Background
In introducing their study, the authors write that "calcifying marine organisms such as molluscs and foraminifera, crustaceans, echinoderms, corals and coccolithophores are predicted to be most vulnerable to decreasing oceanic pH (ocean acidification)." They also, however, say there is a possibility for "increased or maintained calcification under high carbon dioxide conditions," and they go on to experimentally demonstrate the reality of this phenomenon in five different types of calcifying marine animals.

What was done
Working with five different calcifying organisms - two gastropods (the limpet Patella vulgata and the periwinkle Littorina littorea), a bivalve mussel (Mytilus edulis), one crustacean (the cirripede Semibalanus balanoides) and one echinoderm (the brittlestar Amphiura filiformis) - Findlay et al. say they "measured either the calcium (Ca²⁺) concentration in the calcified structures or shell morphological parameters as a proxy for a net change in calcium carbonate in live individuals exposed to lowered pH," where the lower pH of the seawater employed was created by the bubbling of CO2 into header tanks.

What was learned
"Contrary to popular predictions," as the six scientists remark, they found that "the deposition of calcium carbonate can be maintained or even increased in acidified seawater." In fact, they say that four of the five species they studied actually exhibited increased levels of calcium in low pH conditions. In the case of Littorina littorea, for example, they indicate that all morphological shell parameters - width, height, thickness, area, perimeter, aperture area and aperture perimeter - "increased in low pH treatments compared to the control," while noting that "there was ~67% more growth in shell height, ~30% more growth in shell width and ~40% more growth in shell thickness under low pH conditions compared to the control." And in another part of their study, they observed there was a large amount of dissolution taking place on isolated shells and arms of the creatures they studied; but they found that "the presence of a live animal within its calcium carbonate structure offset this dissolution."

What it means
Findlay et al. say their findings demonstrate that "there is a great degree of biological control on calcification with complex links to other physiological processes," and that "increasing evidence in the literature agrees with the results of this [i.e., their] study," noting that: "McDonald et al. (2009) showed calcification in another barnacle species (Amphibalaus amphitrite) to continue, and possibly even increase, under low pH conditions (pH 7.4); Arnold et al. (2009) demonstrated larval lobsters (Homarus gammarus) were able to lay down calcium carbonate structure in pH conditions 0.3 units below the control levels; Checkley et al. (2009) showed young fish have enhanced aragonite otolith growth when grown under elevated CO2; Maier et al. (2009) showed that, although there was a decrease in calcification in cold-water corals, overall they showed a positive net calcification at aragonite saturation states below 1, and longer-term experiments suggest that these corals may actually maintain or even increase calcification over longer timescales at low pH (Schubert et al., 2010)."

So does ocean acidification always depress calcification rates in calcifying marine organisms? Not by a long shot!

References
Arnold, K.E., Findlay, H.S., Spicer, J.I., Daniels, C.L. and Boothroyd, D. 2009. Effects of CO2-related acidification on aspects of the larval development of the European lobster, Homarus gammarus (L.). Biogeosciences 6: 1747-1754.

Checkley, D.M., Dickson, A.G., Takahashi, M., Radish, J.A., Eisenkolb, N. and Asch, R. 2009. Elevated CO2 enhances otolith growth in young fish. Science 324: 1683.

Maier, C., Hegeman, J., Weinbauer, M.G. and Gattuso, J.-P. 2009. Calcification of the cold-water coral Lophelia pertusa under ambient and reduced pH. Biogeosciences 6: 1671-1680.

McDonald, M.R., McClintock, J.B., Amsler, C.D., Rittschof, D., Angus, R.A., Orihuela, B. and Lutostanski K. 2009. Effects of ocean acidification over the life history of the barnacle Amphibalanus amphitrite. Marine Ecology Progress Series 385: 179-187.

Schubert, A., Maier, C., Riebesell, U. and Gattuso, J.-P. 2010. The impact of ocean acidification on calcification rates of Mediterranean cold-water corals. Poster presentation EPOCA Annual Meeting, Bremerhaven, Germany, p. 109.