Description: Deep-water ecosystems are characterized by relatively low carbonate concentration values and, due to ocean acidification (OA), these habitats might be among the first to be exposed to undersaturated conditions in the forthcoming years. However, until now, very few studies have been conducted to test how cold-water coral (CWC) species react to such changes in the seawater chemistry. The present work aims to investigate the mid-term effect of decreased pH on calcification of the two branching CWC species most widely distributed in the Mediterranean, Lophelia pertusa and Madrepora oculata. No significant effects were observed in the skeletal growth rate, microdensity and porosity of both species after 6 months of exposure. However, while the calcification rate of M. oculata was similar for all colony fragments, a heterogeneous skeletal growth pattern was observed in L. pertusa, the younger nubbins showing higher growth rates than the older ones. A higher energy demand is expected in these young, fast-growing fragments and, therefore, a reduction in calcification might be noticed earlier during long-term exposure to acidified conditions.

Description: A 9-month aquarium experiment with the cold-water Dendrophyllia cornigera was conducted to investigate the single and combined effects of warming, acidification and deoxygenation on its ecophysiological response. The experiment took place at the Aquarium finisterrae (A Coruña, Spain) between 2022-05-06 and 2023-02-24. Treatment values for each parameter (current in situ vs. climate change) were: 12 °C and 15 °C (temperature); ~7.99 and 7.69 (pH); ~8.63 mg/L and 6.45 mg/L (dissolved oxygen concentration). A total of eight treatments (with 3 replicates each, 5 L aquaria) were set up. Measurements for pH and total alkalinity (TA) were performed on seawater samples from all the experimental aquaria every 1 – 2 months. Samples for pH were directly collected on cylindrical 10 cm cuvettes and analysed on daily basis. After being thermostated at 25ºC, samples were measured using a manual spectrophotometrical procedure with a Sigma Aldrich impure indicator (Clayton and Byrne, 1993). TA was measured following the double end point potentiometric technique by Pérez and Fraga (1987a) and Pérez et al. (2000). Measurements of Certified Reference Material (Dickson's lab, SIO) were performed in order to control the accuracy of the TA measurements. The uncertainty of TA and pH is about 3 μmol/kg and 0.005 pH units, respectively. In situ additional CO2 system variables were calculated using the R package seacarb (Gattuso et al. 2023) using in situ temperature, salinity, measured pH and TA, the dissociation constants of carbonic acid in seawater (K1 and K2) by Lueker et al. (2000), the equilibrium constant of hydrogen fluoride by Perez and Fraga (1987b), the concentration of total boron by Uppström (1974) formulation, and the stability constant of hydrogen sulphate by Dickson (1990). Mean values for the concentration of silicate and phosphate on the aquaria experiments were used in the CO2 calculations, respectively, mean (and standard) deviations as 2.1 (0.4) and 0.31 (0.03) μmol/kg.