Background
The authors write that "in marine phytoplankton, studies have shown that rising atmospheric CO2 can enhance their growth (Riebesell et al., 1993; Kranz et al., 2010), primary production (Hein and Sand-Jensen, 1997) and nitrogen fixation (Levitan et al., 2007)." But they add that "the impacts of elevated atmospheric CO2 on freshwater habitats are still poorly understood."

What was done
In an effort designed to rectify this latter situation, the four Chinese scientists isolated specimens of the freshwater N2-fixing cyanobacterium (Cylindrospermopsis raciborskii) from a pond near Dianchi Lake in Kunming (China); and they cultured them semi-continuously for 18 days at low and high inorganic phosphorus (Pi) levels (0.022 µM and 22 µM, respectively) in contact with air of either 380 or 1000 ppm CO2, while measuring several important physiological functions of the cyanobacterium.

What was learned
In the case of light-saturated net photosynthesis, the 620-ppm increase in the air's CO2 content resulted in 37% and 74% increases in the low and high Pi treatments, respectively. In the case of biomass growth, the CO2 increase resulted in 26% and 23% increases in the low and high Pi treatments, respectively. And in the case of nitrogen fixation, the CO2 increase resulted in 36% and 14% increases in the low and high Pi treatments, respectively.

What it means
Wu et al. say that the cyanobacterial growth increase they observed "confirms previous studies with other algae (Burkhardt and Riebesell, 1997; Burkhardt et al., 1999; Clark and Flynn, 2000; Kim et al., 2006; Posselt et al., 2009; Kranz et al., 2010)," as well the finding of Chinnasamy et al. (2009) that "the nitrogenase activity of Anabaena fertilissima increased with increasing levels of CO2." Thus, it would appear that in both marine and freshwater ecosystems, continued increases in the air's CO2 content should significantly enhance the wellbeing of various species of phytoplankton.

References
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.

Chinnasamy, S., Ramakrishnan, B., Bhatnagar, A., Goyal, S.K. and Das, K.C. 2009. Carbon and nitrogen fixation by Anabaena fertilissima under elevated CO2 and temperature. Journal of Freshwater Ecology 24: 587-596.

Clark, D.R. and Flynn, K.J. 2000. The relationship between the dissolved inorganic carbon concentration and growth rate in marine phytoplankton. Proceedings of the Royal Society B: Biological Sciences 267: 953-959.

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

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.

Kranz, S.A., Levitan, O., Richter, K.-U., Prasil, O., Berman-Frank, I. and Rost, B. 2010. Combined effects of CO2 and light on the N2-fixing cyanobacterium Trichodesmium IMS101: physiological responses. Plant Physiology 154: 334-345.

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.

Posselt, A.J., Burford, M.A. and Shaw, G. 2009. Pulses of phosphate promote dominance of the toxic cyanophyte Cylindrospermopsis raciborskii in a subtropical water reservoir. Journal of Phycology 45: 540-546.

Riebesell, U., Wolf-Gladrow, D.A. and Smetacek, V. 1993. Carbon dioxide limitation of marine phytoplankton growth rates. Nature 361: 249-251.