We report the results of a land-based experiment testing the cumulative effects of copper (Cu) exposure and ocean acidification (OA). Corals were obtained as by-catch during an experimental long-line fisheries campaign (CONDOR monitoring program, Okeanos-University of the Azores) on the Condor Seamount (38°54′N, 29°05′W), Azores Archipelago, at 200 m depth in October 2019. The experiment was undertaken at the DeepSeaLab aquaria facilities (Okeanos-University of the Azores) where corals were exposed to four OA/Cu-contamination scenarios: (1) ambient pCO2/pH level as measured in situ conditions (385 μatm/ pH 8.09); (2) high pCO2/reduced pH (IPCC RCP8.5 scenario, 1000 μatm/ pH 7.73); (3) ambient pCO2/pH level and additional Cu concentration (60 µg/l); (4) high pCO2/reduced pH and additional Cu concentration (60 µg/l). The pH/pCO2 modification was achieved by bubbling seawater with either pure CO2 (to increase pCO2) or CO2 low air (to decrease pCO2). The copper concentration used in this experiment followed a trial simulating a polymetallic particles plume release during a potential deep-sea mining event. This sublethal concentration was found to be the highest copper concentration dissolved in seawater.Seawater physical-chemical parameters were measured daily in each chamber. Temperature, pH and oxygen saturation were measured manually in each aquaria every day using a Mettler-Toledo S8 glass and a Fibox4 (PreSens) with an Oxygen Probe PSt3.

Species: octocoral Viminella flagellum1. Experimental treatmentsTreatments8.1 + [0]: ambient pH plus no copper addition8.1 + [60]: ambient pH plus Cu concentration of 60 μg/l7.7 + [0]: low pH plus no copper addition7.7 + [60]: low pH plus Cu concentration of 60 μg/lExperiment duration: 9 days