Tropical coral reefs shelter a high rate of marine biodiversity and provide a sizeable amount of ecological services. These ecosystems of interest face tremendous and pervasive challenges from human demography sky-rockets and inherent development pressures, as well as climate change and associated sea level rise. Prospects of coral reefs conservation, critically inevitable, will achieve through the growth of understanding their ecological mechanisms.The basis for the existence of tropical coral reefs is a mutualistic relationship between the coral polyp and its endosymbiotic dinoflagellates, the Zooxanthellae (Symbiodinium spp.). Due to the consequence of global warming coral bleaching became one of the most important threats to coral reefs in the last decade. The higher temperatures and solar radiation disrupt photosynthesis in the coral’s symbiotic algae and result in the production of toxic free oxygen radicals that cause the corals to eject the algae. These stressed corals have lost their major source of energy and are effectively starving. Some corals can regain their algae and recover; but the increased stress often results in lethal coral diseases or reduced coral reproduction and growth during the next year. Surprisingly although bleaching events have been intensively studied worldwide little is known about the origin of algal rejection or the involved signalling pathways.The main objective of this innovative and original proposal is to investigate for the first time the role of the organelles of host and symbiont in coral bleaching via immunoflourescence during stress experiments. In addition, gene expression analysis will elucidate candidate genes that might be involved. This original project will lead to a better understanding of mechanism taking place during bleaching of the coral holobiont and will therefore enhance the European excellence in coral scientific community while studying this so far unknown field.