What was done
The authors grew a non-calcifying strain (PML 92A) of the marine coccolithophorid Emiliania huxleyi (Lohmann) Hay & Mohler in chemostats-cyclostats that were aerated with air of either 360 or 2000 ppm CO2 under both high- and low-light conditions in seawater either replete with or deficient in nitrogen and/or phosphorus, while measuring a suite of physical and biochemical properties of the coccolithophorid populations and the media in which they lived.

What was learned
Leonardos and Geider found that "increased atmospheric CO2 concentration enhances CO2 fixation into organic matter," but "only under certain conditions, namely high light [HL] and nutrient limitation." Under N-limited conditions, for example, they report that particulate organic carbon (POC) "was greatest under HL and elevated CO2 (by up to 46% relative to HL and ambient CO2)." Their work also revealed that "the increase in POC was a consequence of both an increase in cell density and an increase in the cell organic carbon content."

What it means
The two UK researchers state that "enhanced CO2 uptake by phytoplankton such as E. huxleyi, in response to elevated atmospheric CO2, could increase carbon storage in the nitrogen-limited regions of the oceans and thus act as a negative feedback on rising atmospheric CO2 levels." In fact, they calculate that if the results they obtained for E. huxleyi are indicative of the effects of CO2 on primary production in other N-limited phytoplankton, then changes of the magnitude they measured in E. huxleyi due to increased CO2 could increase export production of the oligotrophic ocean by an amount equivalent to the estimated postindustrial increase in the terrestrial carbon sink.

As for corroborating evidence, Leonardos and Geider note that their findings are "consistent with the response of primary productivity to manipulation of aqueous phase CO2 in the oligotrophic North Atlantic (Hein and Sand-Jensen, 1996)," where increases in primary productivity "of up to 100% were observed, although the average increase was 15% to 19%." In addition, they report that "stimulation of carbon fixation by elevated CO2 is already documented for nutrient-limited lake phytoplankton (Urabe et al., 2003)," while other supporting materials can be found in our Subject Index under the heading of Phytoplankton (Growth Response to CO2)."

References
Hein, M. and Sand-Jensen, K. 1997. CO2 increases oceanic primary production. Nature 388: 526-527.

Urabe, J., Togari, J. and Elser, J. 2003. Stoichiometric impacts of increased carbon dioxide on a planktonic herbivore. Global Change Biology 9: 818-825.