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. Metabolic (CA and Hsp70), oxidative damage (SOD, MDA) and detoxification (GST) biomarkers were measured in 4 corals from each treatment at the end of the experiment, using a multiplate spectrophotometer reader.

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