Hypoxia tolerance of the gorgonian coral Eunicella cavolini to variable dissolved oxygen concentrations

The release of CO₂ and other greenhouse gases into the atmosphere, along with changes in land use, triggers global climate change, emerging as one of the most significant threats in the world today. As a result of rising global temperatures, the temperature of seawater is progressively increasing, which also decreases the solubility of oxygen in seawater. In addition, coastal nutrient inputs induce biological oxygen depletion and lead to environmental hypoxia events. Thus, Mediterranean gorgonian corals are exposed to variable deoxygenation levels. To better understand the effects of deoxygenation, we exposed the yellow gorgonian coral Eunicella cavolini to four dissolved oxygen (DO) levels (1.95, 3.64, 5.53 and 7.42 mg L^-1) for 20 days using a fully autonomous experimental setup. No mortality or necrosis was observed throughout the experiment. Polyp activity, alongside reactive oxygen species (ROS), protein, carbohydrate and lipid concentrations and tissue histology remained stable across DO levels, suggesting that these traits are resilient to the range of DO conditions tested, or that any biological response may occur only beyond a physiological threshold not reached in this experiment. Oxygen consumption was greater under hypoxic conditions than under normoxic conditions. These results collectively suggest that the Mediterranean endemic coral E. cavolini is tolerant to medium-term environmental hypoxia.