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The effects of ocean acidification alone or in combination with warming on coral metabolism have been extensively investigated, whereas none of these studies consider that most coral reefs near shore are already impacted by other natural anthropogenic inputs such as metal pollution. It is likely that projected ocean acidification levels will aggravate coral reef health. We first investigated how ocean acidification interacts with one near shore locally abundant metal on the physiology of two major reef-building corals:  Stylophora pistillata  and  Acropora muricata . Two pH levels (pH T  8.02;  p CO 2  366  μ atm and pH T  7.75;  p CO 2  1140  μ atm) and two cobalt concentrations (natural, 0.03  μ g L -1  and polluted, 0.2  μ g L -1 ) were tested during five weeks in aquaria. We found that, for both species, cobalt input decreased significantly their growth rates by 28% while it stimulated their photosystem II, with higher values of rETR max  (relative Electron Transport Rate). Elevated  p CO 2  levels acted differently on the coral rETR max  values and did not affect their growth rates. No consistent interaction was found between  p CO 2  levels and cobalt concentrations. We also measured  in situ  the effect of higher cobalt concentrations (1.06 ± 0.16  μ g L -1 ) on  A .  muricata  using benthic chamber experiments. At this elevated concentration, cobalt decreased simultaneously coral growth and photosynthetic rates, indicating that the toxic threshold for this pollutant has been reached for both host cells and zooxanthellae. Our results from both aquaria and  in situ  experiments, suggest that these coral species are not particularly sensitive to high  p CO 2  conditions but they are to ecologically relevant cobalt concentrations. Our study reveals that some reefs may be yet subjected to deleterious pollution levels, and even if no interaction between  p CO 2  levels and cobalt concentration has been found, it is likely that coral metabolism will be weakened if they are subjected to additional threats such as temperature increase, other heavy metals, and eutrophication.

Responses of Two Scleractinian Corals to Cobalt Pollution and Ocean Acidification