Osmotic stress triggers toxin production by the dinoflagellate Karenia brevis

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.

Treatment of the red tide dinoflagellate Karenia brevis and brevetoxins using USEPA-registered algaecides

The effectiveness of USEPA-registered algaecides for managing algae in lakes and reservoirs has been extensively evaluated in laboratory studies, mesocosm studies and in situ treatment. However, the use of these algaecides in marine environments for the management of dinoflagellates and associated toxins remains largely unknown. Karenia brevis is a toxic dinoflagellate that causes red tides in the Gulf of Mexico. In this study, we investigated the efficacy of six USEPA-registered algaecides, three copper-based and three peroxide-based, on treating toxic K. brevis with a natural bloom density (1.79 × 10⁷ cells/L). Our results indicate that the application of as low as 0.31-0.34 mg Cu/L led to a significant decrease of K. brevis cells within 24 h after treatment, while peroxide-based algaecides required a relatively higher concentration for the effective removal of K. brevis cells (4.89-7.08 mg H₂O₂/L), but still lower than maximum label rate. Total brevetoxin levels 72 h after treatment revealed that 1.00 mg Cu/L for Algimycin® PWF, 6.48 mg H₂O₂/L for PAK® 27 and 7.08 mg H₂O₂/L for Oximycin® P5 had the greatest impact on decreasing toxin levels. The correlation analysis showed that brevetoxin reduction rate was significantly positively related with the peroxide-based algaecide exposure concentration, which is caused by the oxidation of hydroxyl radicals produced by hydrogen peroxide. The degradation dynamics of the three peroxide-based algaecides revealed that salinity, microorganisms and organic matter (≥ 0.2 μm) impact the stability of hydrogen peroxide, and Oximycin® P5 showed the highest stability among tested peroxide-based algaecides with a degradation rate of 0.467 mg/d in natural seawater. Hence, our laboratory work provided new insights into potential emergency treatment methods for immediate mitigation of K. brevis and brevetoxins. More work on the fate and persistence of algaecide active ingredients and phycotoxins, effects of site characteristics, and pilot studies on marine non-targets are still needed before safe application of this method for HABs in marine systems.

The Marine Neurotoxin Brevetoxin (PbTx-2) Inhibits Karenia brevis and Mammalian Thioredoxin Reductases by Targeting Different Residues

The brevetoxins, neurotoxins produced by Karenia brevis, the Florida red tide dinoflagellate, effect fish and wildlife mortalities and adverse public health and economic impacts during recurrent blooms. Knowledge of the biochemical consequences of toxin production for K. brevis could provide insights into an endogenous role of the toxins, yet this aspect has not been thoroughly explored. In addition to neurotoxicity, the most abundant of the brevetoxins, PbTx-2, inhibits mammalian thioredoxin reductase (TrxR). The thioredoxin system, composed of the enzymes TrxR and thioredoxin (Trx), is present in all living organisms and is responsible in part for maintaining cellular redox homeostasis. Herein, we describe the cloning, expression, and semisynthesis of the selenoprotein TrxR from K. brevis (KbTrxR) and reductase activity toward a variety of substrates. Unlike mammalian TrxR, KbTrxR reduces oxidized glutathione (GSSG). We further demonstrate that PbTx-2 is an inhibitor of KbTrxR. Covalent adducts between KbTrxR and rat TrxR were detected by mass spectrometry. While both enzymes are adducted at or near the catalytic centers, the specific residues are distinct. Biochemical differences reported for high and low toxin producing strains of K. brevis are consistent with the inhibition of KbTrxR and suggest that PbTx-2 is an endogenous regulator of this critical enzyme.

Salinity Affects Saxitoxins (STXs) Toxicity in the Dinoflagellate Alexandrium pacificum, with Low Transcription of SXT-Biosynthesis Genes sxtA4 and sxtG

Salinity is an important factor for regulating metabolic processes in aquatic organisms; however, its effects on toxicity and STX biosynthesis gene responses in dinoflagellates require further elucidation. Herein, we evaluated the physiological responses, toxin production, and expression levels of two STX synthesis core genes, sxtA4 and sxtG, in the dinoflagellate Alexandrium pacificum Alex05 under different salinities (20, 25, 30, 35, and 40 psu). Optimal growth was observed at 30 psu (0.12 cell division/d), but cell growth significantly decreased at 20 psu and was irregular at 25 and 40 psu. The cell size increased at lower salinities, with the highest size of 31.5 µm at 20 psu. STXs eq was highest (35.8 fmol/cell) in the exponential phase at 30 psu. GTX4 and C2 were predominant at that time but were replaced by GTX1 and NeoSTX in the stationary phase. However, sxtA4 and sxtG mRNAs were induced, and their patterns were similar in all tested conditions. PCA showed that gene transcriptional levels were not correlated with toxin contents and salinity. These results suggest that A. pacificum may produce the highest amount of toxins at optimal salinity, but sxtA4 and sxtG may be only minimally affected by salinity, even under high salinity stress.

Effect of a short-term salinity stress on the growth, biovolume, toxins, osmolytes and metabolite profiles on three strains of the Dinophysis acuminata-complex (Dinophysis cf. sacculus)

Dinophysis is the main dinoflagellate genus responsible for diarrheic shellfish poisoning (DSP) in human consumers of filter feeding bivalves contaminated with lipophilic diarrheic toxins. Species of this genus have a worldwide distribution driven by environmental conditions (temperature, irradiance, salinity, nutrients etc.), and these factors are sensitive to climate change. The D. acuminata-complex may contain several species, including D. sacculus. The latter has been found in estuaries and semi-enclosed areas, water bodies subjected to quick salinity variations and its natural repartition suggests some tolerance to salinity changes. However, the response of strains of D. acuminata-complex (D. cf. sacculus) subjected to salinity stress and the underlying mechanisms have never been studied in the laboratory. Here, a 24 h hypoosmotic (25) and hyperosmotic (42) stress was performed in vitro in a metabolomic study carried out with three cultivated strains of D. cf. sacculus isolated from the French Atlantic and Mediterranean coasts. Growth rate, biovolume and osmolyte (proline, glycine betaine and dimethylsulfoniopropionate (DMSP)) and toxin contents were measured. Osmolyte contents were higher at the highest salinity, but only a significant increase in glycine betaine was observed between the control (35) and the hyperosmotic treatment. Metabolomics revealed significant and strain-dependent differences in metabolite profiles for different salinities. These results, as well as the absence of effects on growth rate, biovolume, okadaic acid (OA) and pectenotoxin (PTXs) cellular contents, suggest that the D. cf. sacculus strains studied are highly tolerant to salinity variations.

Essential components of the xanthophyll cycle differ in high and low toxin Karenia brevis

The dinoflagellate Karenia brevis, blooms annually in the Gulf of Mexico, producing a suite of neurotoxins known as the brevetoxins. The cellular toxin content of K. brevis, however, is highly variable between or even within strains. Herein, we investigate physiological differences between high (KbHT) and low (KbLT) toxin producing cultures both derived from the Wilson strain, related to energy-dependent quenching (qE) by photosystem II, and reduced thiol content of the proteome. We demonstrate that gene and protein expression of the xanthophyll cycle enzyme diadinoxanthin de-epoxidase (Dde) and monogalactosyldiacylglycerol (MGDG) synthase are not significantly different in the two cultures. Using redox proteomics, we report a significantly higher reduced cysteine content in the low toxin proteome, including plastid localized thioredoxin reductase (Trx) which can result in inactivation of Dde and activation of MGDG synthase. We also report significant differences in the lipidomes of KbHT and KbLT with respect to MGDG, which facilitates the xanthophyll cycle.

Differentiating Two Closely Related Alexandrium Species Using Comparative Quantitative Proteomics

Alexandrium minutum and Alexandrium tamutum are two closely related harmful algal bloom (HAB)-causing species with different toxicity. Using isobaric tags for relative and absolute quantitation (iTRAQ)-based quantitative proteomics and two-dimensional differential gel electrophoresis (2D-DIGE), a comprehensive characterization of the proteomes of A. minutum and A. tamutum was performed to identify the cellular and molecular underpinnings for the dissimilarity between these two species. A total of 1436 proteins and 420 protein spots were identified using iTRAQ-based proteomics and 2D-DIGE, respectively. Both methods revealed little difference (10-12%) between the proteomes of A. minutum and A. tamutum, highlighting that these organisms follow similar cellular and biological processes at the exponential stage. Toxin biosynthetic enzymes were present in both organisms. However, the gonyautoxin-producing A. minutum showed higher levels of osmotic growth proteins, Zn-dependent alcohol dehydrogenase and type-I polyketide synthase compared to the non-toxic A. tamutum. Further, A. tamutum had increased S-adenosylmethionine transferase that may potentially have a negative feedback mechanism to toxin biosynthesis. The complementary proteomics approach provided insights into the biochemistry of these two closely related HAB-causing organisms. The identified proteins are potential biomarkers for organismal toxicity and could be explored for environmental monitoring.

Harmful algae at the complex nexus of eutrophication and climate change

Climate projections suggest-with substantial certainty-that global warming >1.5 °C will occur by mid-century (2050). Population is also projected to increase, amplifying the demands for food, fuel, water and sanitation, which, in turn, escalate nutrient pollution. Global projections of nutrient pollution, however, are less certain than those of climate as there are regionally decreasing trends projected in Europe, and stabilization of nutrient use in North America and Australia. In this review of the effects of eutrophication and climate on harmful algae, some of the complex, subtle, and non-intuitive effects and interactions on the physiology of both harmful and non-harmful taxa are emphasized. In a future ocean, non-harmful diatoms may be disproportionately stressed and mixotrophs advantaged due to changing nutrient stoichiometry and forms of nutrients, temperature, stratification and oceanic pH. Modeling is advancing, but there is much yet to be understood, in terms of physiology, biogeochemistry and trophodynamics and how both harmful and nonharmful taxa may change in an uncertain future driven by anthropogenic activities.

Effect of Salinity on DMSP Production in Gambierdiscus belizeanus (Dinophyceae)

Dimethylsulfoniopropionate (DMSP) is produced by many species of marine phytoplankton and has been reported to provide a variety of beneficial functions including osmoregulation. Dinoflagellates are recognized as major DMSP producers; however, accumulation has been shown to be highly variable in this group. We explored the effect of hyposaline transfer in Gambierdiscus belizeanus between ecologically relevant salinities (36 and 31) on DMSP accumulation, Chl a, cell growth, and cell volume, over 12 d. Our results showed that G. belizeanus maintained an intracellular DMSP content of 16.3 pmol cell^-1 and concentration of 139 mM in both salinities. Although this intracellular concentration was near the median reported for other dinoflagellates, the cellular content achieved by G. belizeanus was the highest reported of any dinoflagellate thus far, owing mainly to its large size. DMSP levels were not significantly affected by salinity treatment but did change over time during the experiment. Salinity, however, did have a significant effect on the ratio of DMSP:Chl a, suggesting that salinity transfer of G. belizeanus induced a physiological response other than DMSP adjustment. A survey of DMSP content in a variety of Gambierdiscus species and strains revealed relatively high DMSP concentrations (1.0-16.4 pmol cell^-1 ) as well as high intrageneric and intraspecific variation. We conclude that, although DMSP may not be involved in long-term (3-12 d) osmoregulation in this species, G. belizeanus and other Gambierdiscus species may be important contributors to DMSP production in tropical benthic microalgal communities due to their large size and high cellular content.

Spatial variability and depuration of tetrodotoxin in the bivalve Paphies australis from New Zealand

Tetrodotoxin (TTX) is a potent neurotoxin responsible for many human intoxications globally. Despite its potency and widespread occurrence in taxonomically diverse species, the primary source of TTX remains uncertain. Paphies australis, an endemic clam found in New Zealand, has been found to contain TTX in several locations. However, it is unknown if this represents endogenous production or accumulation from an external source. To address this question, the concentrations of TTX in whole P. australis and dissected organs (siphons, foot, digestive gland and the ‘rest’) from thirteen sites around New Zealand were determined using liquid chromatography-tandem quadrupole mass spectrometry analysis (LC-MS/MS). Depuration rate of TTX was also investigated by harvesting and measuring concentrations in P. australis maintained in captivity on a toxin-free diet every three to 15 days for 150 days. The LC-MS/MS analyses of the spatial samples showed that TTX was present in P. australis from all regions tested, with significantly (p < 0.001) higher concentrations (15–50 μg kg⁻¹) observed at lower latitudes of the North Island compared with trace levels (0.5–3 μg kg⁻¹) in the South Island of New Zealand. Tetrodotoxin was detected in all the dissected organs but the siphons contained the highest concentrations of TTX at all sites analysed. A linear model of the depuration data identified a significant (p < 0.001) decline in total TTX concentrations in P. australis over the study period. The siphons maintained the highest amount of TTX across the entire depuration study. The digestive glands contained low concentrations at the start of the experiment, but this depurated rapidly and only traces remained after 21 days. These results provide evidence to suggest that P. australis does not produce TTX endogenously but obtains the neurotoxin from an exogenous source (e.g., diet) with the source more prevalent in warmer northern waters. The association of higher TTX concentrations in shellfish with warmer environments raises concerns that this toxin's distribution and abundance could become an increasing human health issue with global warming.

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TTX-containing Paphies australis were maintained in captivity for 150 days and significantly depurated the toxin.

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Thirteen populations of Paphies australis from around New Zealand were collected and tested for TTX.

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All populations tested contained TTX but a significant latitudinal gradient was observed.

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This study provides further evidence of an exogenous source of TTX in marine bivalves.

Taxonomy and toxin production of Gambierdiscus carpenteri (Dinophyceae) in a tropical marine ecosystem: The first record from the Philippines

Morphological and phylogenetic analysis showed that the Gambierdiscus isolate from Bolinao, Philippines belongs to the species of G. carpenteri. It was morphologically more similar to the Merimbula strain than the subtropical Florida Keys strain. Growth and toxin production were also investigated at varying levels of temperature, salinity, and irradiance. Gambierdiscus are known to grow favorably in a low light environment. However, this study showed high growth rates of G. carpenteri even at high irradiance levels. Generally, cells produced more toxins at lower treatment levels. Highest cellular toxin content recorded was 7.48 ± 0.49 pg Pbtx eq/cell at culture conditions of 25 °C, 100 μmol photons m^-2 s^-1, and salinity of 26. Growth rate and toxin production data suggest that cells produced more toxins during the slowest growth at certain range of treatments. This information gives insight into how changes in environmental conditions may affect toxin production and growth of G. carpenteri.

Brevetoxin (PbTx-2) influences the redox status and NPQ of Karenia brevis by way of thioredoxin reductase

The Florida red tide dinoflagellate, Karenia brevis, is the major harmful algal bloom dinoflagellate of the Gulf of Mexico and plays a destructive role in the region. Blooms of K. brevis can produce brevetoxins: ladder-shaped polyether (LSP) compounds, which can lead to adverse human health effects, such as reduced respiratory function through inhalation exposure, or neurotoxic shellfish poisoning through consumption of contaminated shellfish. The endogenous role of the brevetoxins remains uncertain. Recent work has shown that some forms of NADPH dependent thioredoxin reductase (NTR) are inhibited by brevetoxin-2 (PbTx-2). The study presented herein reveals that high toxin and low toxin K. brevis, which have a ten-fold difference in toxin content, also show a significant difference in their ability, not only to produce brevetoxin, but also in their cellular redox status and distribution of xanthophyll cycle pigments. These differences are likely due to the inhibition of NTR by brevetoxin. The work could shed light on the physiological role that brevetoxin fills for K. brevis.

Ladder-Shaped Ion Channel Ligands: Current State of Knowledge

Ciguatoxins (CTX) and brevetoxins (BTX) are polycyclic ethereal compounds biosynthesized by the worldwide distributed planktonic and epibenthic dinoflagellates of Gambierdiscus and Karenia genera, correspondingly. Ciguatera, evoked by CTXs, is a type of ichthyosarcotoxism, which involves a variety of gastrointestinal and neurological symptoms, while BTXs cause so-called neurotoxic shellfish poisoning. Both types of toxins are reviewed together because of similar mechanisms of their action. These are the only molecules known to activate voltage-sensitive Na⁺-channels in mammals through a specific interaction with site 5 of its α-subunit and may compete for it, which results in an increase in neuronal excitability, neurotransmitter release and impairment of synaptic vesicle recycling. Most marine ciguatoxins potentiate Nav channels, but a considerable number of them, such as gambierol and maitotoxin, have been shown to affect another ion channel. Although the extrinsic function of these toxins is probably associated with the function of a feeding deterrent, it was suggested that their intrinsic function is coupled with the regulation of photosynthesis via light-harvesting complex II and thioredoxin. Antagonistic effects of BTXs and brevenal may provide evidence of their participation as positive and negative regulators of this mechanism.

Chemical ecology of marine plankton

Covering: January 2013 to online publication December 2014This review summarizes recent research in the chemical ecology of marine pelagic ecosystems, and aims to provide a comprehensive overview of advances in the field in the time period covered. In order to highlight the role of chemical cues and toxins in plankton ecology this review has been organized by ecological interaction types starting with intraspecific interactions, then interspecific interactions (including facilitation and mutualism, host-parasite, allelopathy, and predator-prey), and finally community and ecosystem-wide interactions.

The voltage-gated sodium channel: a major target of marine neurotoxins

Voltage-gated sodium channels (Nav) are key components for nerve excitability. They initiate and propagate the action potential in excitable cells, throughout the central and peripheral nervous system, thus enabling a variety of physiological functions to be achieved. The rising phase of the action potential is driven by the opening of Nav channels which activate rapidly and carry Na(+) ions in the intracellular medium, and ends with the Na(+) current inactivation. The biophysical properties of these channels have been elucidated, through the use of pharmacological agents that disrupt the molecular mechanism of the channel functioning. Among them, marine toxins produced by venomous animals or microorganisms have been crucial to map the different allosteric binding sites of the channels, understand their mode of action and represent an emerging source of therapeutic agents to alleviate or cure Na(+) channels-linked human diseases. In this article, we review recent discoveries on the molecular and biophysical properties of the Na(+) channel as a target for marine neurotoxins, and present the ongoing developments of pharmacological agents as therapeutic tools.

De novo assembly and characterization of the transcriptome of the toxic dinoflagellate Karenia brevis

Background

Karenia brevis is a harmful algal species that blooms in the Gulf of Mexico and produces brevetoxins that cause neurotoxic shellfish poisoning. Elevated brevetoxin levels in K. brevis cells have been measured during laboratory hypo-osmotic stress treatments. To investigate mechanisms underlying K. brevis osmoacclimation and osmoregulation and establish a valuable resource for gene discovery, we assembled reference transcriptomes for three clones: Wilson-CCFWC268, SP3, and SP1 (a low-toxin producing variant). K. brevis transcriptomes were annotated with gene ontology terms and searched for putative transmembrane proteins that may elucidate cellular responses to hypo-osmotic stress. An analysis of single nucleotide polymorphisms among clones was used to characterize genetic divergence.

Results

K. brevis reference transcriptomes were assembled with 58.5 (Wilson), 78.0 (SP1), and 51.4 million (SP3) paired reads. Transcriptomes contained 86,580 (Wilson), 93,668 (SP1), and 84,309 (SP3) predicted transcripts. Approximately 40% of the transcripts were homologous to proteins in the BLAST nr database with an E value ≤ 1.00E-6. Greater than 80% of the highly conserved CEGMA core eukaryotic genes were identified in each transcriptome, which supports assembly completeness. Seven putative voltage-gated Na⁺ or Ca²⁺ channels, two aquaporin-like proteins, and twelve putative VATPase subunits were discovered in all clones using multiple bioinformatics approaches. Furthermore, 45% (Wilson) and 43% (SP1 and SP3) of the K. brevis putative peptides > 100 amino acids long produced significant hits to a sequence in the NCBI nr protein database. Of these, 77% (Wilson and SP1) and 73% (SP3) were successfully assigned gene ontology functional terms. The predicted single nucleotide polymorphism (SNP) frequencies between clones were 0.0028 (Wilson to SP1), 0.0030 (Wilson to SP3), and 0.0028 (SP1 to SP3).

Conclusions

The K. brevis transcriptomes assembled here provide a foundational resource for gene discovery and future RNA-seq experiments. The identification of ion channels, VATPases, and aquaporins in all three transcriptomes indicates that K. brevis regulates cellular ion and water concentrations via transmembrane proteins. Additionally, > 40,000 unannotated loci may include potentially novel K. brevis genes. Ultimately, the SNPs identified among the three ecologically diverse clones with different toxin profiles may help to elucidate variations in K. brevis brevetoxin production.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-888) contains supplementary material, which is available to authorized users.

Slow science: the value of long ocean biogeochemistry records

Sustained observations (SOs) have provided invaluable information on the ocean's biology and biogeochemistry for over 50 years. They continue to play a vital role in elucidating the functioning of the marine ecosystem, particularly in the light of ongoing climate change. Repeated, consistent observations have provided the opportunity to resolve temporal and/or spatial variability in ocean biogeochemistry, which has driven exploration of the factors controlling biological parameters and processes. Here, I highlight some of the key breakthroughs in biological oceanography that have been enabled by SOs, which include areas such as trophic dynamics, understanding variability, improved biogeochemical models and the role of ocean biology in the global carbon cycle. In the near future, SOs are poised to make progress on several fronts, including detecting climate change effects on ocean biogeochemistry, high-resolution observations of physical-biological interactions and greater observational capability in both the mesopelagic zone and harsh environments, such as the Arctic. We are now entering a new era for biological SOs, one in which our motivations have evolved from the need to acquire basic understanding of the ocean's state and variability, to a need to understand ocean biogeochemistry in the context of increasing pressure in the form of climate change, overfishing and eutrophication.

Metatranscriptome profiling of a harmful algal bloom

Metagenomic methods provide a powerful means to investigate complex ecological phenomena. Developed originally for study of Bacteria and Archaea, the application of these methods to eukaryotic microorganisms is yet to be fully realized. Most prior environmental molecular studies of eukaryotes have relied heavily on PCR amplification with eukaryote-specific primers. Here we apply high throughput short-read sequencing of poly-A selected RNA to capture the metatranscriptome of an estuarine dinoflagellate bloom. To validate the metatranscriptome assembly process we simulated metatranscriptomic datasets using short-read sequencing data from clonal cultures of four algae of varying phylogenetic distance. We find that the proportion of chimeric transcripts reconstructed from community transcriptome sequencing is low, suggesting that metatranscriptomic sequencing can be used to accurately reconstruct the transcripts expressed by bloom-forming communities of eukaryotes. To further validate the bloom metatransciptome assembly we compared it to a transcriptomic assembly from a cultured, clonal isolate of the dominant bloom-causing alga and found that the two assemblies are highly similar. Eukaryote-wide phylogenetic analyses reveal the taxonomic composition of the bloom community, which is comprised of several dinoflagellates, ciliates, animals, and fungi. The assembled metatranscriptome reveals the functional genomic composition of a metabolically active community. Highlighting the potential power of these methods, we found that relative transcript abundance patterns suggest that the dominant dinoflagellate might be expressing toxin biosynthesis related genes at a higher level in the presence of competitors, predators and prey compared to it growing in monoculture.

Osmotic stress does not trigger brevetoxin production in the dinoflagellate Karenia brevis

With the global proliferation of toxic harmful algal bloom species, there is a need to identify the environmental and biological factors that regulate toxin production. One such species, Karenia brevis, forms nearly annual blooms that threaten coastal regions throughout the Gulf of Mexico. This dinoflagellate produces brevetoxins, which are potent neurotoxins that cause neurotoxic shellfish poisoning and respiratory illness in humans, as well as massive fish kills. A recent publication reported that a rapid decrease in salinity increased cellular toxin quotas in K. brevis and hypothesized that brevetoxins serve a role in osmoregulation. This finding implied that salinity shifts could significantly alter the toxic effects of blooms. We repeated the original experiments separately in three different laboratories and found no evidence for increased brevetoxin production in response to low-salinity stress in any of the eight K. brevis strains we tested, including three used in the original study. Thus, we find no support for an osmoregulatory function of brevetoxins. The original publication also stated that there was no known cellular function for brevetoxins. However, there is increasing evidence that brevetoxins promote survival of the dinoflagellates by deterring grazing by zooplankton. Whether they have other as-yet-unidentified cellular functions is currently unknown.

Bioactives from microalgal dinoflagellates

Dinoflagellate microalgae are an important source of marine biotoxins. Bioactives from dinoflagellates are attracting increasing attention because of their impact on the safety of seafood and potential uses in biomedical, toxicological and pharmacological research. Here we review the potential applications of dinoflagellate toxins and the methods for producing them. Only sparing quantities of dinoflagellate toxins are generally available and this hinders bioactivity characterization and evaluation in possible applications. Approaches to production of increased quantities of dinoflagellate bioactives are discussed. Although many dinoflagellates are fragile and grow slowly, controlled culture in bioreactors appears to be generally suitable for producing many of the metabolites of interest.

KARENIA: The biology and ecology of a toxic genus

Karenia is a genus containing at least 12 species of marine unarmored dinoflagellates. Species of the genus can be found throughout the world in both oceanic and coastal waters. They are usually sparse in abundance, but occasionally form large blooms in coastal waters. Most Karenia species produce a variety of toxins that can kill fish and other marine organisms when they bloom. In addition to toxicity, some Karenia blooms cause animal mortalities through the generation of anoxia. At least one species, K. brevis, produces brevetoxin that not only kills fish, marine mammals, and other animals, but also causes Neurotoxic Shellfish Poisoning and respiratory distress in humans. The lipid soluble brevetoxin can biomagnify up the food chain through fish to top carnivores like dolphins, killing them. Karenia dinoflagellates are slow growers, so physical concentrating mechanisms are probably important for the development of blooms. The blooms are highly sporadic in both time and space, although most tend to occur in summer or fall months in frontal regions. At the present time, our understanding of the causes of the blooms and ability to predict them is poor. Given the recent discovery of new species, it is likely that new Karenia species and toxins will be discovered in the future.

Phytoplankton composition, growth and production in the Guadiana estuary (SW Iberia): unraveling changes induced after dam construction

Water quality and quantity problems in the Guadiana estuary due to a recently built dam have been predicted, including an enhancement of cyanobacteria blooms. The main goal of this work was thus to describe the present phytoplankton dynamics in relation to its environmental drivers and to evaluate the effects of damming on phytoplankton in the Guadiana estuary. Sampling campaigns were conducted during 2007-2009 in 4 locations of the Guadiana estuary, covering the salinity gradient. Phytoplankton-related and physical-chemical variables were analyzed. Throughout our study, light availability was mainly controlled by suspended sediments and it was much lower than saturating intensities described for phytoplankton growth. Therefore, light was probably limiting to phytoplankton growth throughout the year, especially in the middle and upper estuarine zones. Nitrogen limitation of phytoplankton growth occurred occasionally throughout the study period, especially during spring and summer. Overall, light and nutrient availability were mainly controlled by river flow; anthropogenic sources of nutrients to the estuary were negligible. Phytoplankton showed a unimodal cycle with biomass maximum in late spring/early summer, and the typical seasonal succession of freshwater phytoplankton (diatoms, green algae, cyanobacteria) was observed. Diatoms were the main component of the phytoplankton community and their variability closely followed nitrate and river flow variability. The relative abundance of the main phytoplankton groups changed in relation to the period before dam construction, with a decrease on cyanobacteria contribution to total abundance. The environmental perturbation induced by dam construction has now stabilized and resulted in an overall decrease in nutrient concentrations, an increase in light availability and a decrease in cyanobacteria abundance.

Dozens of toxin-related genes are expressed in a nontoxic strain of the dinoflagellate Heterocapsa circularisquama

The dinoflagellate Heterocapsa circularisquama is lethal to a variety of marine organisms, in particular, commercially important farmed bivalves. Unlike most dinoflagellate toxins, which are polyketides, the only described toxin from H. circularisquama (H2-a) is a porphyrin derivative that functions in light. It is unknown whether H2-a is produced specifically for its lytic properties. We searched for toxin-related genes in the transcriptome of a nontoxic strain of H. circularisquama, and surprisingly found the richest set of toxin-related genes yet described in dinoflagellates. There are 87 distinct expressed sequence tag contigs that encode polyketide synthases and nonribosomal peptide synthases, as well as 8 contigs that are involved in porphyrin biosynthesis. Phylogenomic analysis shows that many toxin-related genes are widely distributed among dinoflagellates. Our data likely indicate a variety of unknown metabolic functions for the toxin-related genes in H. circularisquama because they were identified in a nontoxic strain raised in unialgal culture.