Toxicology and ecology of Gambierdiscus from Australia : a dinoflagellate genus associated with ciguatera fish poisoning

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Some species from the epibenthic marine microalgal genus Gambierdiscus produce potent neurotoxins, such as ciguatoxins (CTXs) and maitotoxins (MTXs), which can accumulate in the marine food web and cause the human illness Ciguatera Fish Poisoning (CFP). The genus typically has a tropical distribution and is known to occur in the Great Barrier Reef region of north east Australia, although recently, populations have been documented in more temperate locations. In this thesis, a toxicological and ecological approach was used to investigate CFP causing organisms in Australia, with an emphasis on assessing the potential for temperate range extension of the genus in this region. Monoclonal isolates of Gambierdiscus were established from a tropical and a temperate location in eastern Australia and formed the foundation of experimental work. Four species of Gambierdiscus (G. cf. pacificus, G. cf. silvae, G. carpenteri and G. lapillus) were identified from the tropical location and only G. carpenteri was identified at the temperate location. Liquid Chromatography Tandem-Mass Spectrometry (LC-MS/MS) was used to assess whether isolates produced known microalgal CTXs (P-CTX-3B, 3C, 4A and 4B) and MTX-1, but these characterised toxins were not detected in any of the Gambierdiscus strains. Putative MTX-3, however, was detected in all strains, except the temperate G. carpenteri isolates. Using the novel Ca2+ influx bioassay, compounds displaying CTX-like activity were identified in extracts of G. cf. pacificus, G. cf. silvae and G. lapillus, and compounds displaying MTX-like activity were detected in all species tested. Fitness curves across environmental gradients of temperature, salinity and irradiance showed Gambierdiscus species can grow across a broad range of environmental conditions. The environmental niche of the tropical strains was not significantly different from that of the temperate strains, suggesting that tropical toxin producing Gambierdiscus species also have the capacity to occupy temperate locations. Rafting on detached macrophyte fragments that are transported poleward in the East Australian Current was identified as the likely natural long-distance dispersal mechanism for Gambierdiscus species in eastern Australia. The ability of Gambierdiscus to colonise new temperate locations was examined by studying the growth of different strains co-cultured within both natural and artificial epibenthic microalgal communities. These experiments confirmed that it may only require a single pulse of very few cells for successful colonisation of Gambierdiscus. This thesis advances knowledge about the diversity and toxicology of Gambierdiscus in eastern Australia, identifies the potential cause of CFP from this region and provides experimental evidence of the mechanisms that could facilitate temperate range extension of the genus. Results from this thesis therefore provide fundamental information for developing a management strategy to mitigate the risk of human exposure to CFP in eastern Australia.
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