Assessing Resilience of DEep COrals – ARDECO

Two experimental strategies are implemented, in situ and ex situ. In situ, corals are monitored within and at the periphery of their realized niche. Larval traps, colonization and growth devices are deployed. 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

During the first 18 months of the project, we carried out two oceanographic cruises (ChEReef 2021, ChEReef 2022), which enabled us to initiate in-situ experiments, in particular the first deployments of a seabed observatory and growth monitoring devices. The seabed observatory (MARLEY) was deployed in August 2021 to be recovered and then redeployed in August 2022. The observatory allowed us to acquire video data as well as temperature, turbidity and currents. Samples from the 2021 cruise allowed a first characterization of the trophic regimes and the reproductive state of two target species, Desmophylum pertusum and Madrepora oculata. Regarding the localization and quantification of bacteria associated with corals, the first observations by electron microscopy and by fluorescence in situ hybridization showed very few bacteria associated with polyps. New samples were taken during the ChEReef 2022 cruise to continue these investigations.
Regarding the in-situ experiments, the growth monitoring devices (Ranch) were deployed in August 2021 and recovered in August 2022 on two study sites: a flat area near the MARLEY station and along the cliff (at ~800m depth). The measurements show that over the period August 2021 – August 2022, the species D. pertusum has an average growth rate of 2.2 ± 1.8 mm.year-1 and a budding rate of 28 ± 27%. These values are consistent with those measured during the ex-situ experiment conducted in 2022.
Regarding ex-situ experiments (WP2), a first ex-situ experiment was initiated to assess the resilience of D. pertusum to a temperature increase. The experiment was carried out at atmospheric pressure, by subjecting nubbins of D. pertusum to three temperatures: 10°C (control), 13 and 15°C, for two months. The evolution of the microbiota, the rate of prey capture and the activity of the polyps were monitored every 15 days and the growth determined at the end of the 2 months of exposure. Preliminary results show a significant increase in mortality at 15°C, unlike Mediterranean corals, but other biological parameters (growth, prey capture rate) do not seem affected by temperature change. Microbiota analyses are in progress, and will shed light on the causes of mortality. The development of pressurized aquariums, which is coming to an end, will allow us to begin in-situ pressure experiments, at 80 to 150 bars.

Experiments continue in-situ and ex-situ. In situ, growth, reproduction, recruitment and behavior monitoring have been initiated or renewed over the period August 2022 – August 2023. Ex situ, pressure experiments will begin with the objective of evaluating the response of three reef-building corals (Madrepora oculata, Desmophylum pertusum, Solenosmilia variabilis) to ocean warming and acidification scenarios. In parallel, and only at atmospheric pressure, comparisons between Atlantic corals and Mediterranean corals will continue with a focus on the influence of temperature change as well as the cumulative effects of warming and contamination by micro-plastics.

Chemel M., Peru E., Galand P., Lartaud F., 2022. Effects of temperature on deep corals from the Atlantic Ocean. Poster, International Coral Reef Symposium, 3-8 July 2022, Bremen, Germany.
Chemel M., Peru E., Galand P., Lartaud F., 2022. Réchauffement des eaux profondes : les coraux atlantiques sont-ils plus résilients que les méditerranéens ? Poster, Colloque Environnement et Climat, 22-25 juin 2022, Collioure, France.
Amand, L., Menot, L., Tourolle, J., Shillito, B., Zbinden, M., 2023. Pressurised aquaria for long-term experiments on deep-sea corals. Presented at the 8th International Symposium Deep-Sea Corals, Edinburgh, UK.
Chemel, M., Peru, E., Galand, P.E., Lartaud, F., 2023. Effects of temperature on cold-water coral holobiont in the Atlantic Ocean. Presented at the 8th International Symposium Deep-Sea Corals, Edinburgh, UK.
Menot, L., Tourolle, J., Amand, L., Bajjouk, T., Brandily, C., Chemel, M., Edinger, E.N., Ehrhold, A., Fabri, M.C., Fuchs, S., Galand, P.E., Hilario, A., Lartaud, F., Lagadec, J.-R., Michel, L.N., Moreau, B., Olu, K., Pradillon, F., Silva Jacinto, R., Shillito, B., Zbinden, M., 2023. ChEReef?: A multi-disciplinary and multi-scale project to assess and predict the health of deep-sea corals in the Bay of Biscay (NE Atlantic). Presented at the 8th International Symposium Deep-Sea Corals, Edinburgh, UK.

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.