How does rising atmospheric CO2 affect marine organisms?

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Growth Response of a Common Freshwater Microalga to Doubled Atmospheric CO2
Xia, J. and Gao, K.  2003.  Effects of doubled atmospheric CO2 concentration on the photosynthesis and growth of Chlorella pyrenoidosa cultured at varied levels of light.  Fisheries Science 69: 767-771.

The authors of this study note that "Riebesell et al. (1993) reported that the supply of CO2 in seawater limited the growth of marine phytoplankton under optimal light and nutrient conditions, and Hein and Sand-Jensen (1997) demonstrated that doubled atmospheric CO2 concentration stimulated marine productivity."  Hence, they say they decided "to investigate the effects of doubled CO2 concentration of the atmosphere on the growth and photosynthesis of Chlorella pyrenoidosa," a common freshwater microalga.

What was done
Cells of Chlorella pyrenoidosa were obtained from the Freshwater Algae Culture Collection of the Institute of Hydrobiology, the Chinese Academy of Sciences, and cultured in fresh Bristol's solution at low and high light levels (50 and 200 µmol/m2/s, respectively) during 12-hour light periods (each of which was followed by a 12-hour dark period) for a total of 13 days within controlled environment chambers, while the solutions were continuously aerated with air of either 350 or 700 ppm CO2.

What was learned
When the algal cells were harvested in the exponential growth phase, the biomass (cell density) of the twice-ambient CO2 treatment was found to be 10.9% and 8.3% greater than that of the ambient-air treatment in the low- and high-light regimes, respectively, although the authors report that only the high-light result was statistically significant.

What it means
In the words of the authors, "it is supposed that doubled atmospheric CO2 concentration would affect the growth of C. pyrenoidosa when it grows under bright solar radiation, and such an effect would increase by a great extent when the cell density becomes high."  Their data also do not rule out the possibility that the same may be true when the alga grows under not-so-bright conditions.

Hein, M. and Sand-Jensen, K.  1997.  CO2 increases oceanic primary production.  Nature 388: 988-990.

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

Reviewed 24 March 2004