ARDECO

Like tropical coral reefs, cold-water coral (CWC) reefs form complex habitats that locally promote biodiversity and biomass while fixing carbon dioxide in their skeletons. These corals thus provide important ecosystem services. CWC reefs provide support functions, such as spawning grounds and nurseries for fish, and therefore provisioning services such as seafood. They also have a regulating function by buffering ocean acidification. However, because they develop hundreds or even thousands of meters below the sea surface, CWC reefs do not have the same cultural and recreational values as their tropical counterparts. This remoteness, in the depths of the oceans, does not however protect them from human pressures. Deep-sea trawling, chemical contamination or the accumulation of plastic waste are all threats to the health of CWC, to which are added ocean warming and acidification. As a result of warming, corals will be pushed towards ever deeper and colder waters, while acidification will tend to limit their distribution towards shallower and carbonate-rich waters. The two phenomena combined could significantly reduce the fundamental niche of CWC if they are unable to adapt. The ARDECO project will aim at assessing the potential acclimation and predicting the dynamics of the three main species of deep-sea reef-building scleractinians (Desmophylum pertusum, Madrepora oculata and Solenosmilia variablis) in the face of global changes. Two experimental strategies will be implemented, in situ and ex situ. In situ, corals will be transplanted, for a period of one year, from a depth of 1000 meters to shallower areas, on the order of 400 meters, where only coral rubbles remain. Larval traps and colonization devices will be deployed in parallel. The objective of these experiments is to determine the fundamental niche of CWC in the North-East Atlantic, and to assess to what extent their realized niche has been constrained by the intensification of deep-sea trawling in recent decades. Ex situ, corals will be subjected to an increase in temperature and / or a reduction in pH during a period of 6 months in order to test their response to scenarios of global changes by 2100. The main novelty of these experiments will be to simulate the depth at which these species naturally live, thanks to pressurized aquariums developed specifically within the framework of ARDECO. Simulating hydrostatic pressure is indeed fundamental to evaluate the response of deep-sea corals to global changes for two reasons: i) because the dissolution of carbonates increases with hydrostatic pressure, thus aggravating the effects of acidification, and ii) because pressure influences the physiological state of living organisms and neglecting it can induce an experimental bias. The response of corals to environmental changes in situ and ex situ will be assessed by qualifying and quantifying their nutrition, growth, reproduction and behaviour, as well as the nature and function of interactions between coral polyps and their microbiome. The second innovative aspect of ARDECO will thus be to apprehend the response of the coral holobiont to changes in its environment. A last strong component of the project will be to raise awareness among the general public and to inform French and European environmental management bodies in support of their strategies for preserving deep-sea coral reefs.