How does rising atmospheric CO2 affect marine organisms?

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Alkalinity Adjustments and Coral Calcification Rates
Gattuso, J.-P., Frankignoulle, M., Bourge, I., Romaine, S. and Buddemeier, R.W.  1998.  Effect of calcium carbonate saturation of seawater on coral calcification.  Global and Planetary Change 18: 37-46.

What was done
The authors conducted a controlled laboratory experiment to investigate the effect of calcium carbonate saturation state on the calcification rate of two reef-building corals by manipulating the saturation of aragonite.

What was learned
Calcification rates increased logarithmically as a function of aragonite solution state, leveling off at saturation values greater than 300%.  Based upon this and other relationships, the authors calculate that an increase in atmospheric CO2 concentration to a value of 560 ppm may possibly result in a very slight decline in coral calcification; while at an atmospheric CO2 concentration of 1000 ppm, the authors predict only a 14% decrease in calcification rate.

What it means
Although calcification rates in the two corals examined are not expected to decline by any significant amount with a doubling of atmospheric CO2 above pre-industrial values, further CO2 increases may have a significant effect on this phenomenon.  However, as the authors note, it is important to recognize that any decrease in aragonite saturation due to a CO2-induced decrease in pH may be offset by additional CO2-induced weathering of terrestrial carbonates that would release more Ca2+ to the oceans, a scenario that would "likely occur under elevated atmospheric pCO2."  In addition, the authors stress the importance of recognizing the limitations of their predictions of future CO2-induced changes in calcification rates.  They note, for example, that (1) their study utilized only two species of coral and these particular species may not be major reef builders, (2) their study dealt with only short-term effects, while over the longer term "corals display quite impressive acclimation processes to changes in some environmental parameters," and (3) "the saturation state was manipulated by altering the seawater calcium content and not pCO2."  They also note that the rate of zooxanthellae photosynthesis has never been investigated under elevated pCO2 and that increasing the pCO2 content may also increase the photosynthetic activity of these coral symbionts, thus counteracting the decrease in calcification resulting from the lowered aragonite saturation state.  Clearly, as the authors rightly conclude, there is a need to pursue more rigorous experimental studies using appropriately controlled environmental conditions and seawater composition to truly unravel the response of corals to global environmental changes.

Reviewed 1 August 1999