Background
The authors write that "coccolithophores are abundant phytoplankton that are responsible for a large part of modern oceanic carbonate production," and they say that "culture experiments investigating the physiological response of coccolithophere calcification to increased CO2 have yielded contradictory results between and even within species," citing the studies of Riebesell et al. (2000), Langer et al. (2006), Iglesias-Rodriguez et al. (2008) and Langer et al. (2009).

What was done
To further explore the subject and "to assess the influence of the environment on coccolithophore calcification," Beaufort et al. state that they "investigated 180 surface-water and 555 sediment-core samples encompassing a wide spectrum of present and past oceanic conditions," some stretching back in time as much as 40,000 years.

What was learned
The thirteen researchers report that "significant overall correlations of coccolith mass with pH and pCO2 were recorded, but with notable regional variations, indicating that these parameters are not solely responsible for the observed trend." They also report that some cultured strains of coccolithophores "are capable of maintaining calcification (degree and/or rate) over certain carbonate-chemistry ranges, a phenomenon that could contribute to localized within-sample deviations from the broad trend linking coccolith mass to carbonate chemistry." In fact, they say that changes in the relative abundance of taxa were "predominantly responsible for the decrease in coccolith mass with ocean acidification that was seen in modern samples."

Also adding to the complexity of the situation, Beaufort et al. write that "in Patagonian-shelf and Chilean upwelling waters with low CO3^2-, in which the overall trend would predict low coccolith mass," they detected "an unexpectedly highly calcified Emiliania huxleyi morphotype," and they indicate that "the relative abundance of this morphotype increased with decreasing pH along the Pacific transect towards Chile." In addition -- and noting that "because coccolith morphotype is thought to be subject to genetic regulation (Langer et al., 2009)" -- they say "this highly calcified E. huxleyi morphotype may be a genetic entity with an adaptation enabling it to calcify heavily in the relatively acidic upwelling waters."

What it means
The international team of scientists concludes that "the presence of highly calcified E. huxleyi in CO2-rich modern waters demonstrates that prediction of future responses is unlikely to be straightforward," and that "such complexity could account for the lack of an obvious overall direction in the response of coccolithophore calcification over a potentially analogous ocean acidification event about 55 million years ago at the Palaeocene-Eocene Thermal Maximum," citing the study of Gibbs et al. (2006). And these several observations would seem to suggest that rising atmospheric CO2 concentrations will very likely not bring a halt, or even a significant reduction, to coccolithophere calcification in either the decades or centuries to come.

References
Gibbs, S.J., Brown, P.R., Sessa, J.A., Bralower, T.J. and Wilson, P.A. 2006. Nannoplankton extinction and origination across the Paleocene-Eocene Thermal Maximum. Science 314: 1770-1773.

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.

Langer, G., Nehrke, G., Probert, I., Ly, J. and Ziveri, P. 2009. Strain-specific responses of Emiliania huxleyi to changing seawater carbonate chemistry. Biogeosciences Discussions 6: 4361-4383.

Riebesell, U., Zondervan, I., Rost, B., Tortell, P.D., Zeebe, R.E. and Morel, F.M.M. 2000. Reduced calcification of marine plankton in response to increased atmospheric CO2. Nature 407: 364-367.