect was also confirmed by molecular analysis. Twelve new genes, involved in stress response and embryonic developmental processes, were isolated from the sea urchin P. lividus; these genes were found to be functionally interconnected with 11 genes already identified as a stress response of P. lividus embryos to single oxylipins. The expression levels of most of the analyzed genes targeted by oxylipin mixtures were involved in stress, skeletogenesis, development/differentiation, and detoxification processes. This work has important ecological implications, considering that PUAs and HEPEs represent the most abundant oxylipins in bloom-forming diatoms, opening new perspectives in understanding the molecular pathways activated by sea urchins exposed to diatom oxylipins.

Lipoxygenase Pathways in Diatoms: Occurrence and Correlation with Grazer Toxicity in Four Benthic Species

Oxygenated derivatives of fatty acids, collectively called oxylipins, are a highly diverse family of lipoxygenase (LOX) products well described in planktonic diatoms. Here we report the first investigation of these molecules in four benthic diatoms, Cylindrotheca closterium, Nanofrustulum shiloi, Cocconeis scutellum, and Diploneis sp. isolated from the leaves of the seagrass Posidonia oceanica from the Gulf of Naples. Analysis by hyphenated MS techniques revealed that C. closterium, N. shiloi, and C. scutellum produce several polyunsaturated aldehydes (PUAs) and linear oxygenated fatty acids (LOFAs) related to the products of LOX pathways in planktonic species. Diploneis sp. also produced other unidentified fatty acid derivatives that are not related to LOX metabolism. The levels and composition of oxylipins in the benthic species match their negative effects on the reproductive success in the sea urchin Paracentrotus lividus. In agreement with this correlation, the most toxic species N. shiloi revealed the same LOX pathways of Skeletonema marinoi and Thalassiosira rotula, two bloom-forming planktonic diatoms that affect copepod reproduction. Overall, our data highlight for the first time a major role of oxylipins, namely LOFAs, as info-chemicals for benthic diatoms, and open new perspectives in the study of the structuring of benthic communities.

Cell Death in Evolutionary Transitions in Individuality

Programmed cell death (PCD) in cell groups and microbial communities affects population structures, nutrient recycling, and sociobiological interactions. A less explored area is the role played by PCD in the emergence of higher-level individuals. Here, we examine how cell death impacted evolutionary transitions in individuality (ETIs). The focus is on three specific ETIs - the emergence of the eukaryote cell, multicellularity, and social insects - and we review the theoretical and empirical evidence for the role of PCD in these three transitions. We find that PCD likely contributed to many of the processes involved in eukaryogenesis and the transition to multicellularity. PCD is important for the formation of cooperative groups and is a mechanism by which mutual dependencies between individuals evolve. PCD is also a conflict mediator and involved in division of labor in social groups and in the origin of new cell types. In multicellularity, PCD facilitates the transfer of fitness to the higher-level individual. In eusocial insects, PCD of the gonadal cells in workers is the basis for conflict mediation and the division of labor in the colony. In the three ETIs discussed here, PCD likely played an essential role, without which alternate mechanisms would have been necessary for these increases in complexity to occur.

Nitric oxide mediates oxylipin production and grazing defense in diatoms

Diatom blooms are important features of productive marine ecosystems and are known to support higher trophic levels. However, when stressed or wounded, diatoms can produce oxylipin molecules known to inhibit the reproduction and development of copepods and decrease microzooplankton growth rates. Using oxylipin chemical treatments, lipidomic analysis and functional genomic approaches, we provide evidence that nitric oxide (NO) and oxylipin signalling pathways in diatoms respond to protist grazers, resulting in increased defence fitness and survival. Exposure of the diatom Phaeodactylum tricornutum to the dinoflagellate Oxyrrhis marina resulted in NO production by P. tricornutum and pronounced change in its dissolved oxylipin profile. Experimentally elevating levels of NO also resulted in increased oxylipin production, and lower overall grazing rates. Furthermore, O. marina preferentially grazed on P. tricornutum prey with lower levels of NO, suggesting that this molecule and its effect on oxylipin pathways play a key role in prey selection. Exposure of O. marina grazing on P. tricornutum to exogenous oxylipins also decreased grazing rates, which is consistent with a grazing deterrence role for these molecules. These results suggest that NO and oxylipin production help to structure diatom communities, in part by modulating interactions with microzooplankton predators.

trans, trans-2,4-Decadienal, a lipid peroxidation product, induces inflammatory responses via Hsp90- or 14-3-3ζ-dependent mechanisms

Peroxidation of polyunsaturated fatty acids leads to the formation of a large array of lipid-derived electrophiles (LDEs), many of which are important signaling molecules involved in the pathogenesis of human diseases. Previous research has shown that one of such LDEs, trans, trans-2,4-decadienal (tt-DDE), increases inflammation, however, the underlying mechanisms are not well understood. Here we used click chemistry-based proteomics to identify the cellular targets which are required for the pro-inflammatory effects of tt-DDE. We found that treatment with tt-DDE increased cytokine production, JNK phosphorylation, and activation of NF-κB signaling in macrophage cells, and increased severity of dextran sulfate sodium (DSS)-induced colonic inflammation in mice, demonstrating its pro-inflammatory effects in vitro and in vivo. Using click chemistry-based proteomics, we found that tt-DDE directly interacts with Hsp90 and 14-3-3ζ, which are two important proteins involved in inflammation and tumorigenesis. Furthermore, siRNA knockdown of Hsp90 or 14-3-3ζ abolished the pro-inflammatory effects of tt-DDE in macrophage cells. Together, our results support that tt-DDE increases inflammatory responses via Hsp90- and 14-3-3ζ-dependent mechanisms.

Density-dependent oxylipin production in natural diatom communities: possible implications for plankton dynamics

Oxylipins are important signal transduction lipoxygenase-derived products of fatty acids that regulate a variety of physiological and pathological processes in plants and animals. In marine diatoms, these molecules can be highly bioactive, impacting zooplankton grazers, bacteria and other phytoplankton. However, the ultimate cause for oxylipin production in diatoms is still poorly understood, from an evolutionary perspective. Here we analysed production of particulate linear oxygenated fatty acids (LOFAs, previously named non-volatile oxylipins) from natural phytoplankton collected weekly for 1 year. We demonstrate for the first time that diatoms are the main LOFA producers in natural phytoplankton assemblages. Interestingly, LOFA-per-cell production decreased with increasing diatom density and was not due to major changes in diatom community composition. An inverse relation was confirmed at a global scale by analysing diatom lipoxygenase unigenes and metagenomes from Tara Oceans datasets. A network analysis suggested that different LOFAs could contribute to modulate co-variations of different diatom taxa. Overall, we offer new insights in diatom chemical ecology, possibly explaining the evolution of oxylipin synthesis in diatoms.

Microalgal Enzymes with Biotechnological Applications

Enzymes are essential components of biological reactions and play important roles in the scaling and optimization of many industrial processes. Due to the growing commercial demand for new and more efficient enzymes to help further optimize these processes, many studies are now focusing their attention on more renewable and environmentally sustainable sources for the production of these enzymes. Microalgae are very promising from this perspective since they can be cultivated in photobioreactors, allowing the production of high biomass levels in a cost-efficient manner. This is reflected in the increased number of publications in this area, especially in the use of microalgae as a source of novel enzymes. In particular, various microalgal enzymes with different industrial applications (e.g., lipids and biofuel production, healthcare, and bioremediation) have been studied to date, and the modification of enzymatic sequences involved in lipid and carotenoid production has resulted in promising results. However, the entire biosynthetic pathways/systems leading to synthesis of potentially important bioactive compounds have in many cases yet to be fully characterized (e.g., for the synthesis of polyketides). Nonetheless, with recent advances in microalgal genomics and transcriptomic approaches, it is becoming easier to identify sequences encoding targeted enzymes, increasing the likelihood of the identification, heterologous expression, and characterization of these enzymes of interest. This review provides an overview of the state of the art in marine and freshwater microalgal enzymes with potential biotechnological applications and provides future perspectives for this field.

Associated Bacteria Affect Sexual Reproduction by Altering Gene Expression and Metabolic Processes in a Biofilm Inhabiting Diatom

Diatoms are unicellular algae with a fundamental role in global biogeochemical cycles as major primary producers at the base of aquatic food webs. In recent years, chemical communication between diatoms and associated bacteria has emerged as a key factor in diatom ecology, spurred by conceptual and technological advancements to study the mechanisms underlying these interactions. Here, we use a combination of physiological, transcriptomic, and metabolomic approaches to study the influence of naturally co-existing bacteria, Maribacter sp. and Roseovarius sp., on the sexual reproduction of the biofilm inhabiting marine pennate diatom Seminavis robusta. While Maribacter sp. severely reduces the reproductive success of S. robusta cultures, Roseovarius sp. slightly enhances it. Contrary to our expectation, we demonstrate that the effect of the bacterial exudates is not caused by altered cell-cycle regulation prior to the switch to meiosis. Instead, Maribacter sp. exudates cause a reduced production of diproline, the sexual attraction pheromone of S. robusta. Transcriptomic analyses show that this is likely an indirect consequence of altered intracellular metabolic fluxes in the diatom, especially those related to amino acid biosynthesis, oxidative stress response, and biosynthesis of defense molecules. This study provides the first insights into the influence of bacteria on diatom sexual reproduction and adds a new dimension to the complexity of a still understudied phenomenon in natural diatom populations.

Production and fate of fishy odorants produced by two freshwater chrysophyte species under different temperature and light conditions

Fishy odor has become one of the most often encountered aesthetic water quality problems in drinking water. While fresh water algae living in colder water can produce offensive fishy odors, their environmental behaviors remain poorly understood. In this study, two chrysophyte species (Synura uvella and Ochromonas sp.), which are often associated with fishy odor events, were selected to investigate the effect of temperature (8, 16, and 24 °C) and light intensity (10, 41, and 185 μmol photons m^-2 s^-1) on algal growth and odorant production. Five polyunsaturated aldehyde derivatives, including 2,4-heptadienal, 2-octenal, 2,4-octadienal, 2,4-decadienal, and 2,4,7-dectridienal, were identified as fishy/cod liver oil/fatty/rancid descriptors in the cultures of the two algae based on gas chromatography-olfactometry-mass spectrometry and comprehensive two-dimensional gas chromatography mass spectrometry. While biomass yield increased with the increase in temperature for both species, significantly higher odorant yields (production of odorants per cell) were obtained at 8 °C. The total odorant production and cell yield of the odorants decreased with the increase in light intensity from 10 to 185 μmol photons m^-2 s^-1. The biodegradation half-lives for the released odorants were 6-10 h at 8 °C and 2-4 h at 24 °C, whereas the volatilization half-lives were 36-97 d at 8 °C and 6-17 d at 24 °C, suggesting that temperature-dependent biodegradation was an important factor controlling the fate of fishy compounds in aquatic environments. The results of this study will help clarify why most fishy odor events occur in cooler seasons, and provide knowledge related to cold water persistence for the management of fishy odor problems associated with algae.

Why Algae Release Volatile Organic Compounds-The Emission and Roles

A wide spectrum of volatile organic compounds (VOCs) are released from algae in aquatic ecosystems. Environmental factors such as light, temperature, nutrition conditions and abiotic stresses affect their emission. These VOCs can enhance the resistance to abiotic stresses, transfer information between algae, play allelopathic roles, and protect against predators. For homogeneous algae, the VOCs released from algal cells under stress conditions transfer stress information to other cells, and induce the acceptors to make a preparation for the upcoming stresses. For heterogeneous algae and aquatic macrophytes, the VOCs show allelopathic effects on the heterogeneous neighbors, which benefit to the emitter growth and competing for nutrients. In cyanobacterial VOCs, some compounds such as limonene, eucalyptol, β-cyclocitral, α-ionone, β-ionone and geranylacetone have been detected as the allelopathic agents. In addition, VOCs can protect the emitters from predation by predators. It can be speculated that the emission of VOCs is critical for algae coping with the complicated and changeable aquatic ecosystems.

Molecular and Morphological Toxicity of Diatom-Derived Hydroxyacid Mixtures to Sea Urchin Paracentrotus lividus Embryos

Oxylipins such as polyunsaturated aldehydes (PUAs) and hydroxyacids (HEPEs) are signaling molecules derived from the oxidation of polyunsaturated fatty acids. They are common in diatoms that constitute a major group of microalgae in freshwater and oceanic ecosystems. Although HEPEs represent the most common oxylipins produced by diatoms, little information is available on their effects on marine invertebrates, and most of the information has been obtained by testing individual HEPEs. Our previous studies reported that four hydroxyacids, i.e., 5-, 9-, 11-, and 15-HEPE, were able to induce malformations and a marked developmental delay in sea urchin Paracentrotus lividus embryos, which had not been reported for other oxylipins. Here, we tested a mixture of 5-, 9-, 11-, and 15-HEPE at different concentrations for the first time. The results showed that mixtures of HEPEs have synergistic effects that are much more severe compared to those of individual HEPEs: The HEPE mixtures induced malformations in sea urchin embryos at lower concentrations. Increasing HEPE mixture concentrations induced a marked increase in the number of delayed embryos, until all embryos were delayed at the highest concentration tested. At the molecular level, the HEPE mixtures induced variations in the expression of 50 genes involved in different functional processes, mainly down-regulating these genes at the earliest stages of embryonic development. These findings are ecologically significant, considering that during diatom blooms, sea urchins could accumulate HEPEs in concentrations comparable to those tested in the present study.

Algal Oxylipins Mediate the Resistance of Diatoms against Algicidal Bacteria

Algicidal bacteria can lyse microalgal blooms and trigger shifts within plankton communities. Resistant algal species can escape lysis, and have the opportunity to dominate the phytoplankton after a bacterial infection. Despite their important function in ecosystem regulation, little is known about mechanisms of resistance. Here, we show that the diatom Chaetoceros didymus releases eicosanoid oxylipins into the medium, and that the lytic algicidal bacterium, Kordia algicida, induces the production of several wound-activated oxylipins in this resistant diatom. Neither releases nor an induction occurs in the susceptible diatom Skeletonema costatum that is lysed by the bacterium within a few days. Among the upregulated oxylipins, hydroxylated eicosapentaenoic acids (HEPEs) dominate. However, also, resolvins, known lipid mediators in mammals, increase upon exposure of the algae to the algicidal bacteria. The prevailing hydroxylated fatty acid, 15-HEPE, significantly inhibits growth of K. algicida at a concentration of approximately 1 µM. The oxylipin production may represent an independent line of defense of the resistant alga, acting in addition to the previously reported upregulation of proteases.

Patatin-like lipolytic acyl hydrolases and galactolipid metabolism in marine diatoms of the genus Pseudo-nitzschia

Diatoms are eukaryotic microalgae that play a pivotal role in biological and geochemical marine cycles. These microorganisms are at the basis of the trophic chain and their lipids are essential components (e.g. eicosapentaenoic acid, EPA) of aquatic food webs. Galactolipids are the primary lipid components of plastid membranes and form the largest lipid family of diatoms. As source of polyunsaturated fatty acids (PUFAs), these compounds are also involved in the synthesis of lipoxygenase (LOX) products such as non-volatile oxylipins and polyunsaturated aldehydes. Here, we report the first identification of two genes, namely PmLAH1 and PaLAH1, coding for lipolytic enzymes in two diatoms of the genus Pseudo-nitzschia. Functional and modeling studies evidence a patatin-like domain endowed with galactolipase and phospholipase activity at the C-terminus of both proteins. Homologues of Pseudo-nitzschia LAH1 genes were retrieved in other diatom species so far sequenced in agreement with conservation of the functional role of these proteins within the lineage.

Lipoxygenases and Lipoxygenase Products in Marine Diatoms

Marine diatoms negatively affect reproduction and later larval development of dominant zooplankton grazers such as copepods, thereby lowering the recruitment of the next generations of these small crustaceans that are a major food source for larval fish species. The phenomenon has been explained in terms of chemical defense due to grazer-induced synthesis of oxylipins, lipoxygenase-derived oxygenated fatty acid derivatives. Since this first report, studies about diatom oxylipins have multiplied and broadened toward other aspects concerning bloom dynamics, cell growth, and cell differentiation. Diatom oxylipins embrace a number of diverse structures that are recognized as chemical signals in ecological and physiological processes in many other organisms. In diatoms, the most studied examples include polyunsaturated aldehydes (PUAs) and nonvolatile oxylipins (NVOs). The purpose of this chapter is to provide the analytical tools to deal with identification, analysis and biosynthesis of these compounds. Emphasis is given to identification of the enzymatic steps and characterization of the species-specific lipoxygenases even in absence of the availability of molecular information.

First identification of marine diatoms with anti-tuberculosis activity

Marine microalgae are considered a potentially new and valuable source of biologically active compounds for applications in several biotechnology sectors. They can be easily cultured, have short generation times and enable an environmentally-friendly approach to drug discovery by overcoming problems associated with the over-utilization of marine resources and the use of destructive collection practices. Considering the increasing rate of antibiotic-resistance bacteria and infections by fungi, 46 microalgae have been screened in this study for possible antibacterial and antifungal activities. Two different extraction methods have been used in order to increase the probability of finding positive hits. In particular, we screened microalgae in both control and nutrient stress conditions. We also tested different strains for 7 species in order to study potentially different bioactivities due to strain diversity. Results showed that extracts of two diatoms, Skeletonema costatum and Chaetoceros pseudocurvisetus, had anti-tuberculosis activity and were active only when cultured in the control and phosphate-starvation conditions, while the nitrogen starvation condition showed no activity. In addition, we tested both the organic and water extracts and found that only the organic extracts for both diatoms were active. The organic extracts of these two diatom species were not toxic on normal human cell lines.

Two Types of Volatile Polyenes in the Brown Alga Sargassum thunbergii

An essential oil from the brown alga Sargassum thunbergii, prepared by a simultaneous distillation extraction method, contained in two types of volatile polyenes with a terminal double bond such as (6Z,9Z,12Z,15Z,18Z)-1,6,9,12,15,18-henicosahexaene and (6Z,9Z,12Z,15Z)-1,6,9,12,15-henicosapentaene and with their saturated terminal structures such as (3Z,6Z,9Z,12Z,15Z,18Z)-3,6,9,12,15,18-henicosahexaene and (3Z,6Z,9Z,12Z,15Z)-3,6,9,12,15-henicosapentaene. These volatile polyenes were identified by comparison with the GC-MS and NMR spectra of synthetics. The polyenes with the saturated terminal structures were found in the brown algae for the first time.

Evolution and Expansion of the Prokaryote-Like Lipoxygenase Family in the Brown Alga Saccharina japonica

Lipoxygenase (LOX) plays important roles in fatty acid oxidation and lipid mediator biosynthesis. In this study, we give first insights into brown algal LOX evolution. Whole genome searches revealed four, three, and eleven LOXs in Ectocarpus siliculosus, Cladosiphon okamuranus, and Saccharina japonica, respectively. In phylogenetic analyses, LOXs from brown algae form a robust clade with those from prokaryotes, suggesting an ancestral origin and slow evolution. Brown algal LOXs were divided into two clades, C1 and C2 in a phylogenetic tree. Compared to the two species of Ectocarpales, LOX gene expansion occurred in the kelp S. japonica through tandem duplication and segmental duplication. Selection pressure analysis showed that LOX genes in brown algae have undergone strong purifying selection, while the selective constraint in the C2 clade was more relaxed than that in the C1 clade. Furthermore, within each clade, LOXs of S. japonica evolved under more relaxed selection constraints than E. siliculosus and C. okamuranus. Structural modeling showed that unlike LOXs of plants and animals, which contain a β barrel in the N-terminal part of the protein, LOXs in brown algae fold into a single domain. Analysis of previously published transcriptomic data showed that LOXs in E. siliculosus are responsive to hyposaline, hypersaline, oxidative, and copper stresses. Moreover, clear divergence of expression patterns was observed among different life stages, as well as between duplicate gene pairs. In E. siliculosus, all four LOXs are male-biased in immature gametophytes, and mature gametophytes showed significantly higher LOX mRNA levels than immature gametophytes and sporophytes. In S. japonica, however, our RNA-Seq data showed that most LOXs are highly expressed in sporophytes. Even the most recently duplicated gene pairs showed divergent expression patterns, suggesting that functional divergence has likely occurred since LOX genes duplicated, which potentially contributes to the production of various oxylipins in brown algae.

Field sampling marine plankton for biodiscovery

Microalgae and plankton can be a rich source of bioactivity. However, induction of secondary metabolite production in lab conditions can be difficult. One simple way of bypassing this issue is to collect biomass in the field and screen for bioactivity. Therefore, bulk net samples from three areas along the coast of northern Norway and Spitsbergen were collected, extracted and fractionated. Biomass samples from a strain of a mass-cultivated diatom Porosira glacialis were used as a reference for comparison to field samples. Screening for bioactivity was performed with 13 assays within four therapeutic areas: antibacterial, anticancer, antidiabetes and antioxidation. We analysed the metabolic profiles of the samples using high resolution - mass spectroscopy (HR-MS). Principal component analysis showed a marked difference in metabolite profiles between the field samples and the photobioreactor culture; furthermore, the number of active fractions and extent of bioactivity was different in the field compared to the photobioreactor samples. We found varying levels of bioactivity in all samples, indicating that complex marine field samples could be used to investigate bioactivities from otherwise inaccessible sources. Furthermore, we hypothesize that metabolic pathways that would otherwise been silent under controlled growth in monocultures, might have been activated in the field samples.

PUFAs and PUAs production in three benthic diatoms from the northern Adriatic Sea

The production of polyunsaturated aldehydes (PUAs) has been reported by many planktonic diatoms, where they have been implicated in deleterious effects on copepod reproduction and growth of closeby microbes or suggested as infochemicals in shaping plankton interactions. This study investigates the production of PUAs by diatoms commonly occurring in the microphytobenthic communities in temperate regions: Tabularia affinis, Proschkinia complanatoides and Navicula sp. Results highlight the production of PUAs by the three benthic diatoms during stationary and decline phases, with intracellular concentrations from 1.8 to 154.4 fmol cell^-1, which are within the range observed for planktonic species. The existence of a large family of PUAs, including some with four unsaturations, such as decatetraenal, undecatetraenal and tridecatetraenal, was observed. Since particulate and dissolved PUAs were positively correlated, together with cell lysis, equivalent concentrations may be released during late growth stages, which may affect benthic invertebrates grazing on them and other microalgae.

Molecular details of secretory phospholipase A2 from flax (Linum usitatissimum L.) provide insight into its structure and function

Secretory phospholipase A₂ (sPLA₂) are low molecular weight proteins (12-18 kDa) involved in a suite of plant cellular processes imparting growth and development. With myriad roles in physiological and biochemical processes in plants, detailed analysis of sPLA₂ in flax/linseed is meagre. The present work, first in flax, embodies cloning, expression, purification and molecular characterisation of two distinct sPLA₂s (I and II) from flax. PLA₂ activity of the cloned sPLA₂s were biochemically assayed authenticating them as bona fide phospholipase A₂. Physiochemical properties of both the sPLA₂s revealed they are thermostable proteins requiring di-valent cations for optimum activity.While, structural analysis of both the proteins revealed deviations in the amino acid sequence at C- & N-terminal regions; hydropathic study revealed LusPLA₂I as a hydrophobic protein and LusPLA₂II as a hydrophilic protein. Structural analysis of flax sPLA₂s revealed that secondary structure of both the proteins are dominated by α-helix followed by random coils. Modular superimposition of LusPLA₂ isoforms with rice sPLA₂ confirmed monomeric structural preservation among plant phospholipase A₂ and provided insight into structure of folded flax sPLA₂s.

Molecular details of secretory phospholipase A2 from flax (Linum usitatissimum L.) provide insight into its structure and function

Secretory phospholipase A2 (sPLA2) are low molecular weight proteins (12-18 kDa) involved in a suite of plant cellular processes imparting growth and development. With myriad roles in physiological and biochemical processes in plants, detailed analysis of sPLA2 in flax/linseed is meagre. The present work, first in flax, embodies cloning, expression, purification and molecular characterisation of two distinct sPLA2s (I and II) from flax. PLA2 activity of the cloned sPLA2s were biochemically assayed authenticating them as bona fide phospholipase A2. Physiochemical properties of both the sPLA2s revealed they are thermostable proteins requiring di-valent cations for optimum activity.While, structural analysis of both the proteins revealed deviations in the amino acid sequence at C- & N-terminal regions; hydropathic study revealed LusPLA2I as a hydrophobic protein and LusPLA2II as a hydrophilic protein. Structural analysis of flax sPLA2s revealed that secondary structure of both the proteins are dominated by α-helix followed by random coils. Modular superimposition of LusPLA2 isoforms with rice sPLA2 confirmed monomeric structural preservation among plant phospholipase A2 and provided insight into structure of folded flax sPLA2s.

Lipoxygenase in singlet oxygen generation as a response to wounding: in vivo imaging in Arabidopsis thaliana

Wounding, one of the most intensive stresses influencing plants ontogeny and lifespan, can be induced by herbivory as well as by physical factors. Reactive oxygen species play indispensable role both in the local and systemic defense reactions which enable "reprogramming" of metabolic pathways to set new boundaries and physiological equilibrium suitable for survival. In our current study, we provide experimental evidence on the formation of singlet oxygen (¹O₂) after wounding of Arabidopsis leaves. It is shown that ¹O₂ is formed by triplet-triplet energy transfer from triplet carbonyls to molecular oxygen. Using lipoxygenase inhibitor catechol, it is demonstrated that lipid peroxidation is initiated by lipoxygenase. Suppression of ¹O₂ formation in lox2 mutant which lacks chloroplast lipoxygenase indicates that lipoxygenase localized in chloroplast is predominantly responsible for ¹O₂ formation. Interestingly, ¹O₂ formation is solely restricted to chloroplasts localized at the wounding site. Data presented in this study might provide novel insight into wound-induced signaling in the local defense reaction.

Origin and evolution of group XI secretory phospholipase A2 from flax (Linum usitatissimum) based on phylogenetic analysis of conserved domains

Phospholipase A₂ (PLA₂) belongs to class of lipolytic enzymes (EC 3.1.1.4). Lysophosphatidic acid (LPA) and free fatty acids (FFAs) are the products of PLA₂ catalyzed hydrolysis of phosphoglycerides at sn-2 position. LPA and FFA that act as second mediators involved in the development and maturation of plants and animals. Mining of flax genome identified two phospholipase A₂ encoding genes, viz., LusPLA ₂ I and LusPLA ₂ II (Linum usitatissimum secretory phospholipase A₂). Molecular simulation of LusPLA₂s with already characterized plant sPLA₂s revealed the presence of conserved motifs and signature domains necessary to classify them as secretory phospholipase A₂. Phylogenetic analysis of flax sPLA₂ with representative sPLA₂s from other organisms revealed that they evolved rapidly via gene duplication/deletion events and shares a common ancestor. Our study is the first report of detailed phylogenetic analysis for secretory phospholipase A₂ in flax. Comparative genomic analysis of two LusPLA₂s with earlier reported plant sPLA₂s, based on their gene architectures, sequence similarities, and domain structures are presented elucidating the uniqueness of flax sPLA₂.

Defense related decadienal elicits membrane lipid remodeling in the diatom Phaeodactylum tricornutum

Diatoms rapidly release extracellular oxylipins (oxygenated lipids) including polyunsaturated aldehydes in response to herbivory and other stresses. Oxylipins have several defense-related activities including inhibition of reproduction in herbivores and signaling to distant diatoms. Physiological changes in diatoms exposed to varying levels of oxylipins are only beginning to be understood. In this study, Phaeodactylum tricornutum cultures were treated with sublethal concentrations of the polyunsaturated aldehyde trans,trans-2,4-decadienal (DD) to assess effects on lipid composition and membrane permeability. In cells treated with DD for 3 hr, all measured saturated and unsaturated fatty acids significantly decreased (0.46–0.69 fold of levels in solvent control cells) except for 18:2 (decreased but not significantly). The decrease was greater in the polyunsaturated fatty acid pool than the saturated and monounsaturated fatty acid pool. Analysis of lipid classes revealed increased abundances of phosphatidylethanolamine and phosphatidylcholine at 3 and 6 hr. Concomitantly, these and other membrane lipids exhibited increased saturated and monounsaturated acyl chains content relative to polyunsaturated acyl chains compared to control cells. Evidence of decreased plasma membrane permeability in DD treated cells was obtained, based on reduced uptake of two of three dyes relative to control cells. Additionally, cells pre-conditioned with a sublethal DD dose for 3 hr then treated with a lethal DD dose for 2 hr exhibited greater membrane integrity than solvent pre-conditioned control cells that were similarly treated. Taken together, the data are supportive of the hypothesis that membrane remodeling induced by sublethal DD is a key element in the development of cellular resistance in diatoms to varying and potentially toxic levels of polyunsaturated aldehydes in environments impacted by herbivory or other stresses.

Light and temperature effects on bioactivity in diatoms

Isolates of five pelagic North Atlantic marine diatoms (Bacillariophyceae): Attheya longicornis, Chaetoceros socialis, Chaetoceros furcellatus, Skeletonema marinoi and Porosira glacialis were cultivated in large photobioreactors at two light and two temperature regimes to test if this affected bioactivity. We screened for bioactivity in assays representing five different therapeutic areas: diabetes II (PTP1b), cancer (melanoma cells, A2058), anti-oxidants (FRAP), immunomodulation (TNFa) and anti-infection (MRSA, Enterococcus faecalis, Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa). All the diatom strains showed activity in two or more assays. We detected differences in bioactivity both between species and within species cultivated with different light and temperature regimes. Our results demonstrate the potential for a more exhaustive exploitation of diatom metabolites that can be obtained by manipulation of the cultivation conditions.

Specific Metabolites in a Phaeodactylum tricornutum Strain Isolated from Western Norwegian Fjord Water

We have searched for special characteristics in growth, protein expression, fatty acids and volatile organic compounds (VOCs) in a local Phaeodactylum tricornutum Bohlin strain (Bergen Marine Biobank), by comparing it with a common accession strain (CCAP). Differences in growth and expressed proteins were detected between the BMB strain and the CCAP strain, and the BMB strain reached the highest cell densities under the given growth conditions. Fatty acid (FA) analyses showed highest relative eicosapentaenoic acid (EPA) levels in the exponential phase (25.73% and 28.31%), and highest levels of palmitoleic acid (16:1 n-7) in the stationary phase (46.36% and 43.66%) in the BMB and CCAP strain, respectively. The most striking finding of the VOCs analyses was the relatively high levels of ectocarpene, 6-((1E)-butenyl)-1,4-cycloheptadiene, hormosirene, and desmarestene and structurally related compounds, which were exclusively detected in the BMB strain. Many of the VOCs detected in the CCAP and, in particular, in the BMB strain have been reported as antimicrobial agents. We suggest that the array of pheromones and antimicrobial substances could be part of an allelopathic strategy of the BMB strain, dominated by oval cells, thus reflecting the benthic life stage of this morphological form. These findings show the potential for bioactive metabolites in the BMB strain.

Regenerative Astaxanthin Extraction from a Single Microalgal (Haematococcus pluvialis) Cell Using a Gold Nano-Scalpel

Milking of microalgae, the process of reusing the biomass for continuous production of target compounds, can strikingly overcome the time and cost constraints associated with biorefinery. This process can significantly improve production efficiency of highly valuable chemicals, for example, astaxanthin (AXT) from Haematococcus pluvialis. Detailed understanding of the biological process of cell survival and AXT reaccumulation after extraction would be of great help for successful milking. Here we report extraction of AXT from a single cell of H. pluvialis through incision of the cell wall by a gold nanoscalpel (Au-NS), which allows single-cell analysis of wound healing and reaccumulation of AXT. Interestingly, upon the Au-NS incision, the cell could reaccumulate AXT at a rate two times faster than the control cells. Efficient extraction as well as minimal cellular damage, keeping cells alive, could be achieved with the optimized shape and dimensions of Au-NS: a well-defined sharp tip, thickness under 300 nm, and 1-3 μm of width. The demonstration of regenerative extraction of AXT at a single cell level hints toward the potential of a milking process for continuous recovery of target compounds from microalgae while keeping the cells alive.

Determination of Lipid Hydroperoxides in Marine Diatoms by the FOX2 Assay

Ecologically-relevant marine diatoms produce a plethora of bioactive oxylipins deriving from fatty acid oxidation, including aldehydes, hydroxy-fatty acids, epoxy-hydroxy-fatty acids, and oxo-acids. These secondary metabolites have been related to the negative effect of diatoms on copepod reproduction, causing low hatching success and teratogenesis in the offspring during periods of intense diatom blooms. The common intermediates in the formation of oxylipins are fatty acid hydroperoxides. The quantitative measurement of these intermediates can fundamentally contribute to understanding the function and role of lipoxygenase metabolites in diatom-copepod interactions. Here, we describe the successful adaptation of the ferrous oxidation-xylenol orange 2 (FOX2) assay to diatom samples, which showed several advantages over other spectrophotometric and polarographic methods tested in the present work. Using this method we assessed fatty acid hydroperoxide levels in three diatom species: Skeletonema marinoi, Thalassiosira rotula, and Chaetoceros affinis, and discuss results in light of the literature data on their detrimental effects on copepod reproduction.

Metatranscriptomes reveal functional variation in diatom communities from the Antarctic Peninsula

Functional genomics of diatom-dominated communities from the Antarctic Peninsula was studied using comparative metatranscriptomics. Samples obtained from diatom-rich communities in the Bransfield Strait, the western Weddell Sea and sea ice in the Bellingshausen Sea/Wilkins Ice Shelf yielded more than 500K pyrosequencing reads that were combined to produce a global metatranscriptome assembly. Multi-gene phylogenies recovered three distinct communities, and diatom-assigned contigs further indicated little read-sharing between communities, validating an assembly-based annotation and analysis approach. Although functional analysis recovered a core of abundant shared annotations that were expressed across the three diatom communities, over 40% of annotations (but accounting for <10% of sequences) were community-specific. The two pelagic communities differed in their expression of N-metabolism and acquisition genes, which was almost absent in post-bloom conditions in the Weddell Sea community, while enrichment of transporters for ammonia and urea in Bransfield Strait diatoms suggests a physiological stance towards acquisition of reduced N-sources. The depletion of carbohydrate and energy metabolism pathways in sea ice relative to pelagic communities, together with increased light energy dissipation (via LHCSR proteins), photorespiration, and NO3(-) uptake and utilization all pointed to irradiance stress and/or inorganic carbon limitation within sea ice. Ice-binding proteins and cold-shock transcription factors were also enriched in sea ice diatoms. Surprisingly, the abundance of gene transcripts for the translational machinery tracked decreasing environmental temperature across only a 4 °C range, possibly reflecting constraints on translational efficiency and protein production in cold environments.

The molecular ecophysiology of programmed cell death in marine phytoplankton

Planktonic, prokaryotic, and eukaryotic photoautotrophs (phytoplankton) share a diverse and ancient evolutionary history, during which time they have played key roles in regulating marine food webs, biogeochemical cycles, and Earth's climate. Because phytoplankton represent the basis of marine ecosystems, the manner in which they die critically determines the flow and fate of photosynthetically fixed organic matter (and associated elements), ultimately constraining upper-ocean biogeochemistry. Programmed cell death (PCD) and associated pathway genes, which are triggered by a variety of nutrient stressors and are employed by parasitic viruses, play an integral role in determining the cell fate of diverse photoautotrophs in the modern ocean. Indeed, these multifaceted death pathways continue to shape the success and evolutionary trajectory of diverse phytoplankton lineages at sea. Research over the past two decades has employed physiological, biochemical, and genetic techniques to provide a novel, comprehensive, mechanistic understanding of the factors controlling this key process. Here, I discuss the current understanding of the genetics, activation, and regulation of PCD pathways in marine model systems; how PCD evolved in unicellular photoautotrophs; how it mechanistically interfaces with viral infection pathways; how stress signals are sensed and transduced into cellular responses; and how novel molecular and biochemical tools are revealing the impact of PCD genes on the fate of natural phytoplankton assemblages.

Potential polyunsaturated aldehydes in the Strait of Gibraltar under two tidal regimes

Diatoms, a major component of the large-sized phytoplankton, are able to produce and release polyunsaturated aldehydes after cell disruption (potential PUAs or pPUA). These organisms are dominant in the large phytoplankton fraction (>10 µm) in the Strait of Gibraltar, the only connection between the Mediterranean Sea and the Atlantic Ocean. In this area, the hydrodynamics exerts a strong control on the composition and physiological state of the phytoplankton. This environment offers a great opportunity to analyze and compare the little known distribution of larger sized PUA producers in nature and, moreover, to study how environmental variables could affect the ranges and potential distribution of these compounds. Our results showed that, at both tidal regimes studied (Spring and Neap tides), diatoms in the Strait of Gibraltar are able to produce three aldehydes: Heptadienal, Octadienal and Decadienal, with a significant dominance of Decadienal production. The PUA released by mechanical cell disruption of large-sized collected cells (pPUA) ranged from 0.01 to 12.3 pmol from cells in 1 L, and from 0.1 to 9.8 fmol cell⁻¹. Tidal regime affected the abundance, distribution and the level of physiological stress of diatoms in the Strait. During Spring tides, diatoms were more abundant, usually grouped nearer the coastal basin and showed less physiological stress than during Neap tides. Our results suggest a significant general increase in the pPUA productivity with increasing physiological stress for the cell also significantly associated to low nitrate availability.

Impact of the diatom-derived polyunsaturated aldehyde 2-trans,4-trans decadienal on the feeding, survivorship and reproductive success of the calanoid copepod Temora stylifera

Many diatoms, a major class of unicellular algae contributing to over 45% of oceanic primary production, are known to induce deleterious effects on reproductive processes in crustacean copepods. This is mainly due to the production of teratogenic oxylipins, including polyunsaturated aldehydes (PUAs). Here we tested the direct effect of the PUA 2E,4E-decadienal (DD) on feeding activity, survivorship and reproductive success of the calanoid copepod Temora stylifera. DD-inoculated cultures induced high mortality at concentrations above 3 μg mL(-1) compared to controls in both males and females, with males having a higher mortality. Low DD concentrations triggered an increase in female filtration and ingestion rates. Egg production rates and hatching times were also higher in the presence of DD, whereas egg hatching success decreased with increasing DD concentration. Our study shows, for the first time, that the presence of diatom PUAs may increase feeding rates in copepods.