Molecular ecology and toxicity of Alexandrium pacificum in Australian waters

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Species of the genus Alexandrium are one of the most studied dinoflagellates due to their production of the neurotoxins, Paralytic Shellfish Toxins (PSTs). PST-associated Harmful Algal Blooms (HABs) appear to be increasing around the world. The appearance of species of Alexandrium is now frequent in coastal waters of Australia, particularly in Tasmania and New South Wales. The East Australian Current (EAC) flows southward along the coasts of eastern Australia and has been reported as a global ‘climate change hotspot’. Despite such potent neurotoxin production, the ecology, toxicity and population dynamics of Alexandrium species are little known in Australia. In this thesis, I have investigated the first record of PST above the regulatory limit of 0.8 mg/kg produced by Alexandrium pacificum in the commercial aquaculture area of south-eastern Australia. During this unprecedented event, the maximum reported PST concentration in mussel tissue was 7.2 mg/kg STX equivalent. A comparative differential gene expression study was conducted to understand the gene regulation of PST related genes in Alexandrium pacificum. In this study, experiments were performed in the presence and absence of the copepodamide-synthesizing copepod Parvocalanus crassirostris. Using Nanostring gene technology, results identified the up-regulation of the key PST-related gene sxtA, in particular, one paralogue each of domains of sxtA1 and sxtA4. An increased rate of PST production in the two PST-producing strains in the presence of copepods was identified, however it did not influence gene related transcript abundance. This indicated that post-transcriptional regulation processes may be important in regulating PST production in Alexandrium pacificum. In this thesis, the population structure of Alexandrium pacificum was examined in different Australia boundaries currents – the East Australian Current (EAC) and the Leeuwin Current (LC). This study was conducted using Single Nucleotide Polymorphisms (SNPs) as genetic markers and represents the first time the population structure of a phytoplankton species has been examined in Australian waters. Strains from South Australia and Western Australia clustered as a group, and were separated from the strains isolated from the EAC region, indicating the presence of genetic isolation of A. pacificum strains in Australian waters. It suggests that A. pacificum is more likely to represent a long resident population, and it is not a recent bioinvasion in Western Australian waters. The results identified during this study significantly advance the understanding of Alexandrium, especially their abundance, diversity, population structure and the regulation of the PST-related genes.
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