The cyanobacterium Oscillatoria erythraea--a potential source of toxin in the ciguatera food-chain

Generation of Reactive Oxygen Species (ROS) by Harmful Algal Bloom (HAB)-Forming Phytoplankton and Their Potential Impact on Surrounding Living Organisms

Most marine phytoplankton with relatively high ROS generation rates are categorized as harmful algal bloom (HAB)-forming species, among which Chattonella genera is the highest ROS-producing phytoplankton. In this review, we examined marine microalgae with ROS-producing activities, with focus on Chattonella genera. Several studies suggest that Chattonella produces superoxide via the activities of an enzyme similar to NADPH oxidase located on glycocalyx, a cell surface structure, while hydrogen peroxide is generated inside the cell by different pathways. Additionally, hydroxyl radical has been detected in Chattonella cell suspension. By the physical stimulation, such as passing through between the gill lamellas of fish, the glycocalyx is easily discharged from the flagellate cells and attached on the gill surface, where ROS are continuously produced, which might cause gill tissue damage and fish death. Comparative studies using several strains of Chattonella showed that ROS production rate and ichthyotoxicity of Chattonella is well correlated. Furthermore, significant levels of ROS have been reported in other raphidophytes and dinoflagellates, such as Cochlodinium polykrikoides and Karenia mikimotoi. Chattonella is the most extensively studied phytoplankton in terms of ROS production and its biological functions. Therefore, this review examined the potential ecophysiological roles of extracellular ROS production by marine microalgae in aquatic environment.

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.

First evidence of palytoxin and 42-hydroxy-palytoxin in the marine cyanobacterium Trichodesmium

Marine pelagic diazotrophic cyanobacteria of the genus Trichodesmium (Oscillatoriales) are widespread throughout the tropics and subtropics, and are particularly common in the waters of New Caledonia. Blooms of Trichodesmium are suspected to be a potential source of toxins in the ciguatera food chain and were previously reported to contain several types of paralyzing toxins. The toxicity of water-soluble extracts of Trichodesmium spp. were analyzed by mouse bioassay and Neuroblastoma assay and their toxic compounds characterized using liquid chromatography coupled with tandem mass spectrometry techniques. Here, we report the first identification of palytoxin and one of its derivatives, 42-hydroxy-palytoxin, in field samples of Trichodesmium collected in the New Caledonian lagoon. The possible role played by Trichodesmium blooms in the development of clupeotoxism, this human intoxication following the ingestion of plankton-eating fish and classically associated with Ostreopsis blooms, is also discussed.

Detection of ciguatoxin-like and paralysing toxins in Trichodesmium spp. from New Caledonia lagoon

Marine pelagic cyanobacteria Trichodesmium are widespread in the New Caledonia lagoon. Blooms of these Oscillatoriales are suspected to be a potential source of toxins in the ciguatera food chain and were previously reported to contain certain types of paralysing toxins. In the present study, toxicity experiments were conducted on lipid- and water-soluble extracts of freeze-dried samples of these cyanobacteria. Lipid-soluble fractions revealed a ciguatoxin-like activity in both in vivo (mouse bioassay) and in vitro (mouse neuroblastoma cells assay and receptor binding assay using tritiated brevetoxin-3) assays. The water-soluble fractions tested on mice exhibited neurotoxicity with paralytic symptoms. These toxicities have also been observed with benthic filamentous cyanobacteria within the Oscillatoriales order, also collected in New Caledonia. This study provides an unprecedented evidence of the toxicity of Trichodesmium species from the New Caledonia lagoon. This survey also demonstrates the possible role of these cyanobacteria in ciguatera fish poisoning.

Marine toxic cyanobacteria: diversity, environmental responses and hazards

Toxic cyanobacterial blooms have been a primary concern predominantly in the plankton of freshwater bodies. Recently, however, the toxicity of benthic cyanobacteria is increasingly attracting attention of the scientific community and environmental agencies. The occurrence of toxic strains in benthic cyanobacteria is intimately linked to our understanding of the diversity and ecological responses of these organisms under field conditions. To that effect, we are engaged in combined morphotypic and genotypic characterization (polyphasic) of benthic natural populations of cyanobacteria in tropical lagoons and coral reefs, with the objective to provide a reliable reference for further comparative work. The methods of identification based on phenotypic properties and those based on molecular tools for genotypic identification are correlated. The approach is based on identifying the occurrences of cyanobacterial benthic blooms, tested for purity and analyzed by application of molecular tools. The questions addressed include the distinction between marine and freshwater taxa, between populations in geographically separate regions as well as between their potential vs. expressed toxicity.

Ciguatera: Australian perspectives on a global problem

Ciguatera is a global disease caused by the consumption of certain warm-water fish that have accumulated orally effective levels of sodium channel activator toxins (ciguatoxins) through the marine food chain. Symptoms of ciguatera arising from the consumption of ciguateric fish include a range of gastrointestinal, neurological and cardiovascular disturbances. This review examines progress in our understanding of ciguatera from an Australian perspective, especially the laboratory-based research into the problem that was initiated by the late "Bob" Endean at the University of Queensland.

Origin and transfer of toxins involved in ciguatera

1. Ciguatera is a disease caused by sodium channel activator toxins and results from the consumption of warm water fish contaminated by the ciguatoxin class of polyether toxins. 2. Other toxins, including okadaic acid and maitotoxin, have no proven role in causing human illness associated with ciguatera. 3. Ciguatera often affects only a discrete region of a reef, with flare-ups of ciguatera being both temporally and spatially unpredictable. 4. The ciguatoxins likely arise through the biotransformation and acid-catalysed spiroisomerisation of gambiertoxin-4A produced by Gambierdiscus toxicus and it is unlikely that other toxic benthic dinoflagellates are involved. 5. Events leading to a ciguatera outbreak are initiated by environmental and genetic factors that favour the proliferation of gambiertoxins, with an apparent role for anthropomorphic effects; however, the precise factors involved are yet to be determined. 6. The gambiertoxins and/or ciguatoxins are transferred from the benthos to herbivorous species (fish, invertebrates etc) and then to carnivorous fish via marine food chains. 7. Factors influencing the concentration of ciguatoxins that accumulate in fish include the rate of dietary intake, the efficiency of assimilation, the degree and nature of any toxin biotransformation, the rate of depuration, and the rate of growth of fish.