Allelochemicals determine competition and grazing control in Alexandrium catenella.

The production of allelochemicals by the toxigenic dinoflagellate Alexandrium catenella is one of the suggested mechanisms to facilitate its bloom formation and persistence by outcompeting other phototrophic protists and reducing grazing pressure. In Southern California, toxic events caused by A. catenella and paralytic shellfish toxins (PSTs) regularly impact coastal ecosystems; however, the trophic interactions and mechanisms promoting this species in a food web context are still not fully understood. In the present study, we combined a dynamical mathematical model with laboratory experiments to investigate potential toxic and allelochemical effects of an A. catenella strain isolated off the coast of Los Angeles, Southern California, on competitors and a common zooplankton consumer. Experiments were conducted using three toxigenic strains of A. catenella, comparing the new Californian isolate (Alex Cal) to two strains previously described from the North Sea, a lytic (Alex2) and non-lytic (Alex5) strain, testing for donor density-dependent effects on two phytoplankton species (Rhodomonas salina, Tetraselmis sp.) and on the rotifer Brachionus plicatilis. Bioassays revealed a steep decline in competitor and consumer populations with increasing Alex Cal concentrations, indicating an intermediate lytic activity compared to the North Sea strains (lytic Alex2 and non-lytic Alex5). The rotifer fed and grew well on the PST- toxic, but non-lytic Alex5 strain, while its survival significantly decreased with increasing concentrations of the two lytic strains Alex Cal and Alex 2, indicating that negative effects on the rotifer were mediated by allelochemicals rather than PST-toxins. Mixed culture experiments including both competitors and consumers demonstrated that the intensity of allelochemical effects not only depended on the A. catenella density but also on the target density. Negative effects on grazers were alleviated by co-occurring competitors with a lower sensitivity to allelochemicals, thus reducing harmful compounds and allowing grazing control on the dinoflagellate to come into effect again. Results from mixed culture experiments were supported by the mathematical approach used in this study which was calibrated with data from simple monoculture growth, pairwise competition and predator-prey experiments, demonstrating the applicability of this model approach to predict the outcome of more complex food web dynamics at the community level.

Sterolysin from a 1950s culture of Karlodinium veneficum (aka Gymnodinium veneficum Ballantine) forms lethal sterol dependent membrane pores.

In 1957 Abbott and Ballantine described a highly toxic activity from a dinoflagellate isolated from the English Channel in 1949 by Mary Park. From a culture maintained at Plymouth Laboratory since 1950, we have been able to isolate two toxic molecules (abbotoxin and 59-E-Chloro-abbotoxin), determine the planar structures by analysis of HRMS and 1D and 2D NMR spectra, and found them to be karlotoxin (KmTx) congeners. Both toxins kill larval zebrafish with symptoms identical to those described by Abbot and Ballantine for gobies (Gobius virescens). Using surface plasma resonance the sterol binding specificity of karlotoxins is shown to require desmethyl sterols. Our results with black lipid membranes indicate that karlotoxin forms large-conductance channels in the lipid membrane, which are characterized by large ionic conductance, poor ionic selectivity, and a complex gating behavior that exhibits strong voltage dependence and multiple gating patterns. In addition, we show that KmTx 2 pore formation is a highly targeted mechanism involving sterol-specificity. This is the first report of the functional properties of the membrane pores formed by karlotoxins and is consistent with the initial observations of Abbott and Ballantine from 1957.

Quantification of cytosolic 'free' calcium in isolated coral cells with confocal microscopy.

Despite its prominent role as an intracellular messenger in all organisms, cytosolic free calcium ([Ca2+]i) has never been quantified in corals or cnidarians in general. Ratiometric calcium dyes and cell imaging have been key methods in successful research on [Ca2+]i in model systems, and could be applied to corals. Here, we developed a procedure to quantify [Ca2+]i in isolated cells from the model coral species Stylophora pistillata using Indo-1 and confocal microscopy. We quantified [Ca2+]i in coral cells with and without intracellular dinoflagellate symbionts, and verified our procedure on cultured mammalian cells. We then used our procedure to measure changes in [Ca2+]i in coral cells exposed to a classic inhibitor of [Ca2+]i regulation, thapsigargin, and also used it to record elevations in [Ca2+]i in coral cells undergoing apoptosis. Our procedure paves the way for future studies into intracellular calcium in corals and other cnidarians.

Comparative Transcriptomics to Identify RNA Writers and Erasers in Microalgae.

Epitranscriptomics is considered as a new regulatory step in eukaryotes for developmental processes and stress responses. The aim of this study was, for the first time, to identify RNA methyltransferase (writers) and demethylase (erasers) in four investigated species, i.e., the dinoflagellates Alexandrium tamutum and Amphidinium carterae, the diatom Cylindrotheca closterium, and the green alga Tetraselmis suecica. As query sequences for the enzymatic classes of interest, we selected those ones that were previously detected in marine plants, evaluating their expression upon nutrient starvation stress exposure. The hypothesis was that upon stress exposure, the activation/deactivation of specific writers and erasers may occur. In microalgae, we found almost all plant writers and erasers (ALKBH9B, ALKBH10B, MTB, and FIP37), except for three writers (MTA, VIRILIZER, and HAKAI). A sequence similarity search by scanning the corresponding genomes confirmed their presence. Thus, we concluded that the three writer sequences were lacking from the studied transcriptomes probably because they were not expressed in those experimental conditions, rather than a real lack of these genes from their genomes. This study showed that some of them were expressed only in specific culturing conditions. We also investigated their expression in other culturing conditions (i.e., nitrogen depletion, phosphate depletion, and Zinc addition at two different concentrations) in A. carterae, giving new insights into their possible roles in regulating gene expression upon stress.

Microeukaryote metabolism across the western North Atlantic Ocean revealed through autonomous underwater profiling.

Microeukaryotes are key contributors to marine carbon cycling. Their physiology, ecology, and interactions with the chemical environment are poorly understood in offshore ecosystems, and especially in the deep ocean. Using the Autonomous Underwater Vehicle Clio, microbial communities along a 1050 km transect in the western North Atlantic Ocean were surveyed at 10-200 m vertical depth increments to capture metabolic signatures spanning oligotrophic, continental margin, and productive coastal ecosystems. Microeukaryotes were examined using a paired metatranscriptomic and metaproteomic approach. Here we show a diverse surface assemblage consisting of stramenopiles, dinoflagellates and ciliates represented in both the transcript and protein fractions, with foraminifera, radiolaria, picozoa, and discoba proteins enriched at >200 m, and fungal proteins emerging in waters >3000 m. In the broad microeukaryote community, nitrogen stress biomarkers were found at coastal sites, with phosphorus stress biomarkers offshore. This multi-omics dataset broadens our understanding of how microeukaryotic taxa and their functional processes are structured along environmental gradients of temperature, light, and nutrients.

The Effecting Mechanisms of 100 nm Sized Polystyrene Nanoplastics on the Typical Coastal Alexandrium tamarense.

Due to the increase in nanoplastics (NPs) abundance in aquatic environments, their effects on phytoplankton have aroused large research attention. In this study, 100 nm sized polystyrene NPs were chosen to investigate their effecting performance and mechanisms on a typical dinoflagellates Alexandrium tamarense. The results indicated the population growth and photosynthetic efficiencies of A. tamarense were significantly inhibited by NPs exposure, as well as the increase in cellular total carotenoids and paralytic shellfish toxins (PSTs). Meanwhile, the cellar ROS levels increased, corresponding to the increased activities or contents of multiple antioxidant components, including SOD, CAT, GPX, GR, GSH and GSSG. The transcriptional results support the physiological-biochemical results and further revealed the down-regulation of genes encoding the light reaction centers (PSI and PSII) and up-regulation of genes encoding the antioxidant components. Up-regulation of genes encoding key enzymes of the Calvin cycle and glycolytic pathway together with the TCA cycle could accelerate organic carbon and ATP production for A. tamarense cells resistant to NPs stress. Finally, more Glu and acetyl-CoA produced by the enhanced GSH cycle and the glycolytic pathway, respectively, accompanied by the up-regulation of Glu and Arg biosynthesis genes supported the increase in the PST contents under NPs exposure. This study established a data set involving physiological-biochemical changes and gene information about marine dinoflagellates responding to NPs, providing a data basis for further evaluating the ecological risk of NPs in marine environments.

Allelopathy of extracellular chemicals released by Karlodinium veneficum on photosynthesis of Prorocentrum donghaiense.

Dinoflagellates Prorocentrum donghaiense and Karlodinium veneficum are the dominant species of harmful algal blooms in the East China Sea. The role of their allelopathy on the succession of marine phytoplankton populations is a subject of ongoing debate, particularly concerning the formation of blooms. To explore the allelopathy of K. veneficum on P. donghaiense, an investigation was conducted into photosynthetic performance (including PSII functional activities, photosynthetic electron transport chain, energy flux, photosynthetic different genes and photosynthetic performance) and photosynthetic damage-induced oxidative stress (MDA, SOD, and CAT activity). The growth of P. donghaiense was strongly restrained during the initial four days (1-6 folds, CK/CP), but the cells gradually resumed activity at low filtrate concentrations from the eighth day. On the fourth day of the strongest inhibition, allelochemicals reduced representative photosynthetic performance parameters PI and ΦPSII, disrupted related processes of photosynthesis, and elevated the levels of MDA content in P. donghaiense. Simultaneously, P. donghaiense repairs these impairments by up-regulating the expression of 13 photosynthetic genes, modifying photosynthetic processes, and activating antioxidant enzyme activities from the eighth day onward. Overall, this study provides an in-depth overview of allelopathic photosynthetic damage, the relationship between genes and photosynthesis, and the causes of oxidative damage induced by photosynthesis. ENVIRONMENTAL IMPLICATIONS: As a typical HAB species, Karlodinium veneficum is associated with numerous fish poisoning events, which have negative impacts on aquatic ecosystems and human health. Allelochemicals produced by K. veneficum can provide a competitive advantage by interfering with the survival, reproduction and growth of competing species. This study primarily investigated the effects of K. veneficum allelochemicals on the photosynthesis and photosynthetic genes of Prorocentrum donghaiense. Grasping the mechanism of allelochemicals inhibiting microalgae is helpful to better understand the succession process of algal blooms and provide a new scientific basis for effective prevention and control of harmful algal blooms.

Toxin Dynamics among Populations of the Bioluminescent HAB Species Pyrodinium bahamense from the Indian River Lagoon, FL.

Dinoflagellate species that form some of the most frequent toxic blooms are also bioluminescent, yet the two traits are rarely linked when studying bloom development and persistence. P. bahamense is a toxic, bioluminescent dinoflagellate that previously bloomed in Florida with no known record of saxitoxin (STX) production. Over the past 20 years, STX was identified in P. bahamense populations. The goal of this study was to examine toxin dynamics and associated molecular mechanisms in spatially and temporally distinct P. bahamense populations from the Indian River Lagoon, FL. SxtA4 is a key gene required for toxin biosynthesis. SxtA4 genotype analysis was performed on individual cells from multiple sites. Cell abundance, toxin quota cell-1, and sxtA4 and RubisCo (rbcL) transcript abundance were also measured. There was a significant negative correlation between cell abundance and toxin quota cell-1. While the sxtA4+ genotype was dominant at all sites, its frequency varied, but it occurred at 90-100% in many samples. The underlying mechanism for toxin decrease with increased cell abundance remains unknown. However, a strong, statistically significant negative correlation was found between stxA4 transcripts and the sxtA4/rbcL ratio, suggesting cells make fewer sxtA4 transcripts as a bloom progresses. However, the influence of sxtA4- cells must also be considered. Future plans include bioluminescence measurements, normalized to a per cell basis, at sites when toxicity is measured along with concomitant quantification of sxtA4 gene and transcript copy numbers as a means to elucidate whether changes in bloom toxicity are driven more at the genetic (emergence of sxtA4- cells) or transcriptional (repression of sxtA4 in sxtA4+ cells) level. Based on the results of this study, a model is proposed that links the combined traits of toxicity and bioluminescence in P. bahamense bloom development.

Seasonal metabolic dynamics of microeukaryotic plankton: a year-long metatranscriptomic study in a temperate sea.

Seasonal fluctuations profoundly affect marine microeukaryotic plankton composition and metabolism, but accurately tracking these changes has been a long-standing challenge. In this study, we present a year-long metatranscriptomic data set from the Southern Bight of the North Sea, shedding light on the seasonal dynamics in temperate plankton ecosystems. We observe distinct shifts in active plankton species and their metabolic processes in response to seasonal changes. We characterized the metabolic signatures of different seasonal phases in detail, thereby revealing the metabolic versatility of dinoflagellates, the heterotrophic dietary strategy of Phaeocystis during its late-stage blooms, and stark variations in summer and fall diatom abundance and metabolic activity across nearby sampling stations. Our data illuminate the varied contributions of microeukaryotic taxa to biomass production and nutrient cycling at different times of the year and allow delineation of their ecological niches. IMPORTANCE: Ecosystem composition and metabolic functions of temperate marine microeukaryote plankton are strongly influenced by seasonal dynamics. Although monitoring of species composition of microeukaryotes has expanded recently, few methods also contain seasonally resolved information on ecosystem functioning. We generated a year-long spatially resolved metatranscriptomic data set to assess seasonal dynamics of microeukaryote species and their associated metabolic functions in the Southern Bight of the North Sea. Our study underscores the potential of metatranscriptomics as a powerful tool for advancing our understanding of marine ecosystem functionality and resilience in response to environmental changes, emphasizing its potential in continuous marine ecosystem monitoring to enhance our ecological understanding of the ocean's eukaryotic microbiome.

Membrane vectorial lipidomic features of coral host cells' plasma membrane and lipid profiles of their endosymbionts Cladocopium.

The symbiotic relationships between coral animal host and autotrophic dinoflagellates are based on the mutual exchange and tight control of nutritional inputs supporting successful growth. The corals Sinularia heterospiculata and Acropora aspera were cultivated using a flow-through circulation system supplying seawater during cold and warm seasons of the year, then sorted into host cells and symbionts and subjected to phylogenetic, morphological, and advanced lipid analyses. Here we show, that the lipidomes of the dinoflagellates Cladocopium C1/C3 and acroporide-specific Cladocopium hosted by the corals, are determined by lipidomic features of different thermosensitivity and unique betaine- and phospholipid molecular species. Phosphatidylserines and ceramiaminoethylphosphonates are not detected in the symbionts and predominantly localized on the inner leaflet of the S. heterospiculata host plasma membrane. The transmembrane distribution of phosphatidylethanolamines of S. heterospiculata host changes during different seasons of the year, possibly contributing to mutualistic nutritional exchange across this membrane complex to provide the host with a secure adaptive mechanism and ecological benefits.

Distribution and Level of Bioactive Monoacylglycerols in 12 Marine Microalgal Species.

Microalgae are currently considered an attractive source of highly valuable metabolites potentially exploitable as anticancer agents, nutraceuticals and cosmeceuticals and for bioenergy purposes. Their ease of culturing and their high growth rates further promote their use as raw material for the production of specialty products. In the present paper, we focused our attention on specific glycerol-based lipid compounds, monoacylglycerols (MAGs), which displayed in our previous studies a selective cytotoxic activity against the haematological U-937 and the colon HCT-116 cancer cell lines. Here, we performed a quali/quantitative analysis of MAGs and total fatty acids (FAs) along with a profiling of the main lipid classes in a panel of 12 microalgal species, including diatoms and dinoflagellates. Our results highlight an inter- and intraspecific variability of MAG profile in the selected strains. Among them, Skeletonema marinoi (strain FE7) has emerged as the most promising source for possible biotechnological production of MAGs.

Dynamic patterns of carbohydrate metabolism genes in bacterioplankton during marine algal blooms.

Carbohydrates play a pivotal role in nutrient recycling and regulation of algal-bacterial interactions. Despite their ecological significance, the intricate molecular mechanisms governing regulation of phycosphere carbohydrates by bacterial taxa linked with natural algal bloom have yet to be fully elucidated. Here, a comprehensive temporal metagenomic analysis was conducted to explore the carbohydrate-active enzyme (CAZyme) genes in two discrete algal bloom microorganisms (Gymnodinium catenatum and Phaeocystis globosa) across three distinct bloom stages: pre-bloom, peak bloom, and post-bloom. Elevated levels of extracellular carbohydrates, primarily rhamnose, galactose, glucose, and arabinose, were observed during the initial and post-peak stages. The prominent CAZyme families identified-glycoside hydrolases (GH) and carbohydrate-binding modules (CBMs)-were present in both algal bloom occurrences. In the G. catenatum bloom, GH23/24 and CBM13/14 were prevalent during the pre-bloom and peak bloom stages, whereas GH2/3/30 and CBM12/24 exhibited increased prevalence during the post-bloom phase. In contrast, the P. globosa bloom had a dominance of GH13/23 and CBM19 in the initial phase, and this was succeeded by GH3/19/24/30 and CBM54 in the later stages. This gene pool variation-observed distinctly in specific genera-highlighted the dynamic structural shifts in functional resources driven by temporal alterations in available substrates. Additionally, ecological linkage analysis underscored a correlation between carbohydrates (or their related genes) and phycospheric bacteria, hinting at a pattern of bottom-up control. These findings contribute to understanding of the dynamic nature of CAZymes, emphasizing the substantial influence of substrate availability on the metabolic capabilities of algal symbiotic bacteria, especially in terms of carbohydrates.

A comprehensive review on the heterotrophic production of bioactive compounds by microalgae.

Bioactive compounds derived from microalgae have garnered considerable attention as valuable resources for drugs, functional foods, and cosmetics. Among these compounds, photosynthetic pigments and polyunsaturated fatty acids (PUFAs) have gained increasing interest due to their numerous beneficial properties, including anti-oxidant, anti-viral, anti-bacterial, anti-fungal, anti-inflammatory, and anti-tumor effects. Several microalgae species have been identified as rich sources of bioactive compounds, including the Chlorophyceae Dunaliella and Haematococcus, the Bacillariophyta Phaeodactylum and Nitzschia, and the dinoflagellate Crypthecodinium cohnii. However, most of the reported microalgae species primarily grow through autotrophic mechanisms, resulting in low yields and high production costs of bioactive compounds. Consequently, the utilization of heterotrophic microalgae, such as Chromochloris zofingiensis and Nitzschia laevis, has shown significant advantages in the production of astaxanthin and eicosapentaenoic acid (EPA), respectively. These heterotrophic microalgae exhibit superior capabilities in synthesizing target compounds. This comprehensive review provides a thorough examination of the heterotrophic production of bioactive compounds by microalgae. It covers key aspects, including the metabolic pathways involved, the impact of cultivation conditions, and the practical applications of these compounds. The review discusses how heterotrophic cultivation strategies can be optimized to enhance bioactive compound yields, shedding light on the potential of microalgae as a valuable resource for high-value product development.

Microtubule reorganization during mitotic cell division in the dinoflagellate Ostreospis cf. ovata.

Dinoflagellates are marine organisms that undergo seasonal proliferation events known as algal blooms. Vegetative cell proliferation is a main contributing factor in these events. However, mechanistical understanding of mitosis and cytokinesis in dinoflagellates remains rudimentary. Using an optimized immunofluorescence protocol, we analysed changes in microtubule organization occurring during the mitotic cycle of the toxic dinoflagellate Ostreopsis cf. ovata. We find that the flagella and the cortical microtubule array persist throughout the mitotic cycle. Two cytoplasmic microtubule bundles originate from the ventral area, where the basal bodies are located - a cortical bundle and a cytoplasmic bundle. The latter associates with the nucleus in the cell centre before mitosis and with the acentrosomal extranuclear spindle during mitosis. Analysis of tubulin post-translational modifications identifies two populations of spindle microtubules - polar acetylated microtubules, whose length is constant, and central tyrosinated microtubules, which elongate during chromosome segregation. During cell division a microtubule-rich structure forms along the dorsal-ventral axis, associated with the site of cytokinesis, consistent with a cytokinetic mechanism that is independent of the actomyosin ring typical of animal and yeast cells.

Polyphyletic origin of saxitoxin biosynthesis genes in the marine dinoflagellate Alexandrium revealed by comparative transcriptomics.

The marine dinoflagellate Alexandrium is known to form harmful algal blooms, and at least 14 species within the genus can produce saxitoxins (STXs). STX biosynthesis genes (sxt) are individually revealed in toxic dinoflagellates; however, the evolutionary history remains controversial. Herein, we determined the transcriptome sequences of toxic Alexandrium (A. catenella and A. pacificum) and non-toxic Alexandrium (A. fraterculus and A. fragae) and characterized their sxt by focusing on evolutionary events and STX production. Comparative transcriptome analysis revealed higher homology of the sxt in toxic Alexandrium than in non-toxic species. Notably, non-toxic Alexandrium spp. were found to have lost two sxt core genes, namely sxtA4 and sxtG. Expression levels of 28 transcripts related to eight sxt core genes showed that sxtA, sxtG, and sxtI were relatively high (>1.5) in the toxic group compared to the non-toxic group. In contrast, the non-toxic group showed high expression levels in sxtU (1.9) and sxtD (1.7). Phylogenetic tree comparisons revealed distinct evolutionary patterns between 28S rDNA and sxtA, sxtB, sxtI, sxtD, and sxtU. However, similar topology was observed between 28S rDNA, sxtS, and sxtH/T. In the sxtB and sxtI phylogeny trees, toxic Alexandrium and cyanobacteria were clustered together, separating from non-toxic species. These suggest that Alexandrium may acquire sxt genes independently via horizontal gene transfer from toxic cyanobacteria and other multiple sources, demonstrating monocistronic transcripts of sxt in dinoflagellates.

Discovery of a series of portimine-A fatty acid esters in mussels.

Vulcanodinium rugosum is a benthic dinoflagellate known for producing pinnatoxins, pteriatoxins, portimines and kabirimine. In this study, we aimed to identify unknown analogs of these emerging toxins in mussels collected in the Ingril lagoon, France. First, untargeted data acquisitions were conducted by means of liquid chromatography coupled to hybrid quadrupole-orbitrap mass spectrometry. Data processing involved a molecular networking approach, and a workflow dedicated to the identification of biotransformed metabolites. Additionally, targeted analyses by liquid chromatography coupled to triple quadrupole mass spectrometry were also implemented to further investigate and confirm the identification of new compounds. For the first time, a series of 13-O-acyl esters of portimine-A (n = 13) were identified, with fatty acid chains ranging between C12:0 and C22:6. The profile was dominated by the palmitic acid conjugation. This discovery was supported by fractionation experiments combined with the implementation of a hydrolysis reaction, providing further evidence of the metabolite identities. Furthermore, several analogs were semi-synthesized, definitively confirming the discovery of these metabolization products. A new analog of pinnatoxin, with a molecular formula of C42H65NO9, was also identified across the year 2018, with the highest concentration observed in August (4.5 µg/kg). The MS/MS data collected for this compound exhibited strong structural similarities with PnTX-A and PnTX-G, likely indicating a substituent C2H5O2 in the side chain at C33. The discovery of these new analogs will contribute to deeper knowledge of the chemodiversity of toxins produced by V. rugosum or resulting from shellfish metabolism, thereby improving our ability to characterize the risks associated with these emerging toxins.

Genome-wide distribution of 5-hydroxymethyluracil and chromatin accessibility in the Breviolum minutum genome.

BACKGROUND: In dinoflagellates, a unique and extremely divergent genomic and nuclear organization has evolved. The highly unusual features of dinoflagellate nuclei and genomes include permanently condensed liquid crystalline chromosomes, primarily packaged by proteins other than histones, genes organized in very long unidirectional gene arrays, a general absence of transcriptional regulation, high abundance of the otherwise very rare DNA modification 5-hydroxymethyluracil (5-hmU), and many others. While most of these fascinating properties are originally identified in the 1970s and 1980s, they have not yet been investigated using modern genomic tools. RESULTS: In this work, we address some of the outstanding questions regarding dinoflagellate genome organization by mapping the genome-wide distribution of 5-hmU (using both immunoprecipitation-based and basepair-resolution chemical mapping approaches) and of chromatin accessibility in the genome of the Symbiodiniaceae dinoflagellate Breviolum minutum. We find that the 5-hmU modification is preferentially enriched over certain classes of repetitive elements, often coincides with the boundaries between gene arrays, and is generally correlated with decreased chromatin accessibility, the latter otherwise being largely uniform along the genome. We discuss the potential roles of 5-hmU in the functional organization of dinoflagellate genomes and its relationship to the transcriptional landscape of gene arrays. CONCLUSIONS: Our results provide the first window into the 5-hmU and chromatin accessibility landscapes in dinoflagellates.

Environmental Factors Modulate Saxitoxins (STXs) Production in Toxic Dinoflagellate Alexandrium: An Updated Review of STXs and Synthesis Gene Aspects.

The marine dinoflagellate Alexandrium is known to form harmful algal blooms (HABs) and produces saxitoxin (STX) and its derivatives (STXs) that cause paralytic shellfish poisoning (PSP) in humans. Cell growth and cellular metabolism are affected by environmental conditions, including nutrients, temperature, light, and the salinity of aquatic systems. Abiotic factors not only engage in photosynthesis, but also modulate the production of toxic secondary metabolites, such as STXs, in dinoflagellates. STXs production is influenced by a variety of abiotic factors; however, the relationship between the regulation of these abiotic variables and STXs accumulation seems not to be consistent, and sometimes it is controversial. Few studies have suggested that abiotic factors may influence toxicity and STXs-biosynthesis gene (sxt) regulation in toxic Alexandrium, particularly in A. catenella, A. minutum, and A. pacificum. Hence, in this review, we focused on STXs production in toxic Alexandrium with respect to the major abiotic factors, such as temperature, salinity, nutrients, and light intensity. This review informs future research on more sxt genes involved in STXs production in relation to the abiotic factors in toxic dinoflagellates.

Can exposure to Gymnodinium catenatum toxic blooms influence the impacts induced by Neodymium in Mytilus galloprovincialis mussels? What doesn't kill can make them stronger?

The presence in marine shellfish of toxins and pollutants like rare earth elements (REEs) poses a major threat to human well-being, coastal ecosystems, and marine life. Among the REEs, neodymium (Nd) stands out as a widely utilized element and is projected to be among the top five critical elements by 2025. Gymnodinum catenatum is a phytoplankton species commonly associated with the contamination of bivalves with paralytic shellfish toxins. This study evaluated the biological effects of Nd on the mussel species Mytilus galloprovincialis when exposed to G. catenatum cells for fourteen days, followed by a recovery period in uncontaminated seawater for another fourteen days. After co-exposure, mussels showed similar toxin accumulation in the Nd and G. catenatum treatment in comparison with the G. catenatum treatment alone. Increased metabolism and enzymatic defenses were observed in organisms exposed to G. catenatum cells, while Nd inhibited enzyme activity and caused cellular damage. Overall, this study revealed that the combined presence of G. catenatum cells and Nd, produced positive synergistic effects on M. galloprovincialis biochemical responses compared to G. catenatum alone, indicating that organisms' performance may be significantly modulated by the presence of multiple co-occurring stressors, such those related to chemical pollution and harmful algal blooms. ENVIRONMENTAL IMPLICATIONS: Neodymium (Nd) is widely used in green technologies like wind turbines, and this element's potential threats to aquatic environments are almost unknown, especially when co-occurring with other environmental factors such as blooms of toxic algae. This study revealed the cellular impacts induced by Nd in the bioindicator species Mytilus galloprovincialis but further demonstrated that the combination of both stressors can generate a positive defense response in mussels. The present findings also demonstrated that the impacts caused by Nd lasted even after a recovery period while a previous exposure to the toxins generated a faster biochemical improvement by the mussels.

Nutrient depletion and heat stress impair the assimilation of nitrogen compounds in a scleractinian coral.

Concentrations of dissolved nitrogen in seawater can affect the resilience of the cnidarian-dinoflagellate symbiosis to climate change-induced bleaching. However, it is not yet known how the assimilation and translocation of the various nitrogen forms change during heat stress, nor how the symbiosis responds to nutrient depletion, which may occur due to increasing water stratification. Here, the tropical scleractinian coral Stylophora pistillata, in symbiosis with dinoflagellates of the genus Symbiodinium, was grown at different temperatures (26°C, 30°C and 34°C), before being placed in nutrient-replete or -depleted seawater for 24 h. The corals were then incubated with 13C-labelled sodium bicarbonate and different 15N-labelled nitrogen forms (ammonium, urea and dissolved free amino acids) to determine their assimilation rates. We found that nutrient depletion inhibited the assimilation of all nitrogen sources studied and that heat stress reduced the assimilation of ammonium and dissolved free amino acids. However, the host assimilated over 3-fold more urea at 30°C relative to 26°C. Overall, both moderate heat stress (30°C) and nutrient depletion individually decreased the total nitrogen assimilated by the symbiont by 66%, and combined, they decreased assimilation by 79%. This led to the symbiotic algae becoming nitrogen starved, with the C:N ratio increasing by over 3-fold at 34°C, potentially exacerbating the impacts of coral bleaching.