Background
The authors write that "deep-water ecosystems are characterized by relatively low carbonate concentration values," and they say that "due to ocean acidification (OA), these habitats might be among the first to be exposed to under-saturated conditions in the forthcoming years," citing the studies of Guinotte et al. (2006), Steinacher et al. (2009) and Yamamoto et al. (2012).

What was done
In light of this speculated likelihood, Movilla et al. investigated the mid-term (6-month) effects of OA (pH treatments of 8.10 and 7.81 for controlled and acidified conditions, respectively) on the calcification rate, skeleton micro-density and porosity of two branching cold water coral (CWC) species that inhabit deep Mediterranean waters, i.e., Lophelia pertusa and Madrepora oculata. This they did in aquaria containing 50-µm filtered and running natural seawater kept at a temperature of 12°C and in complete darkness, as described by Olariaga et al. (2009), where they fed the corals a mixed diet of frozen Cyclops, Mysis and Artemia five days a week.

What was learned
The eight Spanish researchers report that "no apparent pH-driven effects were observed in the skeletal growth rate, micro-density and porosity of both species compared to control conditions after six months of exposure," which results they say "are in accordance with two previous mid-term studies assessing the effects of OA on these same species, where no differences were observed after 6 months in L. pertusa (Form and Riebesell, 2012) or 9 months of exposure in L. pertusa and M. oculata (Maier et al., 2013)."

What it means
The results of Movilla et al. - and those of the studies they cite - would tend to suggest that all should be well with L. pertusa and M. oculata in a moderately CO2-enriched world of the future.

References
Form, A.U. and Riebesell, U. 2012. Acclimation to ocean acidification during long-term CO2 exposure in the cold-water coral Lophelia pertusa. Global Change Biology 18: 843-853.

Guinotte, J.M., Orr, J.C., Cairns, S.S., Freiwald, A., Morgan, L. and George, R. 2006. Will human-induced changes in seawater chemistry alter the distribution of deep-sea scleractinian corals? Frontiers in Ecology and the Environment 4: 141-146.

Maier, C., Schubert, A., Berzunza-Sanchez, M.M., Weinbauer, M.G., Watremez, P. and Gattuso, J.P. 2013. End of the century pCO2 levels do not impact calcification in Mediterranean cold-water corals. PLOS ONE 8: 10.1371/journal.pone.0062655.

Olariaga, A., Gori, A., Orejas, C. and Gili, J.M. 2009. Development of an autonomous aquarium system for maintaining deep corals. Oceanography 22: 44-45.

Steinacher, M., Joos, F., Frolicher, T.L., Plattner, G.K. and Doney, S.C. 2009. Imminent ocean acidification in the Arctic projected with the NCAR global coupled carbon cycle-climate model. Biogeosciences 6: 515-533.

Yamamoto, A., Kawamiya, M., Ishida, A., Yamanaka, Y. and Watanabe, S. 2012. Impact of rapid sea-ice reduction in the Arctic Ocean on the rate of ocean acidification. Biogeosciences 9: 2365-2375.