Effects of elevated pCO2 and feeding on net calcification and energy budget of the Mediterranean cold-water coral Madrepora oculata

Ocean acidification is a major threat to calcifying marine organisms such as deep-sea cold-water corals (CWC), but related knowledge is scarce. The aragonite saturation threshold (Omega a) for calcification, respiration, and organic matter fluxes was investigated experimentally in the Mediterranean Madrepora oculata (Linnaeus 1758). Over 10 weeks, colonies were maintained under two feeding regimes (uptake of 36.75 and 7.46 µmol C/polyp/week) and exposed in 2 week intervals to a consecutively changing air-CO2 mix (pCO2) of 400, 1600, 800, 2000 and 400 ppm. There was a significant effect of feeding on calcification at initial ambient pCO2, while at consecutive pCO2 treatments feeding had no effect on calcification. Respiration was not significantly affected by feeding or pCO2 levels. Coral skeletons started to dissolve at an average Omega a threshold of 0.92, but recovered and started to calcify again at Omega a> or =1. The surplus energy required to counteract dissolution at elevated pCO2 (> or =1600µatm) was twice that at ambient pCO2. Yet, feeding had no mitigating effect at increasing pCO2 levels. This could be due to the fact that the energy required for calcification is a small fraction (1 to 3%) of the total metabolic energy demand and corals even under low food conditions might therefore still be able to allocate this small portion of energy to calcification. The response and resistance to ocean acidification is consequently not controlled by feeding in this species, but more likely by chemical reaction at the site of calcification and exchange processes between the calicoblastic layer and ambient seawater.