Background
Coccolithophores (i.e., single-celled algae, protists, and phytoplankton that belong to the division of haptophytes) are considered to be "the most productive calcifying organisms on the planet," according to Fiorini et al. (2011); and they say that "they play a crucial role in the marine carbon cycle through calcification and photosynthetic carbon production," citing Rost and Riebesell (2004). More specifically, they note that coccolithophores are responsible "for about half of the global surface ocean calcification," and that they "contribute significantly to the flux of organic matter from the sea surface to deep waters and sediments," as discussed by Klaas and Archer (2002).

What was done
In a study designed to determine how coccolithophores might perform in a CO2-enriched world of the future, Hannisdal et al. looked to the distant past, noting that "the last period of sustained high-CO2 (greenhouse) conditions in earth history ended ~34 Ma [million years ago]." Thus, they combined a comprehensive 50-5 Ma fossil data set on coccolithophore cell size with what they call "a novel measure of ecological prominence" - i.e., the Summed Common Species Occurrence Rate (SCOR) - which parameter, in their words, "captures changes in the extent to which coccolithophores were common and widespread, based on global occurrences in deep-sea sediments," after which they compared the size and SCOR records to state-of-the-art data on climatic and environmental changes from 50 to 5 Ma.

What was learned
The three Nordic researchers discovered what they called, "a striking relationship between macroevolutionary changes in the coccolithophores and estimated changes in atmospheric pCO2 over a period of 50 million years, spanning one of earth's major climatic (greenhouse-icehouse) transitions," wherein "coccolithophores were globally more common and widespread, larger, and more heavily calcified in the pre-34 Ma greenhouse world, and declined in both ecological prominence and body size along with pCO2 during the Oligocene (34-23 Ma)."

What it means
Hannisdal et al. say their results suggest that "atmospheric pCO2 exerted an important long-term control on coccolithophores, either directly through its availability for photosynthesis or indirectly via weathering supply of resources for growth and calcification." In either event - or both - it is clear that their findings are, as they put it, "consistent with inferred coccolith mass increase with rising atmospheric pCO2 over the past two centuries (Halloran et al., 2008; Iglesias-Rodriguez et al., 2008)," as well as theoretical life-history modeling, which has been shown by Irie et al. (2010) "to predict an increase in coccolithophore size and calcification as an adaptive evolutionary response to ocean acidification".

References
Fiorini, S., Middelburg, J.J. and Gattuso, J.-P. 2011. Effects of elevated CO2 partial pressure and temperature on the coccolithophore Syracosphaera pulchra. Aquatic Microbial Ecology 64: 221-232.

Halloran, P.R., Hall, I.R., Colmenero-Hidalgo, E. and Rickaby, R.E.M. 2008. Evidence for a multi-species coccolith volume change over the past two centuries: understanding a potential ocean acidification response. Biogeosciences 5: 1651-1655.

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.

Irie, T., Bessho, K., Findlay, H.S. and Calosi, P. 2010. Increasing costs due to ocean acidification drives phytoplankton to be more heavily calcified: optimal growth strategy of coccolithophores. PLoS ONE 5: 313436.

Klaas, C. and Archer, D.E. 2002. Association of sinking organic matter with various types of mineral ballast in the deep sea: implications for the rain ratio. Global Biogeochemical Cycles 16: 10.1029/2001GB001765.

Rost, B. and Riebesell, U. 2004. Coccolithophores and the biological pump: responses to environmental changes. In: Thierstein, H.R. and Young, J.R. (Eds.). Coccolithophores: From Molecular Processes to Global Impact. Springer, New York, New York, USA, p. 99-125.