Stony corals build hard skeletons of calcium carbonate (CaCO₃) by combining calcium with carbonate ions derived, ultimately, from seawater. The concentration of carbonate ions relative to other carbonate species in seawater is rather low, so corals expend energy to raise the pH of seawater sequestered in an isolated, extracellular compartment where crystal growth occurs. This action converts plentiful bicarbonate ions to the carbonate ions required for calcification, allowing corals to produce CaCO₃ about 100 times faster than it could otherwise form. It is this rapid and efficient production of CaCO₃ crystals that enables corals to build coral reefs.

Ocean acidification reduces the pH and thus the abundance of carbonate ions in seawater. Corals living in acidified seawater continue to produce CaCO₃ and expend as much energy as their counterparts in normal seawater to raise the pH of the calcifying fluid. However, in acidified seawater, corals are unable to elevate the concentration of carbonate ions to the level required for normal skeletal growth. In several experiments, we found that boosting the energetic status of corals by enhanced heterotrophic feeding or moderate increases in inorganic nutrients helped to offset the negative impact of ocean acidification. However, this built-in defense is unlikely to benefit corals as levels of CO₂ in the atmosphere continue to rise. Most climate models predict that the availability of inorganic nutrients and plankton in the surface waters where corals live will decrease as a consequence of global warming. Thus, corals and coral reefs may be significantly more vulnerable to ocean acidification than previously thought.

Cohen, A.L., and M. Holcomb. 2009. Why corals care about ocean acidification: Uncovering the mechanism. Oceanography 22(4):118–127, https://doi.org/10.5670/oceanog.2009.102.

Al-Horani, F.A., S.M. Al-Moghrabi, and D. de Beer. 2003. The mechanism of calcification and its relation to photosynthesis and respiration in the scleractinian coral Galaxea fascicularis. Marine Biology 142:419–426.

Allemand, D., É. Tambutté, D. Zoccola, and S. Tambutté. In press. Coral calcification, cells to reefs. In Coral and Coral Reefs. Z. Dubinsky, ed., Springer.

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

Boyd, P., and S.C. Doney. 2002. The impact of climate change and feedback processes on the ocean carbon cycle. Chapter 7 in Ocean Biogeochemistry: The Role of the Ocean Carbon Cycle in Global Change. International Joint Global Ocean Flux Study Synthesis, M. Fasham, ed., Springer.

Braun, A., and J. Erez. 2004. Preliminary observations on sea water utilization during calcification in scleractinian corals. Paper presented at American Geophysical Union Fall Meeting, December 13–17, San Francisco. Abstract #B14B-04 available online at: http://adsabs.harvard.edu/abs/2004AGUFM.B14B..04B (accessed December 2, 2009).

Cesar, H., L. Burke, and L. Pet-Soede. 2003. The Economics of Worldwide Coral Reef Degradation. Cesar Environmental Economics Consulting, Arnhem, The Netherlands.

Cohen A.L., G.D. Layne, S.R. Hart, and P.S. Lobel. 2001. Kinetic control of skeletal Sr/Ca in a symbiotic coral: Implications for the paleotemperature proxy. Paleoceanography 16(1):20–26.

Cohen, A.L., and T.A. McConnaughey. 2003. Geochemical perspectives on coral mineralization. Pp. 151–187 in Biomineralization. P.M. Dove, S. Weiner, and J.J. deYoreo, eds., Reviews in Mineralogy and Geochemistry, vol. 54, doi:10.2113/0540151, The Mineralogical Society of America, Washington, DC.

Cohen, A.L., D.C. McCorkle, S. de Putron, G.A. Gaetani, and K.A. Rose. 2009. Morphological and compositional changes in the skeletons of new coral recruits reared in acidified seawater: Insights into the biomineralization response to ocean acidification. Geochemistry, Geophysics, Geosystems 10, Q07005, doi:10.1029/2009GC002411.

Cohen, A.L., D.C. McCorkle, S.J. de Putron, and K.A. Rose. 2010. Why corals care about ocean acidification: The role of nutrition. 2010 Ocean Sciences Meeting, Portland, OR (abstract). 

Dennison, W.C., and D.J. Barnes. 1988. Effect of water motion on coral photosynthesis and calcification. Journal of Experimental Marine Biology and Ecology 115:67–77.

Feeley, R.A., S.C. Doney, and S.R. Cooley. 2009. Ocean acidification: Present conditions and future changes in a high-CO₂ world. Oceanography 22(4):36–47.

Garrels, R.M., and M.E. Thompson. 1962. A chemical model for seawater at 25°C and one atmosphere total pressure. American Journal of Science 260:57–66. 

Gaetani, G.A., and A.L. Cohen. 2006. Element partitioning during precipitation of aragonite from seawater: A framework for understanding paleoproxies. Geochemica et Cosmochimica Acta 70:4,617–4,634.

Holcomb, M. 2009. Coral Calcification: Insights From Inorganic Experiments and Coral Responses to Environmental Variables. PhD Thesis, Massachusetts Institute of Technology/Woods Hole Oceanographic Institution Joint Program in Oceanography/Applied Ocean Science and Engineering, 227 pp.

Holcomb, M., A. Cohen, R. Gabitov, and J. Hutter. 2009. Compositional and morphological features of aragonite precipitated experimentally from seawater and biogenically by corals. Geochimica et Cosmochimica Acta 73:4,166–4,179, doi:10.1016/ j.gca.2009.04.015.

Holcomb, M.C., A.L. Cohen, and D.C. McCorkle. 2010. Gender bias in the coral response to ocean acidification. 2010 Ocean Sciences Meeting, Portland, OR (abstract).

Holcomb, M.C., D.C. McCorkle, and A.L. Cohen. In press. Long-term effects of nutrient and CO₂ enrichment on the temperate coral Astrangia poculata (Ellis and Solander, 1786). Journal of Experimental Marine Biology and Ecology. 

Houlbrèque, F., and C. Ferrier-Pagès. 2008. Heterotrophy in tropical scleractinian corals. Biological Reviews 84:1–17, doi:10.1111/j.1469-185X.2008.00058. 

Kastner, M. 1984. Control of dolomite formation. Nature 311:410–411. 

Kleypas, J.A., and K.K. Yates. 2009. Coral reefs and ocean acidification. Oceanography 22(4):108–117.

Langdon, C., and M.J. Atkinson. 2005. Effect of elevated pCO₂ on photosynthesis and calcification of corals and interactions with seasonal change in temperature/irradiance and nutrient enrichment. Journal of Geophysical Research 110, C09S07, doi:10.1029/2004JC002576. 

Lippmann, F. 1973. Sedimentary carbonate minerals. Springer-Verlag, Berlin, 228 pp. 

Lofgren, G. 1974. An experimental study of plagioclase crystal morphology: Isothermal crystallization. American Journal of Science 274:243–273. 

Lofgren, G. 1980. Experimental studies on the dynamic crystallization of silicate melts. Pp. 487–565 in Physics of Magmatic Processes. R.B. Hargraves, ed., Princeton University Press, Princeton, NJ. 

Manzello, D.P., J.A. Kleypas, D. Budd, C.M. Eakin, P.W. Glynn, and C. Langdon. 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 of the United States of America 105, doi:10.1073/pnas.0712167105.

Marubini, F., and P.S. Davies. 1996. Nitrate increases zooxanthellae population density and reduces skeletogenesis in corals. Marine Biology 127:319–328.

Ries, J.B., A.L. Cohen, and D.C. McCorkle. 2009. Marine calcifiers exhibit mixed responses to CO₂-induced ocean acidification. Geology 37(12):1,131–1,134; doi:10.1130/G30210A.

Teng, H.H., P.M. Dove, C.A. Orme, and J.J. de Yoreo. 1998. Thermodynamics of calcite growth: Baseline for understanding biomineral formation. Science 282(5389):724–727, doi:10.1126/science.282.5389.724.

Usdowski, H.-E. 1968. The formation of dolomite in sediments. Pp. 21–32 in Recent Developments in Carbonate Sedimentology in Central Europe. G. Müller and G.M. Friedman, eds., Springer-Verlag, Berlin. 

van Heuven, S., D. Pierrot, E. Lewis, and D.W.R. Wallace. 2009. MATLAB Program Developed for CO₂ System Calculations. ORNL/CDIAC-105b. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, US Department of Energy, Oak Ridge, TN, http://cdiac.ornl.gov/oceans/co2rprt.html.

Veron, J.E.N. 1993. Corals of Australia and the Indo-Pacific. University of Hawaii Press, Honolulu, Hi, 656 pp.

Wells, J.W. 1956. Scleractinia. Pp. F328–F440 in Treatise on Invertebrate Paleontology Part F: Coelenterata. R.C. Moore, ed., Geological Society of America and University of Kansas Press, Lawrence, KS.

Zoccola, D., É. Tambutté, E. Kulhanek, S. Puverel, J.-C. Scimeca, and D. Allemand, and S. Tambutté. 2004. Molecular cloning and localization of a PMCA P-type calcium ATPase from the coral Stylophorapistillata pistillata. Biochimica et Biophysica Acta 1663:117–126.

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