How does rising atmospheric CO2 affect marine organisms?

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Effects of Ocean Acidification on Marine Coccolithophores
Smith, H.E.K., Tyrrell, T., Charalampopoulou, A., Dumousseaud, C., Legge, O.J., Birchenough, S., Pettit, L.R., Garley, R., Hartman, S.E., Hartman, M.C., Sagoo, N., Daniels, C.J., Achterberg, E.P. and Hydes, D.J. 2012. Predominance of heavily calcified coccolithophores at low CaCO3 saturation during winter in the Bay of Biscay. Proceedings of the National Academy of Sciences USA 109: 8845-8849.

The authors write that "coccolithophores are an important component of the Earth system, and, as calcifiers, their possible susceptibility to ocean acidification is of major concern." In addition, they indicate that several experiments on Emiliania huxleyi, which is the world's most common coccolithophore, have observed depressed calcification rates at elevated CO2 concentrations and their associated low pH and CaCO3 saturation states; but they say that other studies have found elevated calcification (Iglesias-Rodriguez et al., 2008; Shi et al., 2009) or no trend (Feng et al., 2008) under such conditions.

This wide array of responses is suggested by them to be due to the fact that "laboratory studies are unrealistic in many respects and, because of their typically short timescales, preclude the possibility of evolutionary adaptation to the imposed change," which they describe as "a key uncertainty in ocean acidification research (Gattuso and Buddemeier, 2000; Langer et al., 2006; Ridgwell et al., 2009)." And in light of these observations, they say it is vital "to complement laboratory experiments with observational studies of coccolithophores living in the natural habitats to which they are evolutionarily adapted," which is precisely what they thus proceeded to do.

What was done
In the words of Smith et al., "seawater carbonate chemistry and other environmental variables were sampled monthly between September 2008 and August 2009 along a 1,000-km route, including over deep oceanic waters in the Bay of Biscay," while abundance assessments were made of two different morphotypes of E. huxleyi, one of which was more heavily calcified than the other.

What was learned
"Whereas a recent observational and paleo study (Beaufort et al., 2011) found a positive correlation between carbonate ion concentration and degree of coccolithophore calcification," to quote the team of 14 researchers, they say that their observational data "exhibit a distinct anti-correlation," such that "as pH and CaCO3 saturation state declined with the onset of winter in the Bay of Biscay, the over-calcified morphotype increased in both percentage [from less than 10% in summer to over 90% in winter] and absolute abundance."

What it means
In the words of Smith et al., "our finding that the most heavily calcified morphotype dominates when conditions are most acidic is contrary to earlier predictions and raises further questions about the fate of coccolithophores in a high-CO2 world."

Beaufort, L., Probert, I., de Garidel-Thoron, T., Bendif, E.M., Ruiz-Pino, D., Metzl, N., Goyet, C., Buchet, N., Coupel, P., Grelaud, M., Rost, B., Rickaby, R.E.M. and de Vargas, C. 2011. Sensitivity of coccolithophores to carbonate chemistry and ocean acidification. Nature 476: 80-83.

Feng, Y., Warner, M.E., Zhang, Y., Sun, J., Fu, F.-X., Rose, J.M. and Hutchins, A. 2008. Interactive effects of increased pCO2, temperature and irradiance on the marine coccolithophore Emiliania huxleyi (Prymnesiophyceae). European Journal of Phycology 43: 87-98.

Gattuso, J.P. and Buddemeier, R.W. 2000. Ocean biogeochemistry. Calcification and CO2. Nature 407: 311-313.

Iglesias-Rodriguez, M.D., Halloran, P.R., Rickaby, R.E.M., Hall, I.R., Colmenero-Hidalgo, E., Gittins, J.R., Green, D.R.H., Tyrrell, T., Gibbs, S.J., von Dassow, P., Rehm, E., Armbrust, E.V. and Boessenkool, K.P. 2008. Phytoplankton calcification in a high-CO2 world. Science 320: 336-340.

Langer, G., Geisen, M., Baumann, K.-H., Klas, J. , Riebesell, U., Thoms, S. and Young, J.R. 2006. Species-specific responses of calcifying algae to changing seawater carbonate chemistry. Geochemistry, Geophysics, Geosystems 7: 10.1029/2005GC001227.

Ridgwell, A, Schmidt, D.N., Turley, C., Brownlee, C., Maldonado, M.T., Tortell, P. and Young, J.R. 2009. From laboratory manipulations to Earth system models: Scaling calcification impacts of ocean acidification. Biogeosciences 6: 2611-2623.

Shi, D., Xu, Y. and Morel, F.M.M. 2009. Effects of the pH/pCO2 control method on medium chemistry and phytoplankton growth. Biogeoscienes 6: 1199-1207.

Reviewed 26 December 2012