What was done
Employing semi-continuous culturing methods that used filtered, microwave-sterilized surface Sargasso seawater that was enriched with phosphate and trace nutrients, the authors "examined the physiological responses of steady-state iron (Fe)-replete and Fe-limited cultures of the biogeochemically critical marine unicellular diazotrophic cyanobacterium Crocosphaera [watsonii] at glacial (190 ppm), current (380 ppm), and projected year 2100 (750 ppm) CO2 levels."

What was learned
When the seawater was replete with iron, daily primary production at 750 ppm CO2 was 21% greater than it was at 380 ppm, while at 190 ppm CO2 it was 38% lower than it was at 380 ppm. When the seawater was iron-limited, however, daily primary production at 750 ppm CO2 was 150% greater than it was at 380 ppm, while at 190 ppm CO2 it was 22% lower than it was at 380 ppm. With respect to N2 fixation, rates varied little among the three CO2 treatments when the seawater was iron-limited; but when the seawater was replete with iron, N2 fixation at 750 ppm CO2 was 60% greater than it was at 380 ppm, while at 190 ppm CO2 it was 33% lower than it was at 380 ppm.

What it means
Fu et al. write that "several studies examining the marine diazotrophic cyanobacterium Trichodesmium have shown significant increases in N2 fixation and photosynthesis in response to elevated CO2 concentration (Hutchins et al., 2007; Levitan et al., 2007; Ramos et al., 2007)," and they say that their data "extend these findings to encompass the marine unicellular N2-fixing cyanobacterium Crocosphaera," which group, they add, "is now recognized as being perhaps equally as important as Trichodesmium to the ocean nitrogen cycle (Montoya et al., 2004)." Consequently, they conclude that "anthropogenic CO2 enrichment could substantially increase global oceanic N2 and CO2 fixation," which two-pronged phenomenon would be a tremendous boon to the marine biosphere.

References
Hutchins, D.A., Fu, F.-X., Zhang, Y., Warner, M.E., Feng, Y., Portune, K., Bernhardt, P.W. and Mulholland, M.R. 2007. CO2 control of Trichodesmium N2 fixation, photosynthesis, growth rates, and elemental ratios: Implications for past, present, and future ocean biogeochemistry. Limnology and Oceanography 52: 1293-1304.

Levitan, O., Rosenberg, G., Setlik, I., Setlikova, E., Grigel, J., Klepetar, J., Prasil, O. and Berman-Frank, I. 2007. Elevated CO2 enhances nitrogen fixation and growth in the marine cyanobacterium Trichodesmium. Global Change Biology 13: 531-538.

Montoya, J.P., Holl, C.M., Zehr, J.P., Hansen, A., Villareal, T.A. and Capone, D.G. 2004. High rates of N2 fixation by unicellular diazotrophs in the oligotrophic Pacific Ocean. Nature 430: 1027-1031.

Ramos, J.B.E., Biswas, H., Schulz, K.G., Laroche, J. and Riebesell, U. 2007. Effect of rising atmospheric carbon dioxide on the marine nitrogen fixer Trichodesmium. Global Biogeochemical Cycles 21: 10.1029/2006GB002898.