Background
The authors state that ocean acidification results from a net uptake of CO2 emissions that causes a decrease in the carbonate ion concentration of the ocean, which has been "forecast to hamper production of biogenic carbonates (aragonite and calcite) in the skeletons, shells and tests of marine taxa (Orr et al., 2005; Moy et al., 2009)," thereby "threatening their long-term viability and severely impacting marine ecosystems." They go on to note, however, that these predictions "are based primarily on modeling studies and short-term laboratory exposure to low-carbonate conditions," citing Riegl et al. (2009), Veron et al. (2009) and Ries et al. (2010). And they say that "their relevance to long-term exposure in the field and the potential for ecological or evolutionary adjustment are uncertain," citing Maynard et al. (2008).

What was done
"To determine the sensitivity of corals and allied taxa to long-term exposure to very low carbonate concentrations," in the words of Thresher et al., they examined in detail "the depth distribution and life-history characteristics of corals and other shell-forming megabenthos along the slopes of deep-sea seamounts and associated structure in the SW Pacific," where the gradient of water chemistry ranged from super-saturated with respect to aragonite and high-magnesium calcite (HMC) to under-saturated, even with respect to calcite.

What was learned
The five researchers report that they "found little evidence that carbonate under-saturation to at least -30% affected the distribution, skeletal composition, or growth rates of corals and other megabenthos on Tasmanian seamounts." In fact, they found that "both solitary scleractinian corals and colonial gorgonians were abundant at depths well below their respective saturation horizons and appeared healthy," while HMC echinoderms were common to as deep as they sampled (4011 m), in water that was approximately 45% under-saturated. They also report that "for both anthozoan and non-anthozoan taxa, there was no obvious difference in species' maximum observed depths as a function of skeletal mineralogy." In other words, the community "was not obviously shifted towards taxa with either less soluble or no skeletal structure at increasing depth." And in light of these observations, they write that "it is not obvious from our data that carbonate saturation state and skeletal mineralogy have any effect on species' depth distributions to the maximum depth sampled," and they say that they also saw "little evidence of an effect of carbonate under-saturation on growth rates and skeletal features."

Commenting further on their findings, Thresher et al. write that "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)." And they say that their own data also "provide no indication that conditions below saturation per se dictate any overall shifts in community composition."

As for why things were as they observed them to be, the researchers note, as highlighted by Cohen and Holcomb (2009), that one or more cell membranes may envelope the organisms' skeletons, largely isolating the calcification process and its associated chemistry from the bulk seawater, citing the studies of McConnaughey (1989), Adkins et al. (2003) and Cohen and McConnaughey (2003), which phenomenon could presumably protect "the skeleton itself from the threat of low carbonate dissolution." In addition, they note that "calcification is energetically expensive, consuming up to 30% of the coral's available resources, and that normal calcification rates can be sustained in relatively low-carbonate environments under elevated feeding or nutrient regimes," as described in detail by Cohen and Holcomb (2009), stating that the likelihood that "elevated food availability could compensate for the higher costs of calcification in heterotrophic deep-sea species appears plausible."

What it means
Whatever the reason or reasons for the various observations of Thresher et al., their data clearly suggest, as they describe it, that "a change in carbonate saturation horizons per se as a result of ocean acidification is likely to have only a slight effect on most of the live deep-sea biogenic calcifiers," which is a most reassuring result.

References
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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.

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