In the framework of the project CARiOCA, financed by ANR (2016-2019), the team from IRD have performed three expeditions in Papua New Guinea onboard the R/V Alis, to study the effect of ocean acidification on coral reefs living at the CO₂ vents of Ambitle Island. Here, corals live at pH values expected for the future because of the submarine, shallow water CO₂ emissions. This unique system emits hot fluids and pure CO₂, offering the timely opportunity to study coral reef responses to climate change in a natural setting.

During the previous missions, the team assessed the area, emission rates and quality, studied the composition and distribution of coral at the study site, their capacity to photosynthesize and respire, as well as tested the opportunity to study the combined effect of acidification and warming by using pipeline driving the hot flux to transplanted corals. During CARIOCA 3 the team transplanted some fragments of corals to measure their calcification rates, which is one of the most affected metabolic response of corals in a changing climate.

From 1^st to 14^th September the R/V Alis had performed the last mission (CARIOCA 4) planned for this project, always at Ambitle Island. The main goal of this cruise was to recover the transplanted corals and measure their calcification (i), sample seawater and biofilm near hot emission to see for any hydrothermal bacteria (ii), measure the metabolic rates of sponges near the CO₂ emissions (iii), and (iv) repeat some onboard measurements of the coral photosynthesis and calcification which gave ambiguous results during the last cruise (v). During the cruise, we also deployed some loggers to better monitor the pH and other physical and chemical parameters at CO₂ emission as made routinely.