What was done
The authors review several basic facts about the process of calcification in zooxanthellate corals.

What was learned
First of all, Cohen and Holcomb remind us of what is perhaps the most fundamental fact of all, i.e., that "calcification is an active, physiological process that requires significant amounts of energy to drive it."

Second, they note that "increased photosynthesis [of coral symbiotic zooxanthellae] means increased photosynthate and more energy for calcification."

Third, they say that Atkinson et al. (1995) have shown that "nutritionally replete zooxanthellate corals in naturally low [aragonite] saturation-state seawaters are capable of accreting skeletons at rates comparable to those achieved by conspecifics in high-saturation-state seawaters."

Fourth, the two researchers report that "today, several reefs, including Galapagos, areas of Pacific Panama, and Jarvis (southern Line Islands), experience levels of aragonite saturation equivalent to that predicted for the open ocean under two times and three times pre-industrial CO2 levels (Manzello et al., 2008; Kathryn Shamberger [PMEL/NOAA] and colleagues, pers. comm., August 2009)," and that "available data on coral colony growth rates on these reefs, albeit limited, suggest that they are equivalent to and sometimes even rival those of conspecifics in areas where aragonite saturation states are naturally high, such as the western Pacific warm pool."

What it means
Probably the most important deduction to flow from these observations is the observable fact, in the words of Cohen and Holcomb, that "naturally elevated levels of inorganic nutrients and, consequently, high levels of primary and secondary production, may already be facilitating high coral calcification rates in regions with naturally high dissolved CO2 levels," which further suggests that earth's corals, with their genetically-diverse symbiotic zooxanthellae, are likely well equipped to deal successfully with whatever increase in the air's CO2 content will ultimately result from the burning of fossil fuels before other energy sources become viable means of providing the bulk of the energy needs of the planet's human populace.

References
Atkinson, M.J., Carlson, B. and Crowe, J.B. 1995. Coral growth in high-nutrient, low pH seawater: A case study in coral growth at the Waikiki aquarium. Coral Reefs 14: 215-233.

Manzello, D.P., Kleypas, J.A., Budd, D., Eakin, C.M., Glynn, P.W. and Langdon, C. 2008. Poorly cemented coral reefs of the eastern Tropical Pacific: Possible insights into reef development in a high-CO2 world. Proceedings of the National Academy of Sciences USA 105: 10.1073/pnas.0712167105.