How does rising atmospheric CO2 affect marine organisms?

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Ocean Acidification and Calcifying Clams
Range, P., Chicharo, M.A., Ben-Hamadou, R., Pilo, D., Matias, D., Joaquim, S., Oliveira, A.P. and Chicharo, L. 2011. Calcification, growth and mortality of juvenile clams Ruditapes decussatus under increased pCO2 and reduced pH: Variable responses to ocean acidification at local scales? Journal of Experimental Marine Biology and Ecology 396: 177-184.

In introducing their study, the authors write that "whether and how ocean acidification will affect marine organisms, ecosystems and the goods and services they provide is currently a topic of great concern," and in their specific case that concern is directed towards juvenile clams of coastal marine ecosystems, since they say these shellfish "link primary productivity with upper trophic levels" and "are also important economic resources for fisheries and aquaculture."

What was done
In an experiment designed to test the effects of increased pCO2 and reduced pH of seawater on the calcification, growth and mortality of juvenile Ruditapes decussatus clams, Range et al. conducted a 75-day controlled CO2 perturbation experiment, where the carbonate chemistry of seawater was manipulated by diffusing pure CO2 into natural seawater to attain two reduced pH levels (by -0.4 and -0.7 pH unit compared to un-manipulated seawater), hypothesizing that under these conditions the juvenile clams would exhibit: (1) reduced net calcification, (2) reduced growth of the shell and soft tissue, and (3) increased mortality.

What was learned
At the conclusion of their experiment, the eight researchers say that they found "no differences among pH treatments in terms of net calcification, size or weight of the clams," disproving the first two of their three hypotheses. Their third hypothesis also proved to be wrong -- doubly wrong, in fact -- for not only was juvenile clam mortality not increased in the low pH seawater, they say that mortality was significantly reduced in the acidified treatments, which was something they describe as a truly "unexpected result."

What it means
The Portuguese scientists conclude their paper by noting that life is intriguingly complex and that "the generalized and intuitively attractive perception that calcification will be the critical process impacted by ocean acidification is being increasingly challenged," citing Widdicombe and Spicer (2008) and Findlay et al. (2009) in this regard. And we note that the results of their own study further contribute to this emerging perception.

Findlay, H.S., Wood, H.L., Kendall, M.A., Spicer, J.I., Twitchett, R.J. and Widdicombe, S. 2009. Calcification, a physiological process to be considered in the context of the whole organism. Biogeosciences Discussions 6: 2267-2284.

Widdicombe, S. and Spicer, J.I. 2008. Predicting the impact of ocean acidification on benthic biodiversity: what can animal physiology tell us? Journal of Experimental Marine Biology and Ecology 366: 187-197.

Reviewed 8 June 2011