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Résumé de la communication de Denis ALLEMAND (Centre Scientifique de Monaco) donnée à l'occasion du séminaire Les constructions en milieu marin du 14 au 19 mars 2016Coral reefs are complex tropical marine ecosystems that live between the 30° North and the 30° South parallels with temperatures around 25-27°C. They make dream touristic destinations that the public imagines with white sandy beaches, palm trees surrounding atolls. Coral reefs form veritable oasis of life in desert ocean and they are only formed by small animals, the coral.Coral reefs, the largest bioconstruction in the world If human architects are building always higher and higher, reef-building corals build the largest bioconstructions of the World, 2600 km long with an area of 344,400 km2 for the Great Barrier Reef in Australia, and 1600 km of length to an area of 24,000 km2 for the New Caledonia Barrier Reef. Remains of past geologic reefs are the basis of our landscapes, while the modern coral reefs started being built about 50 million years. The present total reef area isabout 300,000 km2, i.e. less than 0.2% of the total sea surface.Even if the coral reef surface is small, their biological, ecological and economical importance is huge. Indeed, coral reefs host approximately 30% of known marine species in less than 0.2% of the ocean surface. The fact that coral reefs are hotspot of biodiversity, a source of essential food to nearly 500 million humans, but also a source of income through tourism, explain why they are called true oasis of life in a desert ocean. Last, but not least, they protect the coastline from erosion, making human life possible on many coastal areas. The total value of ecosystem services they provide is estimated to a value of about 30 billion of US $ per year.The major architect in coral reef construction is the coral, an animal belonging to the phylum of Cnidarians.Budding of small millimetric units, called polyps, may form large animal of more than 10 meters in diameter, called colonial (= modular) animals. All the polyps are connected by a common gastrointestinal tract, called coelenteric cavity.Corals are thus responsible for i) firstly, building the housing unit and the coral skeleton, ii) and secondly, creating a habitable living area.Building the housing unit, the coral skeletonThere are just less than 1,500 species of corals. Each has a different skeletal morphology, showing that the shape of the skeleton is written in the genes. How do genes control the formation of the skeleton in an aqueous milieu? The first step of coral skeletogenesis (= biomineralization) is the creation of an enclosed space where the calcification may occur. For this purpose, the coral tissue (=calicoderm) strongly adheres to the skeleton using specialized cells, the desmocytes. Calcification takes place within the extracellular calcifying medium (ECM), between the calicoderm and the skeleton. To isolate ECM from the seawater, the calicoderm cells are connected by a set of proteins, called septate junctions, that make the epithelium almost impermeable to ions and molecules, restraining the diffusion between seawater and ECM. When measured by cell imaging techniques in vivo, physico-chemical parameters within the ECM are different from the ones of seawater or coral cells, demonstrating the close control of the composition of the ECM by the coral.The second step of coral biomineralization is the secretion of a framework of small peptides and macromolecules About 30 40 proteins were already described in corals, while up to 600 seems to exist in some biominerals (such as the eggshell). Their exact role is still largely unknown but inhibition of their synthesis leads immediately to inhibition of calcification. Organic matrix gives some important properties to the biomineral compared to the inorganic mineral such as a higher (up to 300) resistance to fracture. Organic matrix surrounds crystals but is also present within the crystal unit. It seems to shape the biomineral, such as a reinforced concrete.The last step is the formation of a supersaturated environment in calcium carbonate (CaCO3), the mineral brick that makes up the skeleton of the coral. To do this, coral supplies ECM with mineral elements, calcium and bicarbonate, via active transepithelial transport thanks to membrane carriers (calcium pumps, channels, cotransporters). When CaCO3 saturation is sufficient, the nucleation occurs and a nanocrystal is formed within the organic matrix framework. The crystal will thus grow up to the arrest of a cycle of mineralization: a layer of aragonite is therefore deposited over the entire growthsurface of the coral skeleton. The acidity (H+) formed during the mineralization process is eliminated from the ECM by the Ca++ pump through an exchange between Ca++ and H+.Create a habitable living areaTransparency of the tropical waters made it successful for tourists, yet it means that these waters contain almost no nutrients. To colonize space and become self-sufficient, coral will have to create its own food. This is done through the symbiotic process (mutualism). Coral hosts, inside the cells of its tissue, microalgae (about 1 million/cm2) called zooxanthellae, which make photosynthesis for him,transforming CO2 into sugars and producing oxygen. A large part of these sugars are transferred to the host tissues, while animal nitrogen wastes are recycled by the zooxanthellae, leading the coral holobiont (coral and its associated microbes) self-sufficient.The two partners of this association (coral and zooxanthellae) evolved together since at least the Triassic period (230 million of years). Numerous biochemical and morphological adaptations coevolved during this period, such as the capacity of the coral host to absorb actively CO2 to supply photosynthesis of its symbionts, to use UVfilters synthetized by the zooxanthellae for its own protection to avoid sunburns under the tropical sun, or to develop antioxidant defenses to fight against the hyperoxia consecutive to the zooxantellae photosynthesis. Also, symbiosis enhanced the rate of calcification and coral growth (light-enhanced calcification).Curiously, a French company, Symbio2, uses similar process to cultivate microalgae on the façades of buildings in order to both create thermal insulation and provide biomass using vertical facades exposed to sunlight, an urban biomimetic application of corals and coral reefs.ConclusionsWould coral reefs be models for the architect in order to build efficient and economic self-sufficient cities? What is sure is that sometimes man grows coral in a geopolitical goal to maintain its exclusive economic zone (EEZ). This is the case for the Japanese Atoll of Okinotori that gradually disappears due to climate change and sea level rise. If the process continues, Japan will lose its Okinotori atoll and a part of its EEZ equal to the surface of its land territory! It is why Japan started a long process of coral culture and reintroduction to maintain its territory.Summary