Responses of Scleractinian Corals to Ocean Acidification
Krief, S., Hendy, E.J., Fine, M., Yam, R., Meibom, A., Foster, G.L. and Shemesh, A. 2010. Physiological and isotopic responses of scleractinian corals to ocean acidification. Geochimica et Cosmochimica Acta 74: 4988-5001.
In the words of the seven scientists who conducted the study, "following 14 months incubation under reduced pH conditions, all coral fragments survived and added new skeletal calcium carbonate, despite Ωarag values as low as 1.25 and 0.65." This was done, however, at a reduced rate of calcification compared to fragments growing in the normal pH treatment with a Ωarag value of 3.99. Yet in spite of this reduction in skeletal growth, they report that "tissue biomass (measured by protein concentration) was found to be higher in both species after 14 months of growth under increased CO2." And they further note that the same phenomenon had been seen by Fine and Tchernov (2007), who, as they describe it, "reported a dramatic increase (orders of magnitude larger than the present study) in protein concentration following incubation of scleractinian Mediterranean corals (Oculina patagonica and Madracis pharencis) under reduced pH," stating that "these findings imply tissue thickening in response to exposure to high CO2." Also, in a somewhat analogous situation, Krief et al. report that "a decrease in zooxanthellae cell density with decreasing pH was recorded in both species," but that "this trend was accompanied by an increase in chlorophyll concentration per cell at the highest CO2 level."
In discussing their intriguing findings, the Israeli, French and UK researchers say "the inverse response of skeleton deposition and tissue biomass to changing CO2 conditions is consistent with the hypothesis that calcification stimulates zooxanthellae photosynthesis by enhancing CO2 concentration within the coelenteron (McConnaughey and Whelan, 1997)," and they write that "since calcification is an energy-consuming process ... a coral polyp that spends less energy on skeletal growth can instead allocate the energy to tissue biomass," citing Anthony et al. (2002) and Houlbreque et al. (2004). Thus, they suggest that "while reduced calcification rates have traditionally been investigated as a proxy of coral response to environmental stresses, tissue thickness and protein concentrations are a more sensitive indicator of the health of a colony," citing Houlbreque et al. (2004) in this regard as well.
In concluding their paper, Krief et al. say "the long acclimation time of this study allowed the coral colonies to reach a steady state in terms of their physiological responses to elevated CO2," and that "the deposition of skeleton in seawater with Ωarag < 1 demonstrates the ability of both species to calcify by modifying internal pH toward more alkaline conditions." As a result, they further state that "the physiological response to higher CO2/lower pH conditions was significant, but less extreme than reported in previous experiments," suggesting that "scleractinian coral species will be able to acclimate to a high CO2 ocean even if changes in seawater pH are faster and more dramatic than predicted."
Anthony, K.R., Connolly, S.R. and Willis, B.L. 2002. Comparative analysis of energy allocation to tissue and skeletal growth in corals. Limnology and Oceanography 47: 1417-1429.
Fine, M. and Tchernov, D. 2007. Scleractinian coral species survive and recover from decalcification. Science 315: 10.1126/science.1137094.
Houlbreque, F., Tambutte, E., Allemand, D. and Ferrier-Pages, C. 2004. Interactions between zooplankton feeding, photosynthesis and skeletal growth in the scleractinian coral Stylophora pistillata. Journal of Experimental Biology 207: 1461-1469.
McConnaughey, T. and Whelan, J.F. 1997. Calcification generates protons for nutrient and bicarbonate uptake. Earth Science Reviews 42: 95-117.