How does rising atmospheric CO2 affect marine organisms?

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Potentially-Toxic Cyanobacteria vs. Non-Toxic Algae
Holland, D.P., Pantorno, A., Orr, P.T., Stojkovic, S. and Beardall, J. 2012. The impacts of a high CO2 environment on a bicarbonate user: The cyanobacterium Cylindrospermopsis raciborskii. Water Research 46: 1430-1437.

The authors write that the potentially-toxic cyanobacterium Cylindrospermopsis raciborskii - which was originally described as a tropical-subtropical species - "is increasingly found in temperate regions and its range is expanding," while further noting that "climate change is hypothesized to be a factor in this expansion." Thus, they rightly state that "identifying future risk from this, and other nuisance cyanobacteria, is paramount."

What was done
Working with a strain of the cyanobacterium that was originally isolated from a lake near Brisbane (Australia), Holland et al. say they "used continuous (turbidostats) and batch cultures grown under two different light regimes, and adjusted the alkalinity of the media (with an associated change in pH, HCO3- and CO2) to assess the effect of these parameters on the specific growth rate, inorganic carbon acquisition and photosynthetic parameters of C. raciborskii."

What was learned
Although there were insufficient data to confirm results obtained from the low-light experiments, the five Australian researchers report "there was a positive linear relationship in the 'high' light turbidostats between the growth rate and pH," which they describe as being consistent with the fact that "cyanobacteria are reported to have low free-CO2/high pH optima when compared with other microalgae," citing the review of Dokulil and Teubner (2000) in this regard. That is to say, the potentially-toxic C. raciborskii grows more profusely when atmospheric CO2 concentrations are low and water pH is high.

What it means
In light of their experimental findings and those reviewed by Dokulil and Teubner, Holland et al. conclude that "high-CO2/low-pH conditions may therefore change the community structure to favor species that are better adapted to these new growth conditions, such as Chrysophytes" (which are known to produce more than half of the food consumed by aquatic animals), additionally citing Maberly et al. (2009) in this regard.

Dokulil, M.T. and Teubner, K. 2000. Cyanobacterial dominance in lakes. Hydrobiologia 438: 1-12.

Maberly, S.C., Ball, L.A., Raven, J.A. and Sultemeyer, D. 2009. Inorganic carbon acquisition by Chrysophytes. Journal of Phycology 45: 1052-1061.

Reviewed 8 August 2012