My work focuses on quantifying reef carbonate budgets in three bio-regions through calculating the reef’s net calcium carbonate (CaCO3) accretion rate through looking at both net CaCO3 production and erosion rates. Coral calcification is influenced by environmental conditions (e.g. temperature, light and ocean carbon chemistry) and depends on symbiosis with the dinoflagellate algae (Symbiodinium). When corals encounter acute thermal stress, symbiosis breaks down resulting in coral bleaching, which impacts reef calcification in the short and long term. With climate change, rising sea surface temperature (SST) events are predicated to increase in frequency and intensity, affecting reef carbonate budgets and reef capacity to sustain their framework through accretion. My study aims to examine the thermal history (1982-2018) and assess the accretional health of reefs in three bioregions (Persian-Arabian Gulf (PAG), Western Indian Ocean (WIO) & Central Indo-Pacific (CIP), which represent similar coral assemblages but harbour different environmental conditions utilising a standardized Reef Budget methodology which has been adapted to the bioregions in study. In addition, my work includes investigating the genotypes of symbionts associated with the dominant carbonate producer (corals) and eroder species (sponge) across study sites to understand the various associations between the assemblages of Symbiodinium and host in regards to how this relationship impacts future accretion rates of reefs in the face of predicted global environmental change.