How does rising atmospheric CO2 affect marine organisms?

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The Health and Productivity of Diatoms in Acidified Seawater
Crawfurd, K.J., Raven, J.A., Wheeler, G.L., Baxgter, E.J. and Joint, I. 2011. The response of Thalassiosira pseudonana to long-term exposure to increased CO2 and decreased pH. PLoS ONE 6: e26695.

The authors state that diatoms are very important for the productivity of the world's oceans, as they contribute about 45% of global marine primary production (Mann, 1999); and they therefore say it is essential to understand how diatoms and other marine phytoplankton will respond to the higher aqueous CO2 and lower pH conditions that will prevail in their surface waters in the near future, as a result of the dissolution of anthropogenic CO2 in them.

What was done
Crawfurd et al. grew over 100 generations of the diatom Thalassiosira pseudonana in seawater maintained in equilibrium with air of either current (ambient) CO2 concentration or expected end-of-the-century (twice-ambient) CO2 concentration (380 or 760 ppm) - which produced pH values of 8.1 and 7.8, respectively - for a period of three months, after which they evaluated a number of the species' physical and physiological characteristics.

What was learned
Overall, in the words of the five UK researchers, "there were very few effects on T. pseudonana of long-term culture at different pCO2 and pH." In fact, they report that "growth rates were identical in cultures supplied with 780 or 360 ppm CO2," and they note that "similar results have been reported for other diatom species," citing the studies of Tortell et al. (1997), Tortell (2000), Kim et al. (2006) and Shi et al. (2009 ).

What it means
In light of their several findings and the analogous results of several other scientific teams, Crawfurd et al. conclude in the final sentence of their paper that "if all diatoms respond in a similar fashion to T. pseudonana, acidification of this magnitude in the future ocean may have little effect on diatom productivity."

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.

Mann, D.G. 1999. The species concept in diatoms. Phycologia 38: 437-495.

Shi, D., Xu, Y. and Morel, F.M.M. 2009. Effects of the pH/pCO2 control method on medium chemistry and phytoplankton growth. Biogeosciences 6: 1199-1207.

Tortell, P.D., Reinfelder, J.R. and Morel, F.M.M. 1997. Active uptake of bicarbonate by diatoms. Nature 390: 243-244.

Tortell, P.D. 2000. Evolutionary and ecological perspectives on inorganic carbon acquisition in phytoplankton. Limnology and Oceanography 45: 744-750.

Reviewed 30 May 2012