How Corals Are Able to Tolerate Significant Seawater Acidification
Venn, A.A., Tambutte, E., Holcomb, M., Laurent, J., Allemand, D. and Tambutte, S. 2013. Impact of seawater acidification on pH at the tissue-skeleton interface and calcification in reef corals. Proceedings of the National Academy of Sciences USA 110: 1634-1639.
The six scientists from Monaco begin by noting that although several studies of marine calcifiers have investigated how rates of calcification respond to ocean acidification, comparatively few of them broach the question of how ocean acidification impacts the physiological mechanisms that drive calcification, which knowledge is needed to predict how corals and other marine calcifiers will respond and potentially acclimate to ocean acidification.
Continuing, they indicate that in corals, the capacity to regulate pH in the fluid at the tissue-skeleton interface [subcalicoblastic medium (SCM)] and in the calcifying cells [calicoblastic epithelium (CE)] "has been widely proposed to be important in shaping calcification responses to ocean acidification," and they therefore decided to analyze the impact of seawater acidification on pHSCM and pHCE in the coral Stylophora pistillata, "using in vivo imaging of pH in corals exposed to reduced seawater pH and elevated pCO2 in the laboratory for [both] long and short durations," which work included "exposures to levels of acidification and elevated pCO2 many times greater than those predicted to occur at the end of this century [italics added]."
So what did they find?
Venn et al. say they "observed calcification (measured by growth of skeletal crystals and whole colonies) in all our treatments, including treatment pH 7.2, where aragonite was undersaturated." And they say that "this finding agrees with previous work with S. pistillata conducted elsewhere, where net calcification was also observed over a similar range of pH and pCO2 (Krief et al., 2010)." These findings suggest, in their words, that "S. pistillata may have a high tolerance to decreases in seawater pH and changes in seawater chemistry," which leads them to conclude that "maintenance of elevated pHSCM relative to the surrounding seawater may explain how several coral species continue to calcify even in low pH seawater, which is undersaturated with respect to aragonite (this study and Rodolfo-Metalpa et al. (2011) and Cohen et al., (2009))."
Last of all, Venn et al. report that "reductions in calcification rate, both at the level of crystals and whole colonies, were only observed in our lowest pH treatment [pH 7.2] when pH was significantly depressed in the calcifying cells in addition to the SCM." Nevertheless, and "overall," they say their findings suggest that "reef corals may mitigate the effects of seawater acidification by regulating pH in the SCM," which is something they clearly have the capacity to do.
Cohen, A.L., McCorkle, D.C., De Putron, S., Gaetani, G.A. and Rose, K.A. 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: 1-12.
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.
Rodolfo-Metalpa, R., Houlbreque, F., Tambutte, E., Boisson, F., Baggini, C., Patti, F.P., Jeffree, R., Fine, M., Foggo, A., Gattuso, J.P. and Hall-Spencer, J.M. 2011. Coral and mollusk resistance to ocean acidification adversely affected by warming. Nature Climate Change 1: 308-312.