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Shallow water coral reefs are on decline at a global scale and are expected to be lost within this century. The increasing frequency and intensity of heat stress episodes have been identified as the major threat to reefs survival. The impact of high temperature on corals can be exacerbated by regional factors such as pollution of coastal seawater. In the Persian/Arabian Gulf (hereafter: the Gulf) corals are exposed to elevated salinity levels and unusual nutrient concentrations. At the same time, they endure extreme temperature maxima that would kill corals elsewhere on the planet. Despite the crucial role of the lipid composition of zooxanthellae in defining the susceptibility of reef corals to heat stress, the specific lipid signature characterising the symbionts of Gulf corals is as yet completely unexplored. The proposed study will address this knowledge gap by applying a mass spectrometric (MS) approach to compare the lipid profiles of zooxanthellae from Gulf corals flourishing in an extreme temperature habitat (Abu Dhabi, UAE) with those of conspecifics that experience typical tropical conditions in the Indo/South Pacific (Hoga Island, Fiji). We will analyse samples from the two field sites collected at different seasons over a year. In parallel, we will monitor changes in the physicochemical environment over the same period. This strategy will identify key differences in the quantitative / qualitative lipid composition of zooxanthellae associated with coral survival at high temperature and with the dynamic response to changes in environmental conditions. We will use our established laboratory strains of Porites lobata originating from the Gulf and the Pacific to examine how altered temperature, nutrient concentrations and salinity individually affect the lipid complement of zooxanthellae. The experiments under controlled conditions will allow us to test our hypothesis that the unique physicochemical environment of the Gulf promotes thermotolerance of corals by altering the lipid composition of zooxanthellae. The comparative approach of this project will be enabled by the highly complementary expertise of the participating scientists (coral physiology/molecular biology/biochemistry; lipid biology/mass spectrometry & biogeochemistry). The project will benefit from excellent project partners (Burt, Voolstra, Suggett, Smith) that provide substantial in-kind contributions and from the support of a large international consortium of stakeholders from academia, industry, governmental and non-governmental organizations concerned with the coral ecosystems of the Gulf.