Our main hypothesis is that many marine species, which some laboratory studies suggest are sensitive to AO, actually can acclimatise and potentially adapt to future conditions. This hypothesis is supported by the fact that several populations of reef-building corals thrive at CO₂ emission sites in Ambitle where pH conditions are similar to those in future climate scenarios, suggesting that they are capable of acclimatisation or even adaptation in this extreme environment. As part of the ANR-funded CARiOCA project, during the first and second campaigns aboard the R/V Alis (September 2016 and May 2017), we studied coral reefs living at the CO₂ vents on Ambitle Island where corals live at the pH levels expected for 2100. This unique system of vents emits warm fluids and pure CO₂, providing an opportunity to study coral reef responses to climate change in a natural setting. The project proposes to use these unique sites, whose seawater conditions are similar to those expected at the end of this century, to identify the phenotypic traits that enable certain coral species to survive and reproduce in seawater naturally enriched in CO₂. This will provide a better understanding of the mechanisms underlying acclimatisation and adaptation to climate change.

In CARIOCA 3, seawater carbonate chemistry was accurately measured using SEAfet pH sensors, as well as new sampling for metals and trace elements in seawater (collaboration with INGV, Italy). We discovered that the pipeline had been destroyed, probably by a floating tree. A new pipeline was built and some colonies were transplanted. As we had not been able to measure the calcification rates of the corals during the previous missions using the on-board floating weight technique, we carried out a new transplantation of corals at the resurgence and control points. This time, coral fragments were weighed on land using a precision balance powered by a rechargeable battery. Twenty fragments of each of the five selected species were prepared and transplanted between the resurgences and the control sites. An Israeli colleague carried out video transects to assess coral diversity and abundance at the sites. Another Israeli colleague carried out incubations on board to test the combined effect of temperature and warming. One coral species was incubated to assess its ability to feed at different pH levels.