What was done
In a culture experiment with two algal symbiont-bearing, reef-dwelling forams -- Amphisorus kudakajimensis and Calcarina gaudichaudii -- which was conducted in seawater maintained at five different pCO2 concentrations (245, 375, 588, 763 and 907 ppm), the authors measured foram net calcification rates. And in another culture experiment that was conducted in seawater in which bicarbonate ion concentrations were varied, while keeping carbonate ion concentration constant, they did the same thing.

What was learned
In the first experiment, Hikami et al. found that "net calcification of A. kudakajimensis was reduced under higher pCO2, whereas calcification of C. gaudichaudii generally increased with increased pCO2," while in the second experiment, they found that "calcification was not significantly different between treatments in Amphisorus hemprichii, a species closely related to A. kudakajimensis, or in C. gaudichaudii."

What it means
In explaining the results of their first experiment, the nine researchers begin by noting that the upward calcification trend of Calcarina with rising pCO2 "can probably be attributed to the increase in CO2, possibly through enhancement of symbiont photosynthesis, a phenomenon known as the CO2-fertilizing effect (e.g., Ries et al., 2009)." This explanation is probably correct, because, as Hikimi et al. note, Calcarina harbor diatoms, and "both a single-species culture experiment (Wu et al., 2010) and a mesocosm bloom experiment (Engel et al., 2008) have shown that high-CO2 seawater is favorable to diatom growth," which in turn stimulates calcification, while they note that Rost et al. (2006) have reported that dinoflagellates -- which are harbored by Amphisorus -- "use HCO3^- as their carbon source, so their rate of carbon fixation may remain unaffected by fluctuating CO2 levels." As for the second experiment, the sea-water pH varied but little between the different bicarbonate ion concentration treatments, resulting in little variation in the calcification rates of both species. Thus, as they speculate in the concluding sentence of their paper, Hikami et al. feel that the different influences of sea-water chemistry they observed in the two forams may be attributable to the different types of symbiotic algae they hosted, which does indeed seem logical.

References
Engel, A., Schulz, K.G., Riebesell, U., Bellerby, R., Delille, B. and Schartau, M. 2008. Effects of CO2 on particle size distribution and phytoplankton abundance during a mesocosm bloom experiment (PeECE II). Biogeosciences 5: 509-521.

Ries, J.B., Cohen, A.L. and McCorkle, D.C. 2009. Marine calcifiers exhibit mixed responses to CO2-induced ocean acidification. Geology 37: 1131-1134.

Rost, B., Richter, K.-U., Riebesell, U. and Hansen, P.J. 2006. Inorganic carbon acquisition in red-tide dinoflagellates. Plant, Cell, and Environment 29: 810-822.

Wu, Y., Gao, K. and Riebesell, U. 2010. CO2-induced seawater acidification affects physiological performance of the marine diatom Phaeodactylum tricornutum. Biogeosciences 7: 2915-2923.