Background
The authors write that "ocean acidification has been predicted to reduce the ability of marine organisms to produce carbonate skeletons, threatening their long-term viability and severely impacting marine ecosystems," noting in this regard that "corals, as ecosystem engineers, have been identified as particularly vulnerable." However, they state that "these predictions are based primarily on modeling studies and short-term laboratory exposure to low-carbonate conditions." And they therefore logically add that "their relevance to long-term exposure in the field and the potential for ecological or evolutionary adjustment are uncertain."

What was done
In a study that they designed to resolve some of the uncertainties surrounding this subject, Thresher et al. "examined the distribution, growth and skeletal composition of corals and associated megabenthos on seamounts off Tasmania, Australia," which seamounts are said by them to "support an extensive benthic community, dominated at depths <1300 meters by the reef-forming scleractinians Solenosmilia variabilis and Enallopsammia rostrata, and deeper by hormathiid anemones, bathylasmatid barnacles and isidid gorgonians (Koslow et al., 2001; Althaus et al., 2009; Thresher et al., 2011)."

What was learned
In the words of the five researchers, "we found little evidence that carbonate under-saturation to at least -30% affected the distribution, skeletal composition, or growth rates of corals and other megabenthos." Indeed, they indicate that "both solitary scleractinian corals and colonial gorgonians were abundant at depths well below their respective saturation horizons and appeared healthy." Likewise, they report that "high magnesium calcite echinoderms were common as deep as we sampled (4011 meters), in water that was ca. 45% under-saturated."

What it means
Thresher et al. conclude, quite obviously, that "the physiology of the coral appears able to cope with whatever costs or stresses are associated with skeletal accretion in a very low-carbonate environment." And in further support of this conclusion, they say "the observation that the distributions of deep-sea corals are not constrained by carbonate levels below saturation is broadly supported by the literature," noting that "solitary scleractinians have been reported as deep as 6 km (Fautin et al., 2009) and isidid gorgonians as deep as 4 km (Roark et al., 2005)," both of which depths are said by them to be "well below the aragonite and high magnesium calcite saturation horizons, respectively." In addition, they write that "our data provide no indication that conditions below saturation per se dictate any overall shifts in community composition."

So how is it possible? One idea they propose, "as recently highlighted by Cohen and Holcomb (2009)," is that "one or more cell membranes (the scleractinian calicoblastic layer and isidid coenenchyme) envelop the skeleton of live corals when the latter are fully expanded," and they say that "this tissue layer largely or completely isolates the calcification process and chemistry from seawater (McConnaughey, 1989; Adkins et al., 2003; Cohen and McConnaughey, 2003), and presumably the skeleton itself from the threat of low carbonate dissolution." And, we would add, it is also quite likely that the "ecological or evolutionary adjustments" that they referred to earlier in their report may have played a role in the phenomenon.

References
Adkins, J.F., Boyle, E.A., Curry, W.B. and Lutringer, A. 2003. Stable isotopes in deep-sea corals and a new mechanism for 'vital effects.' Geochimica et Cosmochimica Acta 67: 1129-1143.

Althaus, F., Williams, A., Schlacher, T.A., Kloser, R.J., Green, M.A., Barker, B.A., Bax, N.J., Brodie, P. and Schlacher-Hoenlinger, M.A. 2009. Impacts of bottom trawling on deep-coral ecosystems of seamounts are long-lasting. Marine Ecology Progress Series 397: 279-294.

Cohen, A.L. and Holcomb, M. 2009. Why corals care about acidification. Oceanography 22: 118-127.

Cohen, A.L. and McConnaughey, T.A. 2003. Geochemical perspectives on coral mineralization. In: Dove, P.M., Weiner, S. and de Yoreo, J.J. (Eds.). Reviews in Mineralogy and Geochemistry, Volume 54. Mineralogical society of America, New York, New York, USA, p. 151-187.

Fautin, D.G., Guinotte, J.M. and Orr, J.C. 2009. Comparative depth distribution of corallimorpharians and scleractinians (Cnidaria: Anthozoa). Marine Ecology Progress Series 397: 63-70.

Koslow, J.A., Gowlett-Holmes, K., Lowry, J.K., O'Hara, T., Poore, G.C.B. and Williams, A. 2001. Seamount benthic macrofauna off southern Tasmania: community structure and impacts of trawling. Marine Ecology Progress Series 213: 111-125.

McConnaughey, T. 1989. ¹³C and ¹⁸O isotopic disequilibrium in biological carbonates: 1. Patterns. Geochimica et Cosmochimica Acta 53: 151-162.

Roark, E.B., Guilderson, T.P., Flood-Page, S., Dunbar, R.B., Ingram, B.L., Fallon, S.J. and McCulloch, M. 2005. Radiocarbon-based ages and growth rates of bamboo corals from the Gulf of Alaska. Geophysical Research Letters 32: 10.1029/2004GL021919.

Thresher, R.E., Adkins, J., Fallon, S.J., Gowlett-Holmes, K., Althaus, F. and Williams, A. 2011. Extraordinarily high biomass benthic community on southern Ocean seamounts. Scientific Reports (Nature) 1: 10.1038/sprep00119.