Unlocking how corals have acclimatised to thrive within the temperate waters of Sydney Harbour

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Whilst corals are under immense anthropogenic pressure from increasingly altered environments, growing evidence suggests that some coral populations have evolved to thrive within present day marginal environments. Marginal coral populations are therefore significant to understand the adaptive and acclimative capacities required to tolerate future climates. Genetic shifts within the coral microbiome are being increasingly recognised for their importance to holobiont functioning under extremes, and is considered in this thesis in terms of symbiont (Symbiodiniaceae) and bacterial diversity in response to variable environmental conditions. Promising new โ€œomicsโ€ approaches are allowing us to create species-specific metabolite profiles and further uncover the complex mechanisms of cell metabolism under environmental stress. When coupled to measurements of coral physiological variables (photosynthesis, respiration and calcification), the molecular regulation of corals under various environmental conditions can be elucidated. This thesis focuses on two coral species, ๐˜—๐˜ญ๐˜ฆ๐˜ด๐˜ช๐˜ข๐˜ด๐˜ต๐˜ณ๐˜ฆ๐˜ข ๐˜ท๐˜ฆ๐˜ณ๐˜ด๐˜ช๐˜ฑ๐˜ฐ๐˜ณ๐˜ข and ๐˜Š๐˜ฐ๐˜ด๐˜ค๐˜ช๐˜ฏ๐˜ข๐˜ณ๐˜ข๐˜ฆ๐˜ข ๐˜ฎ๐˜ค๐˜ฏ๐˜ฆ๐˜ช๐˜ญ๐˜ญ๐˜ช surviving under highly variable environmental conditions in Sydney Harbour. Specifically, coral heat stress tolerance was investigated during the 2016 El Niรฑo event, which lead to the first report of coral bleaching in Sydney Harbour, showing that these high-latitude corals bleach in a similar way to tropical corals. Sampling for microbial diversity analysis was conducted ahead of coral bleaching in February, during coral bleaching in April and during coral recovery in August. Parallel measurements of coral metabolic rates (photosynthesis, respiration and calcification) were made in aquaria. Only ๐˜—. ๐˜ท๐˜ฆ๐˜ณ๐˜ด๐˜ช๐˜ฑ๐˜ฐ๐˜ณ๐˜ข showed a bleaching response and a switch towards a heterotrophic nutrient acquisition mode during bleaching highlighting the different bleaching susceptibilities of these two-coral species. Microbial community composition showed clear species-specific associations and shifts in diversity and abundance of key bacterial taxa in response to the thermal anomaly event and over a 2-year study with seasonal environmental change. Metabolite profiling conducted using GC-MS showed overall between species similarity with lipid compounds dominating the metabolome of both coral species. Under acute heat stress, increases in fatty acid metabolism significantly correlate to holobiont photosynthesis, suggesting a predictive capacity of metabolomic analysis in determining coral performance under heat stress. The systematic approach implemented in this thesis highlights some of the potential mechanisms of coral persistence in marginal environments. This is relevant as high-latitude regions such as Sydney have been proposed as coral refuge environments with climate change. This research will also open up a new level of biodiversity complexity quantification that is used to designate Sydney Harbourโ€™s high conservation value.
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