Responses of Scleractinian Corals to Ocean Acidification

In July of 2007, Kreif et al. (2010) collected two colonies of massive Porites corals (which form large multi-century-old colonies and calcify relatively slowly) and four colonies of the branching Stylophora pistillata coral (which is short-lived and deposits its skeleton rather rapidly) from a reef adjacent to the Interuniversity Institute for Marine Science in Eilat (Israel) at the northern tip of the Red Sea; and they grew fragments of these corals in 1000-liter tanks through which they pumped Gulf of Eilat seawater that they adjusted to be in equilibrium with air of three different CO₂ concentrations (385, 1904 and 3970 ppm), which led to corresponding pH values of 8.09, 7.49 and 7.19 and corresponding aragonite saturation state (Ω_arag) values of 3.99, 1.25 and 0.65. Then, after an incubation period of six months for S. pistillata and seven months for the Porites corals, several fragments were sampled and analyzed for a number of different coral properties; and fourteen months from the start of the experiment, fragments of each coral species from each CO₂ treatment were analyzed for zooxanthellae cell density, chlorophyll a concentration, and host protein concentration. And what did this work reveal?

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 Ω_arag 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 Ω_arag 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 Ω_arag < 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."

Additional References
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.