Evidence of the bioaccumulation of ciguatoxins in giant clams (Tridacna maxima) exposed to Gambierdiscus spp. cells

Tissue Distribution and Metabolization of Ciguatoxins in an Herbivorous Fish following Experimental Dietary Exposure to Gambierdiscus polynesiensis

Ciguatoxins (CTXs), potent neurotoxins produced by dinoflagellates of the genera Gambierdiscus and Fukuyoa, accumulate in commonly consumed fish species, causing human ciguatera poisoning. Field collections of Pacific reef fish reveal that consumed CTXs undergo oxidative biotransformations, resulting in numerous, often toxified analogs. Following our study showing rapid CTX accumulation in flesh of an herbivorous fish, we used the same laboratory model to examine the tissue distribution and metabolization of Pacific CTXs following long-term dietary exposure. Naso brevirostris consumed cells of Gambierdiscus polynesiensis in a gel food matrix over 16 weeks at a constant dose rate of 0.36 ng CTX3C equiv g-1 fish d-1. CTX toxicity determination of fish tissues showed CTX activity in all tissues of exposed fish (eight tissues plus the carcass), with the highest concentrations in the spleen. Muscle tissue retained the largest proportion of CTXs, with 44% of the total tissue burden. Moreover, relative to our previous study, we found that larger fish with slower growth rates assimilated a higher proportion of ingested toxin in their flesh (13% vs. 2%). Analysis of muscle extracts revealed the presence of CTX3C and CTX3B as well as a biotransformed product showing the m/z transitions of 2,3-dihydroxyCTX3C. This is the first experimental evidence of oxidative transformation of an algal CTX in a model consumer and known vector of CTX into the fish food web. These findings that the flesh intended for human consumption carries the majority of the toxin load, and that growth rates can influence the relationship between exposure and accumulation, have significant implications in risk assessment and the development of regulatory measures aimed at ensuring seafood safety.

Diversity and distribution of benthic dinoflagellates in Tonga include the potentially harmful genera Gambierdiscus and Fukuyoa

Benthic dinoflagellates that can cause illness, such as ciguatera poisoning (CP), are prevalent around the Pacific but are poorly described in many locations. This study represents the first ecological assessment of benthic harmful algae species in the Kingdom of Tonga, a country where CP occurs regularly. Surveys were conducted in June 2016 in the Tongatapu island group, and in June 2017 across three island groups: Ha'apai, Vava'u, and Tongatapu. Shallow subtidal coastal habitats were investigated by measuring water quality parameters and conducting quadrat surveys. Microalgae samples were collected using either macrophyte collection or the artificial substrate method. Benthic dinoflagellates (Gambierdiscus and/or Fukuyoa, Ostreopsis, and Prorocentrum) were counted using light microscopy, followed by molecular analyses (real-time PCR in 2016 and high throughput sequencing (metabarcoding) in 2017) to identify Gambierdiscus and Fukuyoa to species level. Six species were detected from the Tongatapu island group in 2016 (G. australes, G. carpenteri, G. honu, G. pacificus, F. paulensis, and F. ruetzleri) using real-time PCR. Using the metabarcoding approach in 2017, a total of eight species (G. australes, G. carpenteri, G. honu, G. pacificus, G. cheloniae, G. lewisii, G. polynesiensis, and F. yasumotoi) were detected. Species were detected in mixed assemblages of up to six species, with G. pacificus and G. carpenteri being the most frequently observed. Ha'apai had the highest diversity with eight species detected, which identifies this area as a Gambierdiscus diversity 'hotspot'. Vava'u and Tongatapu had three and six species found respectively. Gambierdiscus polynesiensis, a described ciguatoxin producer and proposed causative agent of CP was found only in Ha'apai and Vava'u in 2017, but not in Tongatapu in either year. Ostreopsis spp. and Prorocentrum spp. were also frequently observed, with Prorocentrum most abundant at the majority of sites. In 2016, the highest number of Gambierdiscus and/or Fukuyoa cells were observed on seagrass (Halodule uninervis) from Sopu, Tongatapu. In 2017, the highest numbers of Gambierdiscus and/or Fukuyoa from artificial substrate samples were recorded in the Halimeda dominant habitat at Neiafu Tahi, Vava'u, a low energy site. This raised the question of the effect of wave motion or currents on abundance measurements from artificial substrates. Differences in detection were noticed between macrophytes and artificial substrates, with higher numbers of species found on artificial substrates. This study provides a baseline of benthic dinoflagellate distributions and diversity for Tonga that may be used for future studies and the development of monitoring programmes.

Ciguatera poisoning in French Polynesia: A review of the distribution and toxicity of Gambierdiscus spp., and related impacts on food web components and human health

Ciguatera Poisoning (CP) is a seafood poisoning highly prevalent in French Polynesia. This illness results from the consumption of seafood contaminated with ciguatoxins (CTXs) produced by Gambierdiscus, a benthic dinoflagellate. Ciguatera significantly degrades the health and economic well-being of local communities largely dependent on reef fisheries for their subsistence. French Polynesia has been the site of rich and active CP research since the 1960's. The environmental, toxicological, and epidemiological data obtained in the frame of large-scale field surveys and a country-wide CP case reporting program conducted over the past three decades in the five island groups of French Polynesia are reviewed. Results show toxin production in Gambierdiscus in the natural environment may vary considerably at a temporal and spatial scale, and that several locales clearly represent Gambierdiscus spp. "biodiversity hotspots". Current data also suggest the "hot" species G. polynesiensis could be the primary source of CTXs in local ciguateric biotopes, pending formal confirmation. The prevalence of ciguatoxic fish and the CTX levels observed in several locales were remarkably high, with herbivores and omnivores often as toxic as carnivores. Results also confirm the strong local influence of Gambierdiscus spp. on the CTX toxin profiles characterized across multiple food web components including in CP-prone marine invertebrates. The statistics, obtained in the frame of a long-term epidemiological surveillance program established in 2007, point towards an apparent decline in the number of CP cases in French Polynesia as a whole; however, incidence rates remain dangerously high in some islands. Several of the challenges and opportunities, most notably those linked to the strong cultural ramifications of CP among local communities, that need to be considered to define effective risk management strategies are addressed.

Ciguatera Fish Poisoning in the Caribbean Sea and Atlantic Ocean: Reconciling the Multiplicity of Ciguatoxins and Analytical Chemistry Approach for Public Health Safety

Ciguatera is a major circumtropical poisoning caused by the consumption of marine fish and invertebrates contaminated with ciguatoxins (CTXs): neurotoxins produced by endemic and benthic dinoflagellates which are biotransformed in the fish food-web. We provide a history of ciguatera research conducted over the past 70 years on ciguatoxins from the Pacific Ocean (P-CTXs) and Caribbean Sea (C-CTXs) and describe their main chemical, biochemical, and toxicological properties. Currently, there is no official method for the extraction and quantification of ciguatoxins, regardless their origin, mainly due to limited CTX-certified reference materials. In this review, the extraction and purification procedures of C-CTXs are investigated, considering specific objectives such as isolating reference materials, analysing fish toxin profiles, or ensuring food safety control. Certain in vitro assays may provide sufficient sensitivity to detect C-CTXs at sub-ppb levels in fish, but they do not allow for individual identification of CTXs. Recent advances in analysis using liquid chromatography coupled with low- or high-resolution mass spectrometry provide new opportunities to identify known C-CTXs, to gain structural insights into new analogues, and to quantify C-CTXs. Together, these methods reveal that ciguatera arises from a multiplicity of CTXs, although one major form (C-CTX-1) seems to dominate. However, questions arise regarding the abundance and instability of certain C-CTXs, which are further complicated by the wide array of CTX-producing dinoflagellates and fish vectors. Further research is needed to assess the toxic potential of the new C-CTX and their role in ciguatera fish poisoning. With the identification of C-CTXs in the coastal USA and Eastern Atlantic Ocean, the investigation of ciguatera fish poisoning is now a truly global effort.

Evaluation of relative potency of calibrated ciguatoxin congeners by near-infrared fluorescent receptor binding and neuroblastoma cell-based assays

Ciguatera fish poisoning (CFP) is a foodborne illness affecting > 50,000 people worldwide annually. It is caused by eating marine invertebrates and fish that have accumulated ciguatoxins (CTXs). Recently, the risk of CFP to human health, the local economy, and fishery resources have increased; therefore, detection methods are urgently needed. Functional assays for detecting ciguatoxins in fish include receptor binding (RBA) and neuroblastoma cell-based assay (N2a assay), which can detect all CTX congeners. In this study, we made these assays easier to use. For RBA, an assay was developed using a novel near-infrared fluorescent ligand, PREX710-BTX, to save valuable CTXs. In the N2a assay, a 1-day assay was developed with the same detection performance as the conventional 2-day assay. Additionally, in these assays, we used calibrated CTX standards from the Pacific determined by quantitative NMR for the first time to compare the relative potency of congeners, which differed significantly among previous studies. In the RBA, there was almost no difference in the binding affinity among congeners, showing that the differences in side chains, stereochemistry, and backbone structure of CTXs did not affect the binding affinity. However, this result did not correlate with the toxic equivalency factors (TEFs) based on acute toxicity in mice. In contrast, the N2a assay showed a good correlation with TEFs based on acute toxicity in mice, except for CTX3C. These findings, obtained with calibrated toxin standards, provide important insights into evaluating the total toxicity of CTXs using functional assays.

Toxicity and underlying mechanism of the toxic dinoflagellate Gambierdiscus caribaeus to the fish Oryzias melastigma

Gambierdiscus spp. is mainly responsible for the ciguatera fish poisoning (CFP) around the world. The gambiertoxin produced by Gambierdiscus can be passed through the food chain to form ciguatoxins (CTXs) that cause ciguatoxins poisoning. However, the toxic effects of Gambierdiscus on fish through the food chain and related mechanism remains unclear. In this study, the toxicity of Gambierdiscus caribaeus on the marine medaka (Oryzias melastigma) was investigated, where the simulated food chain toxic algae-food organism-fish (G. caribaeus-Artemia metanauplii-O. melastigma) was set. The results showed that direct or indirect exposure through the food chain of G. caribaeus could affect the swimming behaviour of O. melastigma, manifested as decreased swimming performance and spontaneous abnormal swimming behaviours. Histological observation showed that direct or indirect exposure of G. caribaeus caused different degrees of pathological damage to the gills, intestine and liver tissues of O. melastigma. Transcriptome sequencing and RT-qPCR demonstrated that G. caribaeus exposure could trigger a series of physiological and biochemical responses, mainly reflected in energy metabolism, reproductive system, neural activity, immune stress and drug metabolism in marine medaka. Our finding may provide novel insight into the toxicity of Gambierdiscus on fish.

Origin of Ciguateric Fish: Quantitative Modelling of the Flow of Ciguatoxin through a Marine Food Chain

To begin to understand the impact of food chain dynamics on ciguatera risk, we used published data to model the transfer of ciguatoxins across four trophic levels of a marine food chain in Platypus Bay, Australia. The data to support this first attempt to conceptualize the scale of each trophic transfer step was limited, resulting in broad estimates. The hypothetical scenario we explored generated a low-toxicity 10 kg Spanish mackerel (Scomberomorus commerson) with a flesh concentration of 0.1 µg/kg of Pacific-ciguatoxin-1 (P-CTX-1, also known as CTX1B) from 19.5–78.1 µg of P-CTX-1 equivalents (eq.) that enter the marine food chain from a population of 12–49 million benthic dinoflagellates (Gambierdiscus sp.) producing 1.6 × 10⁻¹² g/cell of the P-CTX-1 precursor, P-CTX-4B. This number of Gambierdiscus could be epiphytic on 22–88 kg of the benthic macroalgae (Cladophora) that carpets the bottom of much of Platypus Bay, with the toxin transferred to an estimated 40,000–160,000 alpheid shrimps in the second trophic level. This large number of shrimps appears unrealistic, but toxic shrimps would likely be consumed by a school of small, blotched javelin fish (Pomadasys maculatus) at the third trophic level, reducing the number of shrimps consumed by each fish. The Spanish mackerel would accumulate a flesh concentration of 0.1 µg/kg P-CTX-1 eq. by preying upon the school of blotched javelin and consuming 3.6–14.4 µg of P-CTX-1 eq. However, published data indicate this burden of toxin could be accumulated by a 10 kg Spanish mackerel from as few as one to three blotched javelin fish, suggesting that much greater amounts of toxin than modelled here must at certain times be produced and transferred through Platypus Bay food chains. This modelling highlights the need for better quantitative estimates of ciguatoxin production, biotransformation, and depuration through marine food chains to improve our understanding and management of ciguatera risk.

Structural Characterization of Maitotoxins Produced by Toxic Gambierdiscus Species

Identifying compounds responsible for the observed toxicity of the Gambierdiscus species is a critical step to ascertaining whether they contribute to ciguatera poisoning. Macroalgae samples were collected during research expeditions to Rarotonga (Cook Islands) and North Meyer Island (Kermadec Islands), from which two new Gambierdiscus species were characterized, G. cheloniae CAWD232 and G. honu CAWD242. Previous chemical and toxicological investigations of these species demonstrated that they did not produce the routinely monitored Pacific ciguatoxins nor maitotoxin-1 (MTX-1), yet were highly toxic to mice via intraperitoneal (i.p.) injection. Bioassay-guided fractionation of methanolic extracts, incorporating wet chemistry and chromatographic techniques, was used to isolate two new MTX analogs; MTX-6 from G. cheloniae CAWD232 and MTX-7 from G. honu CAWD242. Structural characterization of the new MTX analogs used a combination of analytical chemistry techniques, including LC–MS, LC–MS/MS, HR–MS, oxidative cleavage and reduction, and NMR spectroscopy. A substantial portion of the MTX-7 structure was elucidated, and (to a lesser extent) that of MTX-6. Key differences from MTX-1 included monosulfation, additional hydroxyl groups, an extra double bond, and in the case of MTX-7, an additional methyl group. To date, this is the most extensive structural characterization performed on an MTX analog since the complete structure of MTX-1 was published in 1993. MTX-7 was extremely toxic to mice via i.p. injection (LD₅₀ of 0.235 µg/kg), although no toxicity was observed at the highest dose rate via oral administration (155.8 µg/kg). Future research is required to investigate the bioaccumulation and likely biotransformation of the MTX analogs in the marine food web.

Comparative Study on the Performance of Three Detection Methods for the Quantification of Pacific Ciguatoxins in French Polynesian Strains of Gambierdiscus polynesiensis

Gambierdiscus and Fukuyoa dinoflagellates produce a suite of secondary metabolites, including ciguatoxins (CTXs), which bioaccumulate and are further biotransformed in fish and marine invertebrates, causing ciguatera poisoning when consumed by humans. This study is the first to compare the performance of the fluorescent receptor binding assay (fRBA), neuroblastoma cell-based assay (CBA-N2a), and liquid chromatography tandem mass spectrometry (LC-MS/MS) for the quantitative estimation of CTX contents in 30 samples, obtained from four French Polynesian strains of Gambierdiscus polynesiensis. fRBA was applied to Gambierdiscus matrix for the first time, and several parameters of the fRBA protocol were refined. Following liquid/liquid partitioning to separate CTXs from other algal compounds, the variability of CTX contents was estimated using these three methods in three independent experiments. All three assays were significantly correlated with each other, with the highest correlation coefficient (r2 = 0.841) found between fRBA and LC-MS/MS. The CBA-N2a was more sensitive than LC-MS/MS and fRBA, with all assays showing good repeatability. The combined use of fRBA and/or CBA-N2a for screening purposes and LC-MS/MS for confirmation purposes allows for efficient CTX evaluation in Gambierdiscus. These findings, which support future collaborative studies for the inter-laboratory validation of CTX detection methods, will help improve ciguatera risk assessment and management.

Evaluating Age and Growth Relationship to Ciguatoxicity in Five Coral Reef Fish Species from French Polynesia

Ciguatera poisoning (CP) results from the consumption of coral reef fish or marine invertebrates contaminated with potent marine polyether compounds, namely ciguatoxins. In French Polynesia, 220 fish specimens belonging to parrotfish (Chlorurus microrhinos, Scarus forsteni, and Scarus ghobban), surgeonfish (Naso lituratus), and groupers (Epinephelus polyphekadion) were collected from two sites with contrasted risk of CP, i.e., Kaukura Atoll versus Mangareva Island. Fish age and growth were assessed from otoliths’ yearly increments and their ciguatoxic status (negative, suspect, or positive) was evaluated by neuroblastoma cell-based assay. Using permutational multivariate analyses of variance, no significant differences in size and weight were found between negative and suspect specimens while positive specimens showed significantly greater size and weight particularly for E. polyphekadion and S. ghobban. However, eating small or low-weight specimens remains risky due to the high variability in size and weight of positive fish. Overall, no relationship could be evidenced between fish ciguatoxicity and age and growth characteristics. In conclusion, size, weight, age, and growth are not reliable determinants of fish ciguatoxicity which appears to be rather species and/or site-specific, although larger fish pose an increased risk of poisoning. Such findings have important implications in current CP risk management programs.

Electrochemical biosensor for the dual detection of Gambierdiscus australes and Gambierdiscus excentricus in field samples. First report of G. excentricus in the Balearic Islands

Several genera of marine dinoflagellates are known to produce bioactive compounds that affect human health. Among them, Gambierdiscus and Fukuyoa stand out for their ability to produce several toxins, including the potent neurotoxic ciguatoxins (CTXs), which accumulate through the food web. Once fishes contaminated with CTXs are ingested by humans, it can result in an intoxication named ciguatera. Within the two genera, only some species are able to produce toxins, and G. australes and G. excentricus have been highlighted to be the most abundant and toxic. Although the genera Gambierdiscus and Fukuyoa are endemic to tropical areas, their presence in subtropical and temperate regions has been recently recorded. In this work, the combined use of species-specific PCR primers for G. australes and G. excentricus modified with short oligonucleotide tails allowed the development of a multiplex detection system for these two toxin-producing species. Simultaneous detection was achieved using capture probes specific for G. australes and G. excentricus immobilized on maleimide-coated magnetic beads (MBs), separately placed on the working electrodes of a dual electrode array. Additionally, a rapid DNA extraction technique based on a portable bead beater system and MBs was developed, significantly reducing the extraction time (from several hours to 30 min). The developed technique was able to detect as low as 10 cells of both Gambierdiscus species and allowed the first detection of G. excentricus in the Balearic Islands in 8 out of the 12 samples analyzed. Finally, field samples were screened for CTXs with an immunosensor, successfully reporting 13.35 ± 0.5 pg CTX1B equiv. cell^-1 in one sample and traces of toxins in 3 out of the 9 samples analyzed. These developments provide rapid and cost-effective strategies for ciguatera risk assessment, with the aim of guaranteeing seafood safety.

Deep-Water Fish Are Potential Vectors of Ciguatera Poisoning in the Gambier Islands, French Polynesia

Ciguatera poisoning (CP) cases linked to the consumption of deep-water fish occurred in 2003 in the Gambier Islands (French Polynesia). In 2004, on the request of two local fishermen, the presence of ciguatoxins (CTXs) was examined in part of their fish catches, i.e., 22 specimens representing five deep-water fish species. Using the radioactive receptor binding assay (rRBA) and mouse bioassay (MBA), significant CTX levels were detected in seven deep-water specimens in Lutjanidae, Serranidae, and Bramidae families. Following additional purification steps on the remaining liposoluble fractions for 13 of these samples (kept at -20 °C), these latter were reanalyzed in 2018 with improved protocols of the neuroblastoma cell-based assay (CBA-N2a) and liquid chromatography tandem mass spectrometry (LC-MS/MS). Using the CBA-N2a, the highest CTX-like content found in a specimen of Eumegistus illustris (Bramidae) was 2.94 ± 0.27 µg CTX1B eq. kg^-1. Its toxin profile consisted of 52-epi-54-deoxyCTX1B, CTX1B, and 54-deoxyCTX1B, as assessed by LC-MS/MS. This is the first study demonstrating that deep-water fish are potential ciguatera vectors and highlighting the importance of a systematic monitoring of CTXs in all exploited fish species, especially in ciguatera hotspots, including deep-water fish, which constitute a significant portion of the commercial deep-sea fisheries in many Asian-Pacific countries.

Depuration Kinetics and Growth Dilution of Caribbean Ciguatoxin in the Omnivore Lagodon rhomboides: Implications for Trophic Transfer and Ciguatera Risk

Modeling ciguatoxin (CTX) trophic transfer in marine food webs has significant implications for the management of ciguatera poisoning, a circumtropical disease caused by human consumption of CTX-contaminated seafood. Current models associated with CP risk rely on modeling abundance/presence of CTX-producing epi-benthic dinoflagellates, e.g., Gambierdiscus spp., and are based on studies showing that toxin production is site specific and occurs in pulses driven by environmental factors. However, food web models are not yet developed and require parameterizing the CTX exposure cascade in fish which has been traditionally approached through top-down assessment of CTX loads in wild-caught fish. The primary goal of this study was to provide critical knowledge on the kinetics of C-CTX-1 bioaccumulation and depuration in the marine omnivore Lagodon rhomboides. We performed a two-phase, 17 week CTX feeding trial in L. rhomboides where fish were given either a formulated C-CTX-1 (n = 40) or control feed (n = 37) for 20 days, and then switched to a non-toxic diet for up to 14 weeks. Fish were randomly sampled through time with whole muscle, liver, and other pooled viscera dissected for toxin analysis by a sodium channel-dependent MTT-based mouse neuroblastoma (N2a) assay. The CTX levels measured in all tissues increased with time during the exposure period (days 1 to 20), but a decrease in CTX-specific toxicity with depuration time only occurred in viscera extracts. By the end of the depuration, muscle, liver, and viscera samples had mean toxin concentrations of 189%, 128%, and 42%, respectively, compared to fish sampled at the start of the depuration phase. However, a one-compartment model analysis of combined tissues showed total concentration declined to 56%, resulting in an approximate half-life of 97 d (R² = 0.43). Further, applying growth dilution correction models to the overall concentration found that growth was a major factor reducing C-CTX concentrations, and that the body burden was largely unchanged, causing pseudo-elimination and a half-life of 143-148 days (R² = 0.36). These data have important implications for food web CTX models and management of ciguatera poisoning in endemic regions where the frequency of environmental algal toxin pulses may be greater than the growth-corrected half-life of C-CTX in intermediate-trophic-level fish with high site fidelity.

Harmful algal blooms and shellfish in the marine environment: an overview of the main molluscan responses, toxin dynamics, and risks for human health

Besides human health risks, phycotoxins may cause physiological injuries on molluscan shellfish and, consequently, damages to marine ecosystems and global fisheries production. In this way, this review aimed to present an overview of HABs impacts on marine shellfish by evaluating the effects of cultivated molluscs exposure to microalgae and cyanobacteria that form blooms and/or synthesize toxins. More specifically, it was assessed the main molluscan shellfish responses to harmful algae, trophic transfer and dynamics of phycotoxins, and the risks for human health. Of the 2420 results obtained from literature search, 150 scientific publications were selected after thorough inspections for subject adherence. In total, 70 molluscan species and 37 taxa of harmful algae were assessed from retrieved scientific publications. A significant positive correlation was found between the marine production of molluscs and the number of available studies by molluscan category. Molluscan responses to HABs and phycotoxins were categorized and discussed in three sub-sections: effects on grazing and behavior, metabolic and physiological reactions, and fitness consequences. The main histopathological injuries and toxin concentrations in molluscan tissues were also compiled and discussed. Bivalves often accumulate more toxins than gastropods and cephalopods, occasionally exceeding recommended levels for safe consumption, representing a risk for human health. Harmful algae impact on molluscan shellfish are complex to trace and predict; however, considering the perspective of increase in the occurrence and intensity of HABs, the intensification of efforts to expand the knowledge about HABs impacts on marine molluscs is crucial to mitigate the damages on economy and human health.

Identification of Fish Species and Toxins Implicated in a Snapper Food Poisoning Event in Sabah, Malaysia, 2017

In the coastal countries of Southeast Asia, fish is a staple diet and certain fish species are food delicacies to local populations or commercially important to individual communities. Although there have been several suspected cases of ciguatera fish poisoning (CFP) in Southeast Asian countries, few have been confirmed by ciguatoxins identification, resulting in limited information for the correct diagnosis of this food-borne disease. In the present study, ciguatoxin-1B (CTX-1B) in red snapper (Lutjanus bohar) implicated in a CFP case in Sabah, Malaysia, in December 2017 was determined by single-quadrupole selected ion monitoring (SIM) liquid chromatography/mass spectrometry (LC/MS). Continuous consumption of the toxic fish likely resulted in CFP, even when the toxin concentration in the fish consumed was low. The identification of the fish species was performed using the molecular characterization of the mitochondrial cytochrome c oxidase subunit I gene marker, with a phylogenetic analysis of the genus Lutjanus. This is the first report identifying the causative toxin in fish-implicated CFP in Malaysia.

Critical Review and Conceptual and Quantitative Models for the Transfer and Depuration of Ciguatoxins in Fishes

We review and develop conceptual models for the bio-transfer of ciguatoxins in food chains for Platypus Bay and the Great Barrier Reef on the east coast of Australia. Platypus Bay is unique in repeatedly producing ciguateric fishes in Australia, with ciguatoxins produced by benthic dinoflagellates (Gambierdiscus spp.) growing epiphytically on free-living, benthic macroalgae. The Gambierdiscus are consumed by invertebrates living within the macroalgae, which are preyed upon by small carnivorous fishes, which are then preyed upon by Spanish mackerel (Scomberomorus commerson). We hypothesise that Gambierdiscus and/or Fukuyoa species growing on turf algae are the main source of ciguatoxins entering marine food chains to cause ciguatera on the Great Barrier Reef. The abundance of surgeonfish that feed on turf algae may act as a feedback mechanism controlling the flow of ciguatoxins through this marine food chain. If this hypothesis is broadly applicable, then a reduction in herbivory from overharvesting of herbivores could lead to increases in ciguatera by concentrating ciguatoxins through the remaining, smaller population of herbivores. Modelling the dilution of ciguatoxins by somatic growth in Spanish mackerel and coral trout (Plectropomus leopardus) revealed that growth could not significantly reduce the toxicity of fish flesh, except in young fast-growing fishes or legal-sized fishes contaminated with low levels of ciguatoxins. If Spanish mackerel along the east coast of Australia can depurate ciguatoxins, it is most likely with a half-life of ≤1-year. Our review and conceptual models can aid management and research of ciguatera in Australia, and globally.

Acute Toxicity of Gambierone and Quantitative Analysis of Gambierones Produced by Cohabitating Benthic Dinoflagellates

Understanding the toxicity and production rates of the various secondary metabolites produced by Gambierdiscus and cohabitating benthic dinoflagellates is essential to unravelling the complexities associated with ciguatera poisoning. In the present study, a sulphated cyclic polyether, gambierone, was purified from Gambierdiscus cheloniae CAWD232 and its acute toxicity was determined using intraperitoneal injection into mice. It was shown to be of low toxicity with an LD₅₀ of 2.4 mg/kg, 9600 times less toxic than the commonly implicated Pacific ciguatoxin-1B, indicating it is unlikely to play a role in ciguatera poisoning. In addition, the production of gambierone and 44-methylgambierone was assessed from 20 isolates of ten Gambierdiscus, two Coolia and two Fukuyoa species using quantitative liquid chromatography–tandem mass spectrometry. Gambierone was produced by seven Gambierdiscus species, ranging from 1 to 87 pg/cell, and one species from each of the genera Coolia and Fukuyoa, ranging from 2 to 17 pg/cell. The production of 44-methylgambierone ranged from 5 to 270 pg/cell and was ubiquitous to all Gambierdiscus species tested, as well as both species of Coolia and Fukuyoa. The relative production ratio of these two secondary metabolites revealed that only two species produced more gambierone, G. carpenteri CAWD237 and G. cheloniae CAWD232. This represents the first report of gambierone acute toxicity and production by these cohabitating benthic dinoflagellate species. While these results demonstrate that gambierones are unlikely to pose a risk to human health, further research is required to understand if they bioaccumulate in the marine food web.

Deeper insight into Gambierdiscus polynesiensis toxin production relies on specific optimization of high-performance liquid chromatography-high resolution mass spectrometry

Ciguatera food poisoning affects consumer health and fisheries' economies worldwide in tropical zones, and specifically in the Pacific area. The wide variety of ciguatoxins bio-accumulated in fish or shellfish responsible for this neurological illness are produced by marine dinoflagellates of the genus Gambierdiscus and bio-transformed through the food web. The evaluation of the contents of ciguatoxins in strains of Gambierdiscus relies on the availability of standards and on the development of sensitive and specific tools to detect them. There is a need for sensitive methods for the analysis of pacific ciguatoxins with high resolution mass spectrometry to ensure unequivocal identification of all congeners. We have applied a fractional factorial design of experiment 2^^8-3 for the screening of the significance of eight parameters potentially influencing ionization and ion transmission and their interactions to evaluate the behavior of sodium adducts, protonated molecules and first water losses of CTX4A/B, CTX3B/C, 2-OH-CTX3C and 44-methylgambierone on a Q-TOF equipment. The four parameters that allowed to significantly increase the peak areas of ciguatoxins and gambierones (up to a factor ten) were the capillary voltage, the sheath gas temperature, the ion funnel low pressure voltage and the ion funnel exit voltage. The optimized method was applied to revisit the toxin profile of G. polynesiensis (strain TB92) with a confirmation of the presence of M-seco-CTX4A only putatively reported so far and the detection of an isomer of CTX4A. The improvement in toxin detection also allowed to obtain informative high resolution targeted MS/MS spectra revealing high similarity in fragmentation patterns between putative isomer (4) of CTX3C, 2-OH-CTX3C and CTX3B on one side and between CTX4A, M-seco-CTX4A and the putative isomer on the other side, suggesting a relation of constitutional isomerism between them for both isomers.

Improving in vitro ciguatoxin and brevetoxin detection: selecting neuroblastoma (Neuro-2a) cells with lower sensitivity to ouabain and veratridine (OV-LS)

Marine biotoxins accumulating in seafood products pose a risk to human health. These toxins are often potent in minute amounts and contained within complex matrices; requiring sensitive, reliable, and robust methods for their detection. The mouse neuroblastoma (Neuro-2a) cytotoxicity assay (N2a-assay) is a sensitive, high-throughput, in vitro method effective for detecting sodium channel-specific marine biotoxins. The N2a-assay can be conducted to distinguish between specific effects on voltage-gated sodium (Na_V) channels, caused by toxins that activate (e.g., ciguatoxins (CTXs), brevetoxins (PbTxs)) or block (e.g., tetrodotoxins, saxitoxins) the target Na_V. The sensitivity and specificity of the assay to compounds activating the Na_V are achieved through the addition of the pharmaceuticals ouabain (O) and veratridine (V). However, these compounds can be toxic to Neuro-2a cells and their application at insufficient or excessive concentrations can reduce the effectiveness of this assay for marine toxin detection. Therefore, during growth incubation, Neuro-2a cells were exposed to O and V, and surviving cells exhibiting a lower sensitivity to O and V (OV-LS) were propagated. OV-LS Neuro-2a cells were selected for 60-80% survival when exposed to 0.22/0.022 mM O/V during the cytotoxicity assay. At these conditions, OV-LS N2a cells demonstrated a 3.5-fold higher survival rate 71% ± 7.9 SD (n = 232), and lower sensitivity to O/V, compared to the original Neuro-2a cells 20% ± 9.0 SD (n = 16). Additionally, OV-LS N2a cells were 1.3-2.6-fold more sensitive for detecting CTX3C 1.35 pg/ml, CTX1B 2.06 pg/ml, and PbTx-3 3.04 ng/ml compared to Neuro-2a cells using 0.1/0.01 mM O/V. Detection of CTX3C in a complex fish matrix using OV-LS cells was 0.0048 pg CTX3C/mg fish tissue equivalent. This work shows the potential for a significant improvement in sensitivity for CTX3C, CTX1B, and PbTx-3 using the OV-LS N2a-assay.

Marine invertebrate interactions with Harmful Algal Blooms - Implications for One Health

Harmful Algal Blooms (HAB) are natural atypical proliferations of micro or macro algae in either marine or freshwater environments which have significant impacts on human, animal and ecosystem health. The causative HAB organisms are primarily dinoflagellates and diatoms in marine and cyanobacteria within freshwater ecosystems. Several hundred species of HABs, most commonly marine dinoflagellates affect animal and ecosystem health either directly through physical, chemical or biological impacts on surrounding organisms or indirectly through production of algal toxins which transfer through lower-level trophic organisms to higher level predators. Traditionally, a major focus of HABs has concerned their natural production of toxins which bioaccumulate in filter-feeding invertebrates, which with subsequent trophic transfer and biomagnification cause issues throughout the food web, including the human health of seafood consumers. Whilst in many regions of the world, regulations, monitoring and risk management strategies help mitigate against the impacts from HAB/invertebrate toxins upon human health, there is ever-expanding evidence describing enormous impacts upon invertebrate health, as well as the health of higher trophic level organisms and marine ecosystems. This paper provides an overview of HABs and their relationships with aquatic invertebrates, together with a review of their combined impacts on animal, human and ecosystem health. With HAB/invertebrate outbreaks expected in some regions at higher frequency and intensity in the coming decades, we discuss the needs for new science, multi-disciplinary assessment and communication which will be essential for ensuring a continued increasing supply of aquaculture foodstuffs for further generations.

Evidence for the Range Expansion of Ciguatera in French Polynesia: A Revisit of the 2009 Mass-Poisoning Outbreak in Rapa Island (Australes Archipelago)

Ciguatera poisoning (CP) results from the consumption of seafood contaminated with ciguatoxins (CTXs). This disease is highly prevalent in French Polynesia with several well-identified hotspots. Rapa Island, the southernmost inhabited island in the country, was reportedly free of CP until 2007. This study describes the integrated approach used to investigate the etiology of a fatal mass-poisoning outbreak that occurred in Rapa in 2009. Symptoms reported in patients were evocative of ciguatera. Several Gambierdiscus field samples collected from benthic assemblages tested positive by the receptor binding assay (RBA). Additionally, the toxicity screening of ≈250 fish by RBA indicated ≈78% of fish could contain CTXs. The presence of CTXs in fish was confirmed by liquid chromatography tandem mass spectrometry (LC-MS/MS). The potential link between climate change and this range expansion of ciguatera to a subtropical locale of French Polynesia was also examined based on the analysis of temperature time-series data. Results are indicative of a global warming trend in Rapa area. A five-fold reduction in incidence rates was observed between 2009 and 2012, which was due in part to self-regulating behavior among individuals (avoidance of particular fish species and areas). Such observations underscore the prominent role played by community outreach in ciguatera risk management.

Lipophilic toxins occurrence in non-traditional invertebrate vectors from North Atlantic Waters (Azores, Madeira, and Morocco): Update on geographical tendencies and new challenges for monitoring routines

In the last decades, due to monitoring programs and strict legislation poisoning incidents occurrence provoked by ingestion of naturally contaminated marine organisms has decreased. However, climate change and anthropogenic interference contributed to the expansion and establishment of toxic alien species to more temperate ecosystems. In this work, the coasts of Madeira, São Miguel islands and the northwestern Moroccan coast were surveyed for four groups of lipophilic toxins (yessotoxins, azaspiracids, pectenotoxins, and spirolides), searching for new vectors and geographical tendencies. Twenty-four species benthic organisms were screened using UHPLC-MS/MS technique. We report 19 new vectors for these toxins, six of them with commercial interest (P. aspera, P. ordinaria, C. lampas, P. pollicipes, H. tuberculata and P. lividus). Regarding toxin uptake a south-north gradient was detected. This study contributes to the update of monitoring routines and legislation policies, comprising a wider range of vectors, to better serve consumers and ecosystems preservation.

Rapid detection of ciguatoxins in Gambierdiscus and Fukuyoa with immunosensing tools

Consumption of seafood contaminated with ciguatoxins (CTXs) leads to a foodborne disease known as ciguatera. Primary producers of CTXs are epibenthic dinoflagellates of the genera Gambierdiscus and Fukuyoa. In this study, thirteen Gambierdiscus and Fukuyoa strains were cultured, harvested at exponential phase, and CTXs were extracted with an implemented rapid protocol. Microalgal extracts were obtained from pellets with a low cell abundance (20,000 cell/mL) and were then analyzed with magnetic bead (MB)-based immunosensing tools (colorimetric immunoassay and electrochemical immunosensor). It is the first time that these approaches are used to screen Gambierdiscus and Fukuyoa strains, providing not only a global indication of the presence of CTXs, but also the ability to discriminate between two series of congeners (CTX1B and CTX3C). Analysis of the microalgal extracts revealed the presence of CTXs in 11 out of 13 strains and provided new information about Gambierdiscus and Fukuyoa toxin profiles. The use of immunosensing tools in the analysis of microalgal extracts facilitates the elucidation of further knowledge regarding these dinoflagellate genera and can contribute to improved ciguatera risk assessment and management.

Neurological Disturbances of Ciguatera Poisoning: Clinical Features and Pathophysiological Basis

Ciguatera fish poisoning (CFP), the most prevalent seafood poisoning worldwide, is caused by the consumption of tropical and subtropical fish contaminated with potent neurotoxins called ciguatoxins (CTXs). Ciguatera is a complex clinical syndrome in which peripheral neurological signs predominate in the acute phase of the intoxication but also persist or reoccur long afterward. Their recognition is of particular importance in establishing the diagnosis, which is clinically-based and can be a challenge for physicians unfamiliar with CFP. To date, no specific treatment exists. Physiopathologically, the primary targets of CTXs are well identified, as are the secondary events that may contribute to CFP symptomatology. This review describes the clinical features, focusing on the sensory disturbances, and then reports on the neuronal targets and effects of CTXs, as well as the neurophysiological and histological studies that have contributed to existing knowledge of CFP neuropathophysiology at the molecular, neurocellular and nerve levels.

Effects of substratum and depth on benthic harmful dinoflagellate assemblages

Microhabitats influence the distribution and abundance of benthic harmful dinoflagellate (BHAB) species. Currently, much of the information on the relationships between BHABs and microhabitat preferences is based on non-quantitative anecdotal observations, many of which are contradictory. The goal of this study was to better quantify BHAB and microhabitat relationships using a statistically rigorous approach. Between April 2016 to May 2017, a total of 243 artificial substrate samplers were deployed at five locations in the Perhentian Islands, Malaysia while simultaneous photo-quadrat surveys were performed to characterize the benthic substrates present at each sampling site. The screen samplers were retrieved 24 h later and the abundances of five BHAB genera, Gambierdiscus, Ostreopsis, Coolia, Amphidinium, and Prorocentrum were determined. Substrate data were then analyzed using a Bray–Curtis dissimilarity matrix to statistically identify distinct microhabitat types. Although BHABs were associated with a variety of biotic and abiotic substrates, the results of this study demonstrated differing degrees of microhabitat preference. Analysis of the survey results using canonical correspondence analysis explained 70.5% (horizontal first axis) and 21.6% (vertical second axis) of the constrained variation in the distribution of various genera among microhabitat types. Prorocentrum and Coolia appear to have the greatest range being broadly distributed among a wide variety of microhabitats. Amphidinium was always found in low abundances and was widely distributed among microhabitats dominated by hard coral, turf algae, sand and silt, and fleshy algae and reached the highest abundances there. Gambierdiscus and Ostreopsis had more restricted distributions. Gambierdiscus were found preferentially associated with turf algae, hard coral and, to a lesser extent, fleshy macroalgae microhabitats. Ostreopsis, almost always more abundant than Gambierdiscus, preferred the same microhabitats as Gambierdiscus and were found in microbial mats as well. With similar habitat preferences Ostreopsis may serve as an indicator organism for the presence of Gambierdiscus. This study provides insight into how BHAB-specific microhabitat preferences can affect toxicity risks.

Microalgae proteins: production, separation, isolation, quantification, and application in food and feed

Many countries have been experienced an increase in protein consumption due to the population growth and adoption of protein-rich dietaries. Unfortunately, conventional-based protein agroindustry is associated with environmental impacts that might aggravate as the humankind increase. Thus, it is important to screen for novel protein sources that are environmentally friendly. Microalgae farming is a promising alternative to couple the anthropic emissions with the production of food and feed. Some microalgae show protein contents two times higher than conventional protein sources. The use of whole microalgae biomass as a protein source in food and feed is simple and well-established. Conversely, the production of microalgae protein supplements and isolates requires the development of feasible and robust processes able to fractionate the microalgae biomass in different value-added products. Since most of the proteins are inside the microalgae cells, several techniques of disruption have been proposed to increase the efficiency to extract them. After the disruption of the microalgae cells, the proteins can be extracted, concentrated, isolated or purified allowing the development of different products. This critical review addresses the current state of the production of microalgae proteins for multifarious applications, and possibilities to concatenate the production of proteins and advanced biofuels.

Revisiting the Neuroblastoma Cell-Based Assay (CBA-N2a) for the Improved Detection of Marine Toxins Active on Voltage Gated Sodium Channels (VGSCs)

The neuroblastoma cell-based assay (CBA-N2a) is widely used for the detection of marine biotoxins in seafood products, yet a consensus protocol is still lacking. In this study, six key parameters of CBA-N2a were revisited: cell seeding densities, cell layer viability after 26 h growth, MTT incubation time, Ouabain and Veratridine treatment and solvent and matrix effects. A step-by-step protocol was defined identifying five viability controls for the validation of CBA-N2a results. Specific detection of two voltage gated sodium channel activators, pacific ciguatoxin (P-CTX3C) and brevetoxin (PbTx3) and two inhibitors, saxitoxin (STX) and decarbamoylsaxitoxin (dc-STX) was achieved, with EC50 values of 1.7 ± 0.35 pg/mL, 5.8 ± 0.9 ng/mL, 3 ± 0.5 ng/mL and 15.8 ± 3 ng/mL, respectively. When applied to the detection of ciguatoxin (CTX)-like toxicity in fish samples, limit of detection (LOD) and limit of quantification (LOQ) values were 0.031 ± 0.008 and 0.064 ± 0.016 ng P-CTX3C eq/g of flesh, respectively. Intra and inter-assays comparisons of viability controls, LOD, LOQ and toxicity in fish samples gave coefficients of variation (CVs) ranging from 3% to 29%. This improved test adaptable to either high throughput screening or composite toxicity estimation is a useful starting point for a standardization of the CBA-N2a in the field of marine toxin detection.

Transcriptomic analysis of polyketide synthases in a highly ciguatoxic dinoflagellate, Gambierdiscus polynesiensis and low toxicity Gambierdiscus pacificus, from French Polynesia

Marine dinoflagellates produce a diversity of polyketide toxins that are accumulated in marine food webs and are responsible for a variety of seafood poisonings. Reef-associated dinoflagellates of the genus Gambierdiscus produce toxins responsible for ciguatera poisoning (CP), which causes over 50,000 cases of illness annually worldwide. The biosynthetic machinery for dinoflagellate polyketides remains poorly understood. Recent transcriptomic and genomic sequencing projects have revealed the presence of Type I modular polyketide synthases in dinoflagellates, as well as a plethora of single domain transcripts with Type I sequence homology. The current transcriptome analysis compares polyketide synthase (PKS) gene transcripts expressed in two species of Gambierdiscus from French Polynesia: a highly toxic ciguatoxin producer, G. polynesiensis, versus a non-ciguatoxic species G. pacificus, each assembled from approximately 180 million Illumina 125 nt reads using Trinity, and compares their PKS content with previously published data from other Gambierdiscus species and more distantly related dinoflagellates. Both modular and single-domain PKS transcripts were present. Single domain β-ketoacyl synthase (KS) transcripts were highly amplified in both species (98 in G. polynesiensis, 99 in G. pacificus), with smaller numbers of standalone acyl transferase (AT), ketoacyl reductase (KR), dehydratase (DH), enoyl reductase (ER), and thioesterase (TE) domains. G. polynesiensis expressed both a larger number of multidomain PKSs, and larger numbers of modules per transcript, than the non-ciguatoxic G. pacificus. The largest PKS transcript in G. polynesiensis encoded a 10,516 aa, 7 module protein, predicted to synthesize part of the polyether backbone. Transcripts and gene models representing portions of this PKS are present in other species, suggesting that its function may be performed in those species by multiple interacting proteins. This study contributes to the building consensus that dinoflagellates utilize a combination of Type I modular and single domain PKS proteins, in an as yet undefined manner, to synthesize polyketides.

Uptake and Depuration Kinetics of Pacific Ciguatoxins in Orange-Spotted Grouper (Epinephelus coioides)

Ciguatoxins (CTXs), produced by toxic benthic dinoflagellates, can bioaccumulate in marine organisms at higher trophic levels. The current study evaluated the uptake and depuration kinetics of some of the most potent CTXs, Pacific CTX-1, -2, and -3 (P-CTX-1, -2, and -3), in orange-spotted grouper (Epinephelus coioides) exposed to 1 ng P-CTXs g^-1 fish daily. Over a 30 d exposure, P-CTX-1, -2, and -3 were consistently detected in various tissues of exposed fish, and the concentrations of the total P-CTXs in tissues generally ranked following the order of liver, intestine, gill, skin, brain, and muscle. Relatively higher uptake rates of P-CTX-1 in the groupers were observed compared with those of P-CTX-2 and -3. The depuration rate constants of P-CTX-1, -2, and -3 in different tissues were (0.996-16.5) × 10^-2, (1.51-16.1) × 10^-2, and (0.557-10.6) × 10^-2 d^-1, respectively. The accumulation efficiencies of P-CTX-1, -2, and -3 in whole groupers were 6.13%, 2.61%, and 1.15%, respectively. The increasing proportion of P-CTX-1 and the decreasing proportion of P-CTX-2 and -3 over the exposure phase suggest a likely biotransformation of P-CTX-2 and -3 to P-CTX-1, leading to higher levels of P-CTX-1 in fish and possibly a higher risk of CTXs in long-term exposed fish.

Global impact of ciguatoxins and ciguatera fish poisoning on fish, fisheries and consumers

Ciguatera fish poisoning (CFP) is one of the most devastating food-borne illnesses caused by fish consumption. Ciguatoxins (CTXs) are potent neurotoxins synthesized by the benthic microalgae Gambierdiscus spp. and Fukuyoa spp. that are transmitted to fish by grazing and predation. Despite the high incidence of CFP, affecting an estimated number of 50,000 persons per year in tropical and subtropical latitudes, the factors underlying CTXs occurrence are still not well understood. Toxin transfer and dynamics in fish and food-webs are complex. Feeding habits and metabolic pathways determine the toxin profile and toxicity of fish, and migratory species may transport and spread the hazard. Furthermore, CTX effect on fish may be a limiting factor for fish recruitment and toxin prevalence. Recently, new occurrences of Gambierdiscus spp. in temperate areas have been concomitant with the detection of toxic fish and CFP incidents in non-endemic areas. CFP cases in Europe have led to implementation of monitoring programs and fisheries restrictions with considerable impact on local economies. More than 400 species of fish can be vectors of CTXs, and most of them are high-valued commercial species. Thus, the risk uncertainty and the spread of Gambierdiscus have serious consequences for fisheries and food safety. Here, we present a critical review of CTXs impacts on fish, fisheries, and humans, based on the current knowledge on CFP incidence and CTXs prevalence in microalgae and fish.

Climate change and harmful benthic microalgae

Sea surface temperatures in the world's oceans are projected to warm by 0.4-1.4 °C by mid twenty-first century causing many tropical and sub-tropical harmful dinoflagellate genera like Gambierdiscus, Fukuyoa and Ostreopsis (benthic harmful algal bloom species, BHABs) to exhibit higher growth rates over much of their current geographic range, resulting in higher population densities. The primary exception to this trend will be in the tropics where temperatures exceed species-specific upper thermal tolerances (30-31 °C) beyond which growth slows significantly. As surface waters warm, migration to deeper habitats is expected to provide refuge. Range extensions of several degrees of latitude also are anticipated, but only where species-specific habitat requirements can be met (e.g., temperature, suitable substrate, low turbulence, light, salinity, pH). The current understanding of habitat requirements that determine species distributions are reviewed to provide fuller understanding of how individual species will respond to climate change from the present to 2055 while addressing the paucity of information on environmental factors controlling small-scale distribution in localized habitats. Based on the available information, we hypothesized how complex environmental interactions can influence abundance and potential range extensions of BHAB species in different biogeographic regions and identify sentinel sites appropriate for long-term monitoring programs to detect range extensions and reduce human health risks.

Intraspecific Variability in the Toxin Production and Toxin Profiles of In Vitro Cultures of Gambierdiscus polynesiensis (Dinophyceae) from French Polynesia

Ciguatera poisoning (CP) is a foodborne disease caused by the consumption of seafood contaminated with ciguatoxins (CTXs) produced by dinoflagellates in the genera Gambierdiscus and Fukuyoa. The toxin production and toxin profiles were explored in four clones of G. polynesiensis originating from different islands in French Polynesia with contrasted CP risk: RIK7 (Mangareva, Gambier), NHA4 (Nuku Hiva, Marquesas), RAI-1 (Raivavae, Australes), and RG92 (Rangiroa, Tuamotu). Productions of CTXs, maitotoxins (MTXs), and gambierone group analogs were examined at exponential and stationary growth phases using the neuroblastoma cell-based assay and liquid chromatography–tandem mass spectrometry. While none of the strains was found to produce known MTX compounds, all strains showed high overall P-CTX production ranging from 1.1 ± 0.1 to 4.6 ± 0.7 pg cell⁻¹. In total, nine P-CTX analogs were detected, depending on strain and growth phase. The production of gambierone, as well as 44-methylgamberione, was also confirmed in G. polynesiensis. This study highlighted: (i) intraspecific variations in toxin production and profiles between clones from distinct geographic origins and (ii) the noticeable increase in toxin production of both CTXs, in particular CTX4A/B, and gambierone group analogs from the exponential to the stationary phase.

Ciguatera poisoning in French Polynesia: insights into the novel trends of an ancient disease

Ciguatera is a non-bacterial seafood poisoning highly prevalent in French Polynesia where it constitutes a major health issue and a major threat to food sustainability and food security for local populations. Ciguatera results from the bioaccumulation in marine food webs of toxins known as ciguatoxins, originating from benthic dinoflagellates in the genera Gambierdiscus and Fukuyoa. Ciguatera is characterized by a complex array of gastrointestinal, neurological and cardiovascular symptoms. The effective management of patients is significantly hampered by the occurrence of atypical forms and/or chronic sequelae in some patients, and the lack of both a confirmatory diagnosis test and a specific antidote. In addition, recent findings have outlined the implication of novel species of the causative organisms as well as new vectors, namely marine invertebrates, in ciguatera outbreaks. Another novel trend relates to the geographical expansion of this disease to previously unaffected areas, not only in certain island groups of French Polynesia but also in temperate regions worldwide, as a likely consequence of the effects of climate change.

Identification of Causative Ciguatoxins in Red Snappers Lutjanus bohar Implicated in Ciguatera Fish Poisonings in Vietnam

Ciguatera fish poisoning (CFP) is a type of food poisoning caused by the consumption of a variety of toxic ciguatera fish species in the tropical and subtropical waters. Although there have been a large number of suspected CFP cases in the Southeast Asian countries, few were confirmed with causative ciguatoxins (CTXs), and reliable information on the symptoms still remains rather limited. In the present study, CTXs in red snapper Lutjanus bohar, implicated in two suspected CFP cases in Vietnam in 2014 and 2016, were determined by use of the single-quadrupole selected ion monitoring (SIM) liquid chromatography/mass spectrometry (LC/MS). Ciguatoxin-1B (CTX-1B), 54-deoxyCTX-1B, and 52-epi-54-deoxyCTX-1B were detected in the red snapper by our LC/MS method. Moreover, CTX-1B, 54-deoxyCTX-1B, and 52-epi-54-deoxyCTX-1B were further identified by the time of flight (TOF) LC/MS with the exact mass spectrum. The CTX profile of the red snapper in Vietnam is similar to those of ciguatera fish from Australia, Okinawa Islands in Japan, Kiribati, and Hong Kong. This is the first comprehensive report unambiguously identifying the causative toxins in fish implicated with reliable information on the poisoning symptoms in CFP in Vietnam and/or Southeast Asian countries.

Habitat complexity affects benthic harmful dinoflagellate assemblages in the fringing reef of Rawa Island, Malaysia

Few studies have investigated the effect of fine-scale habitat differences on the dynamics of benthic harmful dinoflagellate assemblages. To determine how these microhabitat differences affect the distribution and abundance of the major benthic harmful dinoflagellate genera in a tropical coral reef ecosystem, a field study was undertaken between April-September 2015 and January 2016 on the shallow reef flat of the fringing reef of Rawa Island, Terengganu, Malaysia. Sampling of benthic dinoflagellates was carried out using an artificial substrate sampling method (fiberglass screens). Benthic microhabitats surrounding the sampling screens were characterized simultaneously from photographs of a 0.25-m² quadrat based on categories of bottom substrate types. Five taxonomic groups of benthic dinoflagellates, Ostreopsis, Gambierdiscus, Prorocentrum, Amphidinium, and Coolia were identified, and cells were enumerated using a light microscope. The results showed Gambierdiscus was less abundant than other genera throughout the study period, with maximum abundance of 1.2 × 10³ cells 100 cm^-2. While most taxa were present on reefs with high coral cover, higher cell abundances were observed in reefs with high turf algal cover and coral rubble, with the exception of Ostreopsis, where the abundance reached a maximum of 3.4 × 10⁴ cells 100 cm^-2 in habitats with high coral cover. Microhabitat heterogeneity was identified as a key factor governing the benthic harmful dinoflagellate assemblages and may account for much of the observed variability in dominant taxa. This finding has significant implications for the role of variability in the benthic harmful algal bloom (BHAB) outbreaks and the potential in identifying BHAB-related toxin transfer pathways and the key vectors in the food webs.

Experimental evidence of dietary ciguatoxin accumulation in an herbivorous coral reef fish

Ciguatoxins (CTXs) are potent algal toxins that cause widespread ciguatera poisoning and are found ubiquitously in coral reef food webs. Here we developed an environmentally-relevant, experimental model of CTX trophic transfer involving dietary exposure of herbivorous fish to the CTX-producing microalgae Gambierdiscus polynesiensis. Juvenile Naso brevirostris were fed a gel-food embedded with microalgae for 16 weeks (89 cells g^-1 fish daily, 0.4 μg CTX3C equiv kg^-1 fish). CTXs in muscle tissue were detectable after 2 weeks at levels above the threshold for human intoxication (1.2 ± 0.2 μg CTX3C equiv kg^-1). Although tissue CTX concentrations stabilized after 8 weeks (∼3 ± 0.5 μg CTX3C equiv kg^-1), muscle toxin burden (total μg CTX in muscle tissue) continued to increase linearly through the end of the experiment (16 weeks). Toxin accumulation was therefore continuous, yet masked by somatic growth dilution. The observed CTX concentrations, accumulation rates, and general absence of behavioural signs of intoxication are consistent with field observations and indicate that this method of dietary exposure may be used to develop predictive models of tissue-specific CTX uptake, metabolism and depuration. Results also imply that slow-growing fish may accumulate higher CTX flesh concentrations than fast-growing fish, which has important implications for global seafood safety.

Tissue Distribution and Elimination of Ciguatoxins in Tridacna maxima (Tridacnidae, Bivalvia) Fed Gambierdiscus polynesiensis

Ciguatera is a foodborne disease caused by the consumption of seafood contaminated with ciguatoxins (CTXs). Ciguatera-like poisoning events involving giant clams (Tridacna maxima) are reported occasionally from Pacific islands communities. The present study aimed at providing insights into CTXs tissue distribution and detoxification rate in giant clams exposed to toxic cells of Gambierdiscus polynesiensis, in the framework of seafood safety assessment. In a first experiment, three groups of tissue (viscera, flesh and mantle) were dissected from exposed individuals, and analyzed for their toxicity using the neuroblastoma cell-based assay (CBA-N2a) and liquid chromatography-tandem mass spectrometry (LC-MS/MS) analyses. The viscera, flesh, and mantle were shown to retain 65%, 25%, and 10% of the total toxin burden, respectively. All tissues reached levels above the safety limit recommended for human consumption, suggesting that evisceration alone, a practice widely used among local populations, is not enough to ensure seafood safety. In a second experiment, the toxin content in contaminated giant clams was followed at different time points (0, 2, 4, and 6 days post-exposure). Observations suggest that no toxin elimination is visible in T. maxima throughout 6 days of detoxification.

Toxicological Investigations on the Sea Urchin Tripneustes gratilla (Toxopneustidae, Echinoid) from Anaho Bay (Nuku Hiva, French Polynesia): Evidence for the Presence of Pacific Ciguatoxins

The sea urchin Tripneustes gratilla (Toxopneustidae, Echinoids) is a source of protein for many islanders in the Indo-West Pacific. It was previously reported to occasionally cause ciguatera-like poisoning; however, the exact nature of the causative agent was not confirmed. In April and July 2015, ciguatera poisonings were reported following the consumption of T.gratilla in Anaho Bay (Nuku Hiva Island, Marquesas archipelago, French Polynesia). Patient symptomatology was recorded and sea urchin samples were collected from Anaho Bay in July 2015 and November 2016. Toxicity analysis using the neuroblastoma cell-based assay (CBA-N2a) detected the presence of ciguatoxins (CTXs) in T.gratilla samples. Gambierdiscus species were predominant in the benthic assemblages of Anaho Bay, and G.polynesiensis was highly prevalent in in vitro cultures according to qPCR results. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analyses revealed that P-CTX-3B was the major ciguatoxin congener in toxic sea urchin samples, followed by 51-OH-P-CTX-3C, P-CTX-3C, P-CTX-4A, and P-CTX-4B. Between July 2015 and November 2016, the toxin content in T.gratilla decreased, but was consistently above the safety limit allowed for human consumption. This study provides evidence of CTX bioaccumulation in T.gratilla as a cause of ciguatera-like poisoning associated with a documented symptomatology.

Ciguatoxins activate the Calcineurin signalling pathway in Yeasts: Potential for development of an alternative detection tool?

Ciguatoxins (CTXs) are lipid-soluble polyether compounds produced by dinoflagellates from the genus Gambierdiscus spp. typically found in tropical and subtropical zones. This endemic area is however rapidly expanding due to environmental perturbations, and both toxic Gambierdiscus spp. and ciguatoxic fishes have been recently identified in the North Atlantic Ocean (Madeira and Canary islands) and Mediterranean Sea. Ciguatoxins bind to Voltage Gated Sodium Channels on the membranes of sensory neurons, causing Ciguatera Fish Poisoning (CFP) in humans, a disease characterized by a complex array of gastrointestinal, neurological, neuropsychological, and cardiovascular symptoms. Although CFP is the most frequently reported non bacterial food-borne poisoning worldwide, there is still no simple and quick way of detecting CTXs in contaminated samples. In the prospect to engineer rapid and easy-to-use CTXs live cells-based tests, we have studied the effects of CTXs on the yeast Saccharomyces cerevisiae, a unicellular model which displays a remarkable conservation of cellular signalling pathways with higher eukaryotes. Taking advantage of this high level of conservation, yeast strains have been genetically modified to encode specific transcriptional reporters responding to CTXs exposure. These yeast strains were further exposed to different concentrations of either purified CTX or micro-algal extracts containing CTXs. Our data establish that CTXs are not cytotoxic to yeast cells even at concentrations as high as 1μM, and cause an increase in the level of free intracellular calcium in yeast cells. Concomitantly, a dose-dependent activation of the calcineurin signalling pathway is observed, as assessed by measuring the activity of specific transcriptional reporters in the engineered yeast strains. These findings offer promising prospects regarding the potential development of a yeast cells-based test that could supplement or, in some instances, replace current methods for the routine detection of CTXs in seafood products.

Tectus niloticus (Tegulidae, Gastropod) as a Novel Vector of Ciguatera Poisoning: Clinical Characterization and Follow-Up of a Mass Poisoning Event in Nuku Hiva Island (French Polynesia)

Ciguatera fish poisoning (CFP) is the most prevalent non-bacterial food-borne form of poisoning in French Polynesia, which results from the consumption of coral reef fish naturally contaminated with ciguatoxins produced by dinoflagellates in the genus Gambierdiscus. Since the early 2000s, this French territory has also witnessed the emergence of atypical forms of ciguatera, known as ciguatera shellfish poisoning (CSP), associated with the consumption of marine invertebrates. In June 2014, nine tourists simultaneously developed a major and persistent poisoning syndrome following the consumption of the gastropod Tectus niloticus collected in Anaho, a secluded bay of Nuku Hiva Island (Marquesas Archipelago, French Polynesia). The unusual nature and severity of this event prompted a multidisciplinary investigation in order to characterize the etiology and document the short/long-term health consequences of this mass-poisoning event. This paper presents the results of clinical investigations based on hospital medical records, medical follow-up conducted six and 20 months post-poisoning, including a case description. This study is the first to describe the medical signature of T. niloticus poisoning in French Polynesia and contributed to alerting local authorities about the potential health hazards associated with the consumption of this gastropod, which is highly prized by local communities in Pacific island countries and territories.

Dinoflagellate cyst abundance is positively correlated to sediment organic carbon in Sydney Harbour and Botany Bay, NSW, Australia

There is growing public concern about the global expansion of harmful algal bloom species (HABs), with dinoflagellate microalgae comprising the major portion of the harmful taxa. These motile, unicellular organisms have a lifecycle involving sexual reproduction and resting cyst formation whereby cysts can germinate from sediments and 'seed' planktonic populations. Thus, investigation of dinoflagellate cyst (dinocyst) distribution in sediments can provide significant insights into HAB dynamics and contribute to indices of habitat quality. Species composition and abundance of dinocysts in relation to sediment characteristics were studied at 18 stations in two densely populated temperate Australian estuaries, Sydney Harbour (Parramatta River/Port Jackson; PS) and Botany Bay (including Georges River; GB). Eighteen dinocyst taxa were identified, dominated by Protoceratium reticulatum and Gonyaulax sp.1 in the PS estuary, together with Archaeperidinium minutum and Gonyaulax sp.1 in the GB estuary. Cysts of Alexandrium catenella, which is one of the causative species of paralytic shellfish poisoning (PSP), were also detected in both estuaries. Out of the measured sediment characteristics (TOC, Cd, Cr, Cu, Fe, Pb, Mn, Ni, Zn and polycyclic aromatic hydrocarbons), TOC was the parameter explaining most of the variation in dinocyst assemblages and was positively correlated to most of the heavy metals. Given the significant relationship between sediment TOC and dinocyst abundance and heavy metal concentrations, this study suggests that sediment TOC could be broadly used in risk management for potential development of algal blooms and sediment contamination in these estuaries.

Application of solid phase adsorption toxin tracking (SPATT) devices for the field detection of Gambierdiscus toxins

Ciguatera fish poisoning is a food-borne illness caused by the consumption of seafood contaminated with ciguatoxins (CTXs) produced by dinoflagellates in the Gambierdiscus genus. Since most surveillance programs currently rely on the survey of Gambierdiscus cell densities and species composition, supplementary toxin-based methods allowing the time- and spatially integrated sampling of toxins in ciguateric environments are needed for a more reliable assessment and management of the risks associated with Gambierdiscus proliferation. Solid Phase Adsorption Toxin Tracking (SPATT) filters use porous synthetic resins capable of adsorbing toxins directly from the water column. To assess the ability of these passive monitoring devices to retain Gambierdiscus toxins, SPATT bags filled with 10g of HP20 resin were deployed for 48h in two French Polynesian locations at high (Nuku Hiva Island) vs. low to moderate (Kaukura Atoll) risk of ciguatera. CTXs could be detected in SPATT bags extracts from Nuku Hiva Island, as assessed by the mouse neuroblastoma cell-based assay (CBA-N2a) and liquid chromatography - tandem mass spectrometry (LC-MS/MS) analyses. Results of in vitro experiments suggest that the saturation limit of CTXs on HP20 resin, for a deployment time of 48h, is ≃ 55ng P-CTX-3C equiv. g^-1 resin. Despite the non detection of maitotoxin (MTX), LC-MS/MS analyses showed that two other compounds also produced by Gambierdiscus species were retained on SPATT bags, i.e. iso-P-CTX-3B/C and a putative MTX analogue, known as MTX-3. This study, the first to demonstrate the suitability of SPATT technology for the in situ monitoring of Gambierdiscus toxins, highlights the potential application of this tool for routine ciguatera risk assessment and management programs.

Tectus niloticus (Tegulidae, Gastropod) as a Novel Vector of Ciguatera Poisoning: Detection of Pacific Ciguatoxins in Toxic Samples from Nuku Hiva Island (French Polynesia)

Ciguatera fish poisoning (CFP) is a foodborne disease caused by the consumption of seafood (fish and marine invertebrates) contaminated with ciguatoxins (CTXs) produced by dinoflagellates in the genus Gambierdiscus. The report of a CFP-like mass-poisoning outbreak following the consumption of Tectus niloticus (Tegulidae, Gastropod) from Anaho Bay on Nuku Hiva Island (Marquesas archipelago, French Polynesia) prompted field investigations to assess the presence of CTXs in T. niloticus. Samples were collected from Anaho Bay, 1, 6 and 28 months after this poisoning outbreak, as well as in Taiohae and Taipivai bays. Toxicity analysis using the neuroblastoma cell-based assay (CBA-N2a) detected the presence of CTXs only in Anaho Bay T. niloticus samples. This is consistent with qPCR results on window screen samples indicating the presence of Gambierdiscus communities dominated by the species G. polynesiensis in Anaho Bay. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analyses revealed that P-CTX-3B was the major congener, followed by P-CTX-3C, P-CTX-4A and P-CTX-4B in toxic samples. Between July 2014 and November 2016, toxin content in T. niloticus progressively decreased, but was consistently above the safety limit recommended for human consumption. This study confirms for the first time T. niloticus as a novel vector of CFP in French Polynesia.

The Epiphytic Genus Gambierdiscus (Dinophyceae) in the Kermadec Islands and Zealandia Regions of the Southwestern Pacific and the Associated Risk of Ciguatera Fish Poisoning

Species in the genus Gambierdiscus produce ciguatoxins (CTXs) and/or maitotoxins (MTXs), which may cause ciguatera fish poisoning (CFP) in humans if contaminated fish are consumed. Species of Gambierdiscus have previously been isolated from macroalgae at Rangitahua (Raoul Island and North Meyer Islands, northern Kermadec Islands), and the opportunity was taken to sample for Gambierdiscus at the more southerly Macauley Island during an expedition in 2016. Gambierdiscus cells were isolated, cultured, and DNA extracted and sequenced to determine the species present. Bulk cultures were tested for CTXs and MTXs by liquid chromatography-mass spectrometry (LC-MS/MS). The species isolated were G. australes, which produced MTX-1 (ranging from 3 to 36 pg/cell), and G. polynesiensis, which produced neither MTX-1 nor, unusually, any known CTXs. Isolates of both species produced putative MTX-3. The risk of fish, particularly herbivorous fish, causing CFP in the Zealandia and Kermadec Islands region is real, although in mainland New Zealand the risk is currently low. Both Gambierdiscus and Fukuyoa have been recorded in the sub-tropical northern region of New Zealand, and so the risk may increase with warming seas and shift in the distribution of Gambierdiscus species.

Toxicity screening of 13 Gambierdiscus strains using neuro-2a and erythrocyte lysis bioassays

Species in the epi-benthic dinoflagellate genus Gambierdiscus produce ciguatoxins (CTXs) and maitotoxins (MTXs), which are among the most potent marine toxins known. Consumption of fish contaminated with sufficient quantities of CTXs causes Ciguatera Fish Poisoning (CFP), the largest cause of non-bacterial food poisoning worldwide. Maitotoxins, which can be found in the digestive system of fish, could also contribute to CFP if such tissues are consumed. Recently, an increasing number of Gambierdiscus species have been identified; yet, little is known about the variation in toxicity among Gambierdiscus strains or species. This study is the first assessment of relative CTX- and MTX-toxicity of Gambierdiscus species from areas as widespread as the North-Eastern Atlantic Ocean, Pacific Ocean and the Mediterranean Sea. A total of 13 strains were screened: (i) seven Pacific strains of G. australes, G. balechii, G. caribaeus, G. carpenteri, G. pacificus, G. scabrosus and one strain of an undetermined species (Gambierdiscus sp. Viet Nam), (ii) five strains from the North-Eastern Atlantic Ocean (two G. australes, a single G. excentricus and two G. silvae strains), and (iii) one G. carolinianus strain from the Mediterranean Sea. Cell pellets of Gambierdiscus were extracted with methanol and the crude extracts partitioned into a CTX-containing dichloromethane fraction and a MTX-containing aqueous methanol fraction. CTX-toxicity was estimated using the neuro-2a cytoxicity assay, and MTX-toxicity via a human erythrocyte lysis assay. Different species were grouped into different ratios of CTX- and MTX-toxicity, however, the ratio was not related to the geographical origin of species (Atlantic, Mediterranean, Pacific). All strains showed MTX-toxicity, ranging from 1.5 to 86pg MTX equivalents (eq) cell^-1. All but one of the strains showed relatively low CTX-toxicity ranging from 0.6 to 50 fg CTX3C eq cell^-1. The exception was the highly toxic G. excentricus strain from the Canary Islands, which produced 1426 fg CTX3C eq cell^-1. As was true for CTX, the highest MTX-toxicity was also found in G. excentricus. Thus, the present study confirmed that at least one species from the Atlantic Ocean demonstrates similar toxicity as the most toxic strains from the Pacific, even if the metabolites in fish have so far been shown to be more toxic in the Pacific Ocean.