How does rising atmospheric CO2 affect marine organisms?

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Coccolithophores in the Bay of Biscay
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 "laboratory studies are unrealistic in many respects and, because of their typically short timescales, preclude the possibility of evolutionary adaptation to the imposed change, a key uncertainty in OA [ocean acidification] research," citing Gattuso and Buddemeier (2000), Langer et al. (2006) and Ridgwell et al. (2009). And, therefore, 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."

What was done
Focusing on two morphotypes (over-calcified and normal) of the world's most abundant coccolithophore species (Emiliania huxleyi), Smith et al. assessed their numbers, along with seawater carbonate chemistry and other environmental variables, at monthly intervals between September 2008 and August 2009 along a 1,000-km route, including over deep oceanic waters in the Bay of Biscay.

What was learned
The fourteen researchers say their data "clearly show" that "there is a pronounced seasonality in the morphotypes of E. huxleyi," reporting "surprisingly" that "the over-calcified morphotype was found to dominate the E. huxleyi population in winter," in spite of the fact that seawater pH and CaCO3 saturation were lowest in winter. And the heavily-calcified form of E. huxleyi dominated dramatically, shifting from less than10% of the total E. huxleyi population in summer to more than 90% of the population in winter.

What it means
Smith et al. say they "do not suggest that the changing carbonate chemistry was necessarily responsible for this shift in morphotypes." However, they suggest that "if it was not, then the alternative is that carbonate chemistry is not the sole and overriding control over coccolithophore calcification," which should, in their words, "seriously call into question" the contention of some that "ocean acidification will lead to a replacement of heavily-calcified coccolithophores by lightly-calcified ones."

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

Langer, G., Geisen, M., Baumann, K.-H, Kläs, 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.

Reviewed 7 November 2012