What was done
The authors grew mono-cultures of six species of marine microalgae (Chlorella sp., Dunaliella salina, Isochrysis galbana Parke 8701, Nitzschia closterium, Platymanas subcordiformis, and Platymanas sp.) in Erlenmeyer flasks under 14:10 dark:light 24-hour cycles through which air of either 360 or 5000 ppm CO2 was continuously bubbled for a period of five days, during which time they measured algal cell densities at one-day intervals in order to calculate and compare algal specific growth rates.

What was learned
All six species responded positively to the ultra-high CO2 enrichment of the air that was continuously bubbled through the medium in which they grew, with the ratios of their specific growth rates in the CO2-enriched compared to ambient-air treatments being 1.15 for Chlorella sp., 1.35 for Dunaliella salina, 1.35 for Isochrysis galbana, 1.40 for Nitzschia closterium, 1.47 for Platymanas subcordiformis, and 1.60 for Platymanas sp.

What it means
In bold subject-heading print, Yu et al. conclude that a "high concentration of CO2 promotes the growth of microalgae," citing not only their results, but the findings of a number of other researchers as well, including Lin (1991), Nobutaka et al. (1992), Riebesell et al. (1993), Hein and Sand-Jensen (1997), Liang and Yonemoto (1999), Hu and Gao (2001), and Xia and Gao (2001, 2002), noting that "possibly, the CO2 enrichment made the chloroplast[s] more developed," such that "both the photosynthesis and cell division rate were increased," and, therefore, that "the algal cell density was enhanced."

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

Hu, H. and Gao, K. 2001. Effects of doubled atmospheric CO2 on the growth and photosynthesis of Chaetoceros muelleri. Acta Hydrobiologica Sinica 25: 636-638.

Liang, W. and Yonemoto, T. 1999. The culture of Gonidium (Chlorella Ellipsoidea) using light and the study of its growth. Journal of Wuhan Yejin University of Science & Technology 22: 248-251.

Lin, H. 1991. Comparison of Spirulina subsalsa with other Spirulina species. Acta Hydrobiologica Sinica 15: 27-34.

Nobutaka, H., Toshifumi, T., Yoshiharu F. et al. 1992. Tolerance of microalgae to high CO2 and high temperature. Phytochemistry 31: 3345-3348.

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

Xia, J. and Gao, K. 2001. Effects of high CO2 concentration on growth and photosynthesis of Spirulina maxima. Acta Hydrobiologica Sinica 25: 474-480.

Xia, J. and Gao, K. 2002. Effects of CO2 enrichment on microstructure and ultrastructure of two species of fresh water green algae. Acta Botanica Sinica 44: 527-531.