Coral reefs shelter one of the largest biodiversity on earth and provide US$36 Billion per year of ecosystem services. According to current experimental evidence, they could disappear or be drastically decimated within the coming decades if CO2 emissions remain unabated. Yet, we recently discovered a remarkable site, which the only natural analogue of future climatic conditions exhibiting the three main parameters that drive climate change in the oceans: lower pH (<7.8) and oxygen (-20 to 30%), and warmer temperatures (+ 0.5 to 3°C). While these conditions are generally recognized as unfavourable to corals, there, a rich and abundant coral reef has developed. This novel natural analogue brings new hope for the future of coral reefs and provides a unique natural laboratory to explore how corals could keep pace with climate change.

Our overarching goal is to address the following question, "How do changes in coral functional metabolism and gene expression (the host and associated microbiome) enable corals to thrive into extreme mangrove waters in New Caledonia?"; we used a combination of state-of-the-art sequencing technology and physiological rate measurements to specifically address the objectives: (a) Identify the physiological trait changes in the whole holobiont metabolic rates (i.e., respiration, photosynthetic, excretion and calcification rates as well as protein and lipid profiles), the endosymbiont (i.e. photosynthetic efficiency, chlorophyll symbiont density, cell division rates, etc); (b) Identify and contrast the biodiversity of coral endosymbiont and microbiome communities across the reef-mangrove habitat continuum; (c) Examine the functional genetic response expressed by the coral "holobiont" (the host and associated microbiome) for species common to reef and mangrove habitats; (d) Identify how changes in gene expression (objectives b-c) relate to differences in key metabolic traits that govern fitness across habitats.

In addressing these objectives, we will shed new and unique light on how coral holobionts 'genetically reprogram' themselves, and how this is linked to the emergent physiological properties that enable them to thrive in habitats with coupled extremes of temperature and pH. The results will also make it possible to identify the species most likely to resist in the future, which is particularly important for coral reef conservation and assisted evolution projects aimed at improving coral resistance.