What was done
Noting that "cyanobacteria such as Synechococcus and Prochlorococcus have a major impact on the global carbon cycle and contribute up to 50% of fixed carbon in marine systems (Partensky et al., 1999)," the authors determined "how CO2 and temperature individually and together affect the physiology of these two species under identical growth conditions" by growing stock cultures of the two picocyanobacteria in 1-L bottles of autoclaved and filtered seawater maintained at temperatures of either 20 or 24°C in equilibrium with air of either 380 or 750 ppm CO2.

What was learned
The five researchers discovered that the growth rate and the maximum photosynthetic rate in Synechococcus "increased ~2.3-fold and 4-fold, respectively, relative to ambient conditions, but remained unchanged in Prochlorococcus," in the high-temperature and high-CO2 treatment.

What it means
Fu et al. say their observations "could be taken to mean that in the future, rising temperature and CO2 would stimulate growth or photosynthesis of [the] Synechococcus isolate but would have much less effect on [the] Procholrococcus strain," and that such a result could "potentially influence competition between particular Synechococcus and Prochlorococcus ecotypes." However, they add that "we need to be very cautions about inferring ecosystem-scale shifts in broad taxonomic groups like picocyanobacteria from studies using only two isolates." In harmony with their findings for Synechococcus, they additionally report that stimulation of algal growth rates by elevated CO2 has also been observed by Burkhardt and Riebesell (1997), Burkhardt et al. (1999), Yang and Gao (2003), Beardall and Raven (2004) and Kim et al. (2006).

References
Beardall, J. and Raven, J.A. 2004. Potential effects of global change on microalgal photosynthesis, growth and ecology. Phycologia 43: 26-40.

Burkhardt, S. and Riebesell, U. 1997. CO2 availability effects elemental composition (C:N:P) of the marine diatom Skeletonema costatum. Marine Ecology Progress Series 155: 67-76.

Burkhardt, S., Riebesell, U. and Zondervan, I. 1999. Effects of growth rate, CO2 concentration, and cell size on the stable carbon isotope fractions in marine phytoplankton. Geochimica et Cosmochimica Acta 63: 3729-3741.

Kim, J.-M., Lee, K., Shin, K., Kang, J.-H., Lee, H.-W., Kim, M., Jang, P.-G. and Jang, M.C. 2006. The effect of seawater CO2 concentration on growth of a natural phytoplankton assemblage in a controlled mesocosm experiment. Limnology and Oceanography 51: 1629-1636.

Partensky, F., Blanchot, J. and Vaulot, D. 1999. Differential distribution and ecology of Prochlorococcus and Synechococcus in oceanic waters: a review. In: Charpy, L. and Larkum, A.W.D. (Eds.) Marine Cyanobacteria. Institute of Oceanography, Monaco, pp. 457-475.

Yang, Y. and Gao, K. 2003. Effects of CO2 concentrations on the freshwater microalgae, Chlamydomonas reinhardtii, Chlorella pyrenoidosa and Scenedesmus obliquus (Chlorophyta). Journal of Applied Phycology 15: 379-389.