Allelopathic interactions between the benthic toxic dinoflagellate Ostreopsis cf. ovata and a co-occurring diatom

Ecophysiological responses of Ostreopsis towards temperature: A case study of benthic HAB facing ocean warming

Reports of the benthic dinoflagellate Ostreopsis spp. have been increasing in the last decades, especially in temperate areas. In a context of global warming, evidences of the effects of increasing sea temperatures on its physiology and its distribution are still lacking and need to be investigated. In this study, the influence of temperature on growth, ecophysiology and toxicity was assessed for several strains of O. cf. siamensis from the Bay of Biscay (NE Atlantic) and O. cf. ovata from NW Mediterranean Sea. Cultures were acclimated to temperatures ranging from 14.5 °C to 32 °C in order to study the whole range of each strain-specific thermal niche. Acclimation was successful for temperatures ranging from 14.5 °C to 25 °C for O. cf. siamensis and from 19 °C to 32 °C for O. cf. ovata, with the highest growth rates measured at 22 °C (0.54-1.06 d-1) and 28 °C (0.52-0.75 d-1), respectively. The analysis of cellular content of pigments and lipids revealed some aspects of thermal acclimation processes in Ostreopsis cells. Specific capacities of O. cf. siamensis to cope with stress of cold temperatures were linked with the activation of a xanthophyll cycle based on diadinoxanthin. Lipids (neutral reserve lipids and polar ones) also revealed species-specific variations, with increases in cellular content noted under extreme temperature conditions. Variations in toxicity were assessed through the Artemia franciscana bioassay. For both species, a decrease in toxicity was observed when temperature dropped under the optimal temperature for growth. No PLTX-like compounds were detected in O. cf. siamensis strains. Thus, the main part of the lethal effect observed on A. franciscana was dependent on currently unknown compounds. From a multiclonal approach, this work allowed for defining specificities in the thermal niche and acclimation strategies of O. cf. siamensis and O. cf. ovata towards temperature. Potential impacts of climate change on the toxic risk associated with Ostreopsis blooms in both NW Mediterranean Sea and NE Atlantic coast is further discussed, taking into account variations in the geographic distribution, growth abilities and toxicity of each species.

Algal blooms in the ocean: hot spots for chemically mediated microbial interactions

The cycling of major nutrients in the ocean is affected by large-scale phytoplankton blooms, which are hot spots of microbial life. Diverse microbial interactions regulate bloom dynamics. At the single-cell level, interactions between microorganisms are mediated by small molecules in the chemical crosstalk that determines the type of interaction, ranging from mutualism to pathogenicity. Algae interact with viruses, bacteria, parasites, grazers and other algae to modulate algal cell fate, and these interactions are dependent on the environmental context. Recent advances in mass spectrometry and single-cell technologies have led to the discovery of a growing number of infochemicals - metabolites that convey information - revealing the ability of algal cells to govern biotic interactions in the ocean. The diversity of infochemicals seems to account for the specificity in cellular response during microbial communication. Given the immense impact of algal blooms on biogeochemical cycles and climate regulation, a major challenge is to elucidate how microscale interactions control the fate of carbon and the recycling of major elements in the ocean. In this Review, we discuss microbial interactions and the role of infochemicals in algal blooms. We further explore factors that can impact microbial interactions and the available tools to decipher them in the natural environment.

Towards sustainable diatom biorefinery: Recent trends in cultivation and applications

Diatoms, with their complex cellular architecture, have been recognized as a source of limitless potential. These microbes are common in freshwater and marine habitats and are essential for primary production and carbon sequestration. They are excellent at utilizing nutrients, providing a sustainable method of treating wastewater while also producing biomass rich in beneficial substances like vitamins, carotenoids, polysaccharides, lipids, omega-3 fatty acids, pigments, and novel bioactive molecules. Additionally, they are highly efficient organisms that can be employed to monitor the environment by acting as trustworthy indicators of water quality. This comprehensive review explores the multifaceted applications of diatoms in a variety of fields, such as bioremediation, aquaculture, value-added products, and other applications. The review set out on a path towards greener, more sustainable methods amicable to both industry and the environment by utilizing theenormous diverse biotechnological potentials of diatoms.

Multiply charged ion profiles in the UHPLC-HRMS analysis of palytoxin analogues from Ostreopsis cf. ovata blooms

Analogues of palytoxin (PLTX), one of the most potent marine biotoxins, are produced by some species of the marine dinoflagellates of the genus Ostreopsis. The proliferation of these species in different coastal zones represents a potential threat of seafood poisoning in humans because the produced toxins can be transferred through marine food webs. Thus, the determination of the concentration of PLTX analogues (ovatoxins-OVTXs, ostreocins-OSTs and isobaric PLTX) in different matrices (seawater, marine fauna, etc.) is necessary to protect human health. This study is addressed to overcome some of the challenges that the chemical complexity of these molecules poses to their quantification by ultra-high-performance liquid chromatography high-resolution mass spectrometry-based techniques (UHPLC-HRMS). In particular, the mass spectra of the palytoxin analogues show the presence of a large number of ions (including mono- and multiply charged ions) whose nature, relative abundances and behavior can lead to quantitation errors if the correct ions are not selected. In this work, the variability of the PLTX and OVTX profiles under different instrument conditions, including the use of diverse electrospray generation sources and different quantitation methods, is studied. Moreover, the extraction protocol in seawater containing Ostreopsis sp. ovata cells is also evaluated. The use of a heated electrospray operating at 350 °C and a quantitative method including ions from different multiply charged species provides a more robust and reliable method for overcoming the problems due to the variability in the toxin's mass spectrum profile. A single MeOH : H2O (80 : 20, v/v) extraction is proposed as the best and reliable procedure. The overall method proposed was applied to quantify OVTXs (-a to -g) and iso-PLTX along the 2019 Ostreopsis cf. ovata bloom. The cells contained a total toxin concentration of up to 20.39 pg per cell.

Putative biosynthesis mechanism of the neurotoxin β-N-methylamino-L-alanine in marine diatoms based on a transcriptomics approach

The neurotoxin β-N-methylamino-L-alanine (BMAA) has been presumed as an environmental cause of human neurodegenerative disorders, such as Alzheimer's disease. Marine diatoms Thalassiosira minima are demonstrated here to produce BMAA-containing proteins in axenic culture while the isomer diaminobutyric acid was bacterially produced. In the co-culture with Cyanobacterium aponinum, diatom growth was inhibited but the biosynthesis of BMAA-containing proteins was stimulated up to seven times higher than that of the control group by cell-cell interactions. The stimulation effect was not caused by the cyanobacterial filtrate. Nitrogen deprivation also doubled the BMAA content of T. minima cells. Transcriptome analysis of the diatom in mixed culture revealed that pathways involved in T. minima metabolism and cellular functions were mainly influenced, including KEGG pathways valine and leucine/isoleucine degradation, endocytosis, pantothenate and CoA biosynthesis, and SNARE interactions in vesicular transport. Based on the expression changes of genes related to protein biosynthesis, it was hypothesized that ubiquitination and autophagy suppression, and limited COPII vesicles transport accuracy and efficiency were responsible for biosynthesis of BMAA-containing proteins in T. minima. This study represents a first application of transcriptomics to investigate the biological processes associated with BMAA biosynthesis in diatoms.

Rapid Biotic and Abiotic Transformation of Toxins produced by Ostreopsis. cf. ovata

The dinoflagellate Ostreopsis cf. ovata produces several families of toxic polyketides. Despite only a few field measurements of these phycotoxins in seawater and aerosols, they are believed to be responsible for dermatitis and the toxic inhalations reported during blooms of this species. Therefore, the stability of these compounds in seawater is essential to understanding the causes of these symptoms, however, this has never been assessed. In the current study, the optimization of a solid phase extraction (SPE) procedure was first performed to ensure the most efficient extraction of all phycotoxins known to be produced by this strain, including the recently described liguriatoxins. The SPE cartridge SDBL® under non acidified conditions offered the best option. The stability of the ovatoxins and the liguriatoxins under biotic and abiotic stress was assessed by exposing the spent medium of a culture of Ostreopsis cf. ovata to its bacterial consortium and natural sunlight. A rapid biotic transformation was detected for both families of compounds. When exposed to bacteria, the half-lives of the ovatoxins were reached before 10 h and at 36 h, 97% of these toxins had been transformed. The half-lives of the liguriatoxins were 10 h under these conditions. Photolysis (abiotic degradation) of the ovatoxins (T1/2 < 36 h) was faster than for the liguriatoxins (T1/2 > 62 h). Although none of the catabolites of these phycotoxins were thoroughly identified, an untargeted metabolomics approach combined with molecular networking highlighted the presence of several compounds exhibiting structural similarities with the ovatoxins. Additional work should confirm the preliminary findings on these potential ovatoxins’ catabolites and their biological properties. The rapid transformation of O. cf. ovata’s phycotoxins introduces questions concerning their presence in seawater and their dispersion in the sea spray aerosols. The compounds involved in the toxic inhalations and dermatitis often experienced by beachgoers may stem from the catabolites of these toxins or even unrelated and as yet unidentified compounds.

Environmental, human health and socioeconomic impacts of Ostreopsis spp. Blooms in the NW Mediterranean

This paper summarizes the research conducted by the partners of the EU co-funded CoCliME project to ascertain the ecological, human health and economic impacts of Ostreopsis (mainly O. cf. ovata) blooms in the NW Mediterranean coasts of France, Monaco and Spain. This knowledge is necessary to design strategies to prevent, mitigate and, if necessary, adapt to the impacts of these events in the future and in other regions. Ostreopsis proliferations in the Mediterranean have been related to massive mortalities of benthic organisms and to symptoms of respiratory and cutaneous irritation in humans. A six-year epidemiologic study in a Ostreopsis hot spot in Catalonia and the accumulated experience of the French Mediterranean National Ostreopsis Surveillance Network confirm the main effects of these blooms on human health in the NW Mediterranean. The impacts are associated to direct exposure to seawater with high Ostreopsis cell concentrations and to inhalation of aerosols containing unknown irritative chemicals produced under certain circumstances during the blooms. A series of mild acute symptoms, affecting the entire body as well as the ophthalmic, digestive, respiratory and dermatologic systems have been identified. A main remaining challenge is to ascertain the effects of the chronic exposure to toxic Ostreopsis blooms. Still, the mechanisms involved in the deletereous effects of Ostreopsis blooms are poorly understood. Characterizing the chemical nature of the harmful compounds synthesized by Ostreopsis as well as the role of the mucus by which cells attach to benthic surfaces, requires new technical approaches (e.g., metabolomics) and realistic and standardized ecotoxicology tests. It is also necessary to investigate how palytoxin analogues produced by O. cf. ovata could be transferred through the marine food webs, and to evaluate the real risk of seafood poisonings in the area. On the other hand, the implementation of beach monitoring and surveillance systems in the summer constitutes an effective strategy to prevent the impacts of Ostreopsis on human health. In spite of the confirmed noxious effects, a survey of tourists and residents in Nice and Monaco to ascertain the socioeconomic costs of Ostreopsis blooms indicated that the occurrence of these events and their impacts are poorly known by the general public. In relationship with a plausible near future increase of Ostreopsis blooms in the NW Mediterranean coast, this survey showed that a substantial part of the population might continue to go to the beaches during Ostreopsis proliferations and thus could be exposed to health risks. In contrast, some people would not visit the affected areas, with the potential subsequent negative impacts on coastal recreational and touristic activities. However, at this stage, it is too early to accurately assess all the economic impacts that a potentially increasing frequency and biogeographic expansion of the events might cause in the future.

Mechanism of the allelopathic effect of macroalgae Gracilaria bailiniae on Nitzschia closterium

With the rapid development of the seaweed industry in China, the scale and production of its commercial seaweed are ranked among the most significant worldwide. Consequently, the control of algal blooms, especially fouling diatoms, during macroalgae industrialisation is an important issue. Many diatom bloom studies have focused on physical and chemical controls, with limited economic and eco-friendly biological controls reported. In our study, Gracilaria bailiniae fresh thalli and aqueous extract profoundly suppressed Nitzschia closterium growth (50% inhibition concentration of the fourth day (IC_50-4 day) was 0.667 × 10^-3 g·mL^-1 and 3.889 × 10^-3 g·mL^-1, respectively). The cellular morphology changes of N. closterium exposed to the G. bailiniae aqueous extract were severe atrophies and plasmolysis and dissolution of endocellular structures. To explore more potential allelochemicals to control N. closterium, the intracellular compounds of G. bailiniae were detected and screened. Three organic acids (citrate, hydroxyethanesulfonic acid (HA) and taurine) had allelopathic potential against N. closterium. Our results showed that citrate and HA markedly suppressed N. closterium (IC_50-4 day: 1.035 mM and 1.151 mM, respectively); however, taurine poorly suppressed N. closterium (IC_50-4 day: 2.500 mM). Therefore, HA is one of the main allelopathic compounds in G. bailiniae. Further, the allelopathic mechanism of HA against the N. closterium photosynthetic system broke its photosynthetic apparatus (oxygen-evolving complex, reaction centres, the effective antenna size and the donor side of photosystem II) and hindered electron transport. The experimental results provide a new and eco-friendly strategy to control diatom blooms.

Metabolomic Insights of the Effects of Bacterial Algicide IRI-160AA on Dinoflagellate Karlodinium veneficum

Shewanella sp. IRI-160 is an algicidal bacterium that secretes an algicide, IRI-160AA. This algicide specifically targets dinoflagellates, while having no adverse effects on other algal species tested. Dinoflagellates exposed to IRI-160AA exhibited increased production of reactive oxygen species (ROS), DNA damage, and cell cycle arrest, implying a programmed pathway leading to cell death (PCD). Here, a metabolomic analysis was conducted on dinoflagellate Karlodinium veneficum and a control cryptophyte species Rhodomonas exposed to IRI-160AA to investigate the cellular mechanisms behind the physiological effects and the specificity of this algicide. Results of this research supported previous observations about physiological responses to the algicide. A suite of metabolites was identified that increased in the cell pellets of K. veneficum but not in Rhodomonas, including oxidative stress biomarkers, antioxidants, and compounds involved in DNA damage and PCD. Overall, the results of this study illustrated the metabolomic mechanisms underlying the algicidal effects of IRI-160AA on dinoflagellates. This research also provided insights and future directions for studies on the cellular response of dinoflagellates exposed to antagonistic bacteria in the environment.

Towards the Prediction of Favourable Conditions for the Harmful Algal Bloom Onset of Ostreopsis ovata in the Ligurian Sea Based on Satellite and Model Data

In recent years, the occurrence of Ostreopsis ovata (O. ovata) Harmful Algal Blooms (HAB) has increased in the coasts of the Ligurian Sea, causing problems to the marine environment and human health. Favourable conditions for O. ovata bloom are triggered by many drivers, many of which are still under investigation, but we hypothesize that this phenomenon can be simulated using a reduced number of major meteo-marine factors, namely water temperature and remixing. Satellite and model data obtained and derived from Copernicus service, namely Sea Surface Temperature (SST) and Significant Wave Height (SWH), were therefore investigated as possible proxies of these local factors. A simplified conceptual model, built on such proxies, was developed and applied to yield a synthetic indicator informative on O. ovata abundance. The model was tested in two study areas in the Ligurian Sea, Marina di Pisa and Marina di Massa in Tuscany, Italy. The results obtained show that the synthetic indicator is able to account for about 35% of the temporal variability of O. ovata bloom occurrence in the two study areas.

On the Hunt for New Toxin Families Produced by a Mediterranean Strain of the Benthic Dinoflagellate Ostreopsis cf. ovata

Ostreopsis cf. ovata is a benthic dinoflagellate known to produce palytoxin (PLTX) and its analogues. Recent investigations suggested the production of unknown toxins by a Mediterranean strain. In the present work, two new families of toxins, potentially novel in their structures, were purified from this same Mediterranean strain of Ostreopsis cf. ovata. The low amount of material isolated only allowed for acquisition of high-resolution mass spectrometry data and the evaluation of their cytotoxicity to human lung cancer cells. Based on their HRMS data, none of these new compounds appear to be close PLTX analogues, although their mass spectra suggest poly-hydroxylated long chain compounds of high molecular weight (1370–2143 Da). The cell cytotoxicity concentrations (CC₅₀) of these new purified toxins ranged between 0.68 and 3.12 µg/mL, and this was enhanced when they were tested as mixtures, suggesting synergistic effects of Ostreopsis toxins. The two families of compounds were named the liguriatoxins (LGTX) and rivieratoxins (RVTX), with each family containing three members. Additional work on purification is needed to fully characterize the structures of these six new dinoflagellate toxins.

The allelopathy and underlying mechanism of Skeletonema costatum on Karenia mikimotoi integrating transcriptomics profiling

The roles of allelopathy for succession of marine phytoplankton communities remain controversial, especially for the development of blooms. Physiological parameters measurement (Fv/Fm value, MDA content, SOD activity, Na⁺/K⁺, Ca²⁺/ Mg²⁺-ATPase activity, cell size, chlorophyll content, apoptosis and cell cycle) and whole transcriptome profiling analysis were used to investigate allelopathy effect of Skeletonema costatum on Karenia mikimotoi. Filtrate and extracts from S. costatum culture inhibited the growth of K. mikimotoi. Allelopathic effects were dose-dependent for filtrate culture and extract culture. K. mikimotoi scavenged excessive ROS and adapted to the stress fastly and easily, so oxidative damage was not the main cause of the growth inhibition. Allelochemicals of S. costatum were found to influence the structure and function of cell membrane of K. mikimotoi by damaging membrane structure till to cell necrosis, which caused high mortality. Coupled with the sensitivity of algal cells to environmental stress and restricted cell cycle, allelopathy was suggested to be deeply detrimental to the development of competition algal population.

Citizen science in the monitoring of Ostreopsis ovata blooms in southern Italy: A five-year study

The toxic benthic dinoflagellate Ostreopsis ovata causes harmful algal blooms. During five years, citizens have monitored blooms of O. cf. ovata along the coast of Molfetta city facing the Adriatic Sea. Coscinodiscus spp., Licmophora spp., Gyrosigma spp. and Achnantes spp. diatoms were also quantified. O. cf. ovata was detected from spring up to winter, however, blooms always occurred in summer. Correlation with nine weather parameters was relatively strong with seawater temperature, dew point and air temperature. Dew point has never been reported before as key parameter. Blooms of O. cf. ovata were preceded by lag period during which conditions permitted proliferation but no proliferation observed. Furthermore, dew point, seawater and air temperature only moderately correlated with proliferation of Coscinodiscus spp. However, correlation between blooms of O. cf. ovata and Coscinodiscus spp. was relatively strong. Correlation between proliferation of O. cf. ovata and Gyrosigma spp. was very weak, while moderate and negative with Licmophora spp. or Achnantes spp.

Effects of dinoflagellate Gymnodinium catenatum on swimming behavior and expression of heat shock protein (hsp) genes in the brine shrimp Artemia franciscana

To understand the effects of the toxic marine dinoflagellate, Gymnodinium catenatum, on the brine shrimp, Artemia franciscana, we examined the acute toxicity and swimming behavior parameters such as swimming speed, swimming distance, and swimming path trajectory with transcriptional regulation of heat shock protein (hsp) genes in response to G. catenatum exposure. Mortality was not observed in response to G. catenatum. In the case of swimming behavior parameters, swimming speed and swimming distance were significantly decreased (P < 0.05) for 5 min at three concentrations (240, 360, and 600 cells/mL) of G. catenatum, whereas no significant change in swimming path trajectory was observed, suggesting that G. catenatum has potential adverse effects on the swimming behavior of A. franciscana. Additionally, the four A. franciscana-hsp genes (hsp26, hsp40, hsp70, and hsp90) were upregulated in response to G. catenatum. In particular, A. franciscana-hsp40 was significantly upregulated in response to 600 cells/mL G. catenatum, suggesting that A. franciscana-hsp genes are highly associated with cellular defense mechanisms and that A. franciscana-hsp40 is a potential biomarker for G. catenatum exposure. Overall, this study improves our understanding of the effects of G. catenatum on the swimming behavior and cellular defense mechanisms of A. franciscana.

Allelochemicals of Alexandrium minutum: Kinetics of membrane disruption and photosynthesis inhibition in a co-occurring diatom

Allelopathy is an efficient strategy by which some microalgae can outcompete other species. Allelochemicals from the toxic dinoflagellate Alexandrium minutum have deleterious effects on diatoms, inhibiting metabolism and photosynthesis and therefore give a competitive advantage to the dinoflagellate. The precise mechanisms of allelochemical interactions and the molecular target of allelochemicals remain however unknown. To understand the mechanisms, the short-term effects of A. minutum allelochemicals on the physiology of the diatom Chaetoceros muelleri were investigated. The effects of a culture filtrate were measured on the diatom cytoplasmic membrane integrity (polarity and permeability) using flow-cytometry and on the photosynthetic performance using fluorescence and absorption spectroscopy. Within 10 min, the unknown allelochemicals induced a depolarization of the cytoplasmic membranes and an impairment of photosynthesis through the inhibition of the plastoquinone-mediated electron transfer between photosystem II and cytochrome b₆f. At longer time of exposure, the cytoplasmic membranes were permeable and the integrity of photosystems I, II and cytochrome b₆f was compromised. Our demonstration of the essential role of membranes in this allelochemical interaction provides new insights for the elucidation of the nature of the allelochemicals. The relationship between cytoplasmic membranes and the inhibition of the photosynthetic electron transfer remains however unclear and warrants further investigation.

Effects of paralytic shellfish poisoning toxin-producing dinoflagellate Gymnodinium catenatum on the marine copepod Tigriopus japonicus

To understand how the marine copepod Tigriopus japonicus responds to the toxic marine dinoflagellate Gymnodinium catenatum, we assessed acute toxicity and investigated swimming behavior parameters (e.g., swimming speed, swimming path trajectory, and swimming distance) in response to G. catenatum exposure. In addition, the mRNA expression levels of detoxification-related genes (e.g., phase I cytochrome P450 [CYP] and phase II glutathione-S transferase [GST]) were measured in G. catenatum-exposed copepods. No significant change in survival was observed in response to G. catenatum, but swimming speed was significantly decreased (P < 0.05) at a high concentration of G. catenatum (600 cells/mL). Furthermore, the swimming distance was significantly decreased (P < 0.05) compared to that of the control at 600 cells/mL G. catenatum, while no significant change in swimming path trajectory was observed, suggesting that G. catenatum potentially has adverse effects on the swimming behavior of T. japonicus. In addition, the transcriptional regulation of T. japonicus CYPs and -GSTs were significantly upregulated and downregulated (P < 0.05), respectively, in response to G. catenatum. In particular, certain genes (e.g., CYPs [CYP307E1, CYP3041A1, and CYP3024A2] and GSTs [GST-kappa, GST-mu5, and GST-omega]) were significantly induced (P < 0.05) by G. catenatum, suggesting that these genes likely play a critical role in detoxification mechanisms and might be useful as potential molecular biomarkers in response to G. catenatum exposure. Overall, these results elucidate the potential impacts of the dinoflagellate G. catenatum on the swimming behavior and detoxification system of the marine copepod T. japonicus.

The first characterization of airborne cyanobacteria and microalgae in the Adriatic Sea region

The presence of airborne cyanobacteria and microalgae as well as their negative impacts on human health have been documented by many researchers worldwide. However, studies on cyanobacteria and microalgae are few compared with those on bacteria and viruses. Research is especially lacking on the presence and taxonomic composition of cyanobacteria and microalgae near economically important water bodies with much tourism, such as the Adriatic Sea region. Here, we present the first characterization of the airborne cyanobacteria and microalgae in this area. Sampling conducted between 11^th and 15^th June 2017 revealed a total of 15 taxa of airborne cyanobacteria and microalgae. Inhalation of many of the detected taxa, including Synechocystis sp., Synechococcus sp., Bracteacoccus sp., Chlorella sp., Chlorococcum sp., Stichococcus sp., and Amphora sp., poses potential threats to human health. Aside from two green algae, all identified organisms were capable of producing harmful metabolites, including toxins. Moreover, we documented the presence of the cyanobacterium Snowella sp. and the green alga Tetrastrum sp., taxa that had not been previously documented in the atmosphere by other researchers. Our study shows that the Adriatic Sea region seems to be a productive location for future research on airborne cyanobacteria and microalgae in the context of their impacts on human health, especially during the peak of tourism activity.

Atypical Membrane-Anchored Cytokine MIF in a Marine Dinoflagellate

Macrophage Migration Inhibitory Factors (MIF) are pivotal cytokines/chemokines for vertebrate immune systems. MIFs are typically soluble single-domain proteins that are conserved across plant, fungal, protist, and metazoan kingdoms, but their functions have not been determined in most phylogenetic groups. Here, we describe an atypical multidomain MIF protein. The marine dinoflagellate Lingulodinium polyedra produces a transmembrane protein with an extra-cytoplasmic MIF domain, which localizes to cell-wall-associated membranes and vesicular bodies. This protein is also present in the membranes of extracellular vesicles accumulating at the secretory pores of the cells. Upon exposure to biotic stress, L. polyedra exhibits reduced expression of the MIF gene and reduced abundance of the surface-associated protein. The presence of LpMIF in the membranes of secreted extracellular vesicles evokes the fascinating possibility that LpMIF may participate in intercellular communication and/or interactions between free-living organisms in multispecies planktonic communities.

Multiple Roles of Diatom-Derived Oxylipins within Marine Environments and Their Potential Biotechnological Applications

The chemical ecology of marine diatoms has been the subject of several studies in the last decades, due to the discovery of oxylipins with multiple simultaneous functions including roles in chemical defence (antipredator, allelopathic and antibacterial compounds) and/or cell-to-cell signalling. Diatoms represent a fundamental compartment of marine ecosystems because they contribute to about 45% of global primary production even if they represent only 1% of the Earth’s photosynthetic biomass. The discovery that they produce several toxic metabolites deriving from the oxidation of polyunsaturated fatty acids, known as oxylipins, has changed our perspectives about secondary metabolites shaping plant–plant and plant–animal interactions in the oceans. More recently, their possible biotechnological potential has been evaluated, with promising results on their potential as anticancer compounds. Here, we focus on some recent findings in this field obtained in the last decade, investigating the role of diatom oxylipins in cell-to-cell communication and their negative impact on marine biota. Moreover, we also explore and discuss the possible biotechnological applications of diatom oxylipins.

Efficient, fast and inexpensive bioassay to monitor benthic microalgae toxicity: Application to Ostreopsis species

Even though HPLC-MS is commonly used to quantify the toxin content of Ostreopsis spp. cells, there is a need to develop easy-to-use toxicological tests to set thresholds during Ostreopsis spp. blooms. The crustacean Artemia has been widely used to evaluate the presence and toxicity of chemicals and biological contaminants and we anticipated that it could also be useful to test Ostreopsis spp. toxicity. Its relevance was first assessed by investigating the variability of the toxic effects among Ostreopsis spp. strains and throughout the dinoflagellate life cycle in combination with chemical analyses of the toxinic content by UHPLC-HRMS. After testing the toxicity of fractions prepared from Ostreopsis spp. cells, the known ova- and paly-toxins were not the only toxic metabolites to Artemia franciscana, indicating that other toxic compounds synthesized by Ostreopsis spp. still remain to be identified. To extend the bioassay to in situ monitoring, the toxicity of the benthic microalgal consortium was tested during a natural bloom of Ostreopsis cf. ovata in the NW Mediterranean Sea. The results highlight the accuracy and sensitivity of the ecotoxicological assay with Artemia franciscana to assess the toxicity of Ostreopsis spp. blooms.

Isolation of anti-algal substances from Cylindrotheca closterium and their inhibition activity on bloom-forming Prorocentrum donghaiense

This paper studied the allelopathic effect of Cylindrotheca closterium on the growth of Prorocentrum donghaiense, the model of harmful algal blooms in aquatic environment, by the co-culture tests and bioassay-guided fraction methods. The growth of P. donghaiense in co-cultures was observably suppressed by C. closterium, and P. donghaiense biomass in C/P = 3: 1 group increased slowly with a low growth rate of 0.18 d^-1 after 4 days. Petroleum ether (PE) extract derived from C. closterium filtrates was isolated by C18 column and the allelopathy of all isolated fractions for P. donghaiense was investigated. After 96 h cultivation, the inhibition ratio of PE-Ⅲ and PE-Ⅷ fractions on P. donghaiense could reach up to 70.2% and 64.3% at the concentration of 10-fold when compared to control, while the other fractions displayed relatively low inhibitory effects on P. donghaiense. PE-Ⅲ and PE-Ⅷ fractions also decreased the chlorophyll content and maximum quantum yield of photosystem II (Fv/Fm) of P. donghaiense cells. The activities of superoxide dismutase (SOD), one of antioxidant enzymes, reduced around 8.3% and 13.7% following exposure to 2-fold PE-Ⅲ and PE-Ⅷ, and was significantly decreased following higher exposure concentrations. After 96 h of 10-fold PE-Ⅲ and PE-Ⅷ treatments, Catalase (CAT) activity reduced to 44.86% and 46.42% of that observed in the control group. At the same time, a significant increase in malondialdehyde (MDA) contents was observed. These findings suggested that PE-Ⅲ and PE-Ⅷ fractions contained main allelochemicals and possibly acted as promising algistatic agents for emergency handling of P. donghaiense blooms.

Omics Analysis for Dinoflagellates Biology Research

Dinoflagellates are important primary producers for marine ecosystems and are also responsible for certain essential components in human foods. However, they are also notorious for their ability to form harmful algal blooms, and cause shellfish poisoning. Although much work has been devoted to dinoflagellates in recent decades, our understanding of them at a molecular level is still limited owing to some of their challenging biological properties, such as large genome size, permanently condensed liquid-crystalline chromosomes, and the 10-fold lower ratio of protein to DNA than other eukaryotic species. In recent years, omics technologies, such as genomics, transcriptomics, proteomics, and metabolomics, have been applied to the study of marine dinoflagellates and have uncovered many new physiological and metabolic characteristics of dinoflagellates. In this article, we review recent application of omics technologies in revealing some of the unusual features of dinoflagellate genomes and molecular mechanisms relevant to their biology, including the mechanism of harmful algal bloom formations, toxin biosynthesis, symbiosis, lipid biosynthesis, as well as species identification and evolution. We also discuss the challenges and provide prospective further study directions and applications of dinoflagellates.

Variation in allelopathy of extracellular compounds produced by Cylindrotheca closterium against the harmful-algal-bloom dinoflagellate Prorocentrum donghaiense

Allelopathy between algae is an ecological strategy that can facilitate or inhibit the occurrence of algal blooms. The role of allelopathic effects of marine microalgae Cylindrotheca closterium in other phytoplankton population dynamics are still limited. In the current study, the effects of cell-free filtrates of diatom Cylindrotheca closterium on two common dinoflagellates (Prorocentrum donghaiense and Prorocentrum cordatum), a chrysophyceae (Isochrysis galbana) and a diatom (Chaetoceros curvisetus) were investigated within controlled laboratory experiments. It was observed that the growth of P. donghaiense was significantly suppressed and approximately 80% cells disappeared after 8-d exposure, while the other three algae was less sensitive. P. donghaiense was very sensitive to the exudates of C. closterium from the stationary phase by comparing various percentage (10, 30, 50, 70 and 100%) of filtrates. In addition, the allelopathic effects of extracellular compounds of C. closterium extracted by three different organic solvents (ethyl acetate, chloroform and petroleum ether) on P. donghaiense were explored by determining cell density, chlorophyll content and maximum photosystem II (PSII) quantum yield (F_v/F_m). It was found that the compounds extracted by ethyl acetate and chloroform appeared to exhibit less toxicity on P. donghaiense than that of petroleum ether. The present results indicated that the allelochemicals released by C. closterium might be concentrated effectively in the petroleum ether extraction phase, which provided a new perspective for controlling the red tides of P. donghaiense in the East China Sea by means of the ecological inhibitors extracted.