How does rising atmospheric CO2 affect marine organisms?

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Photosynthesis and Calcification in Coral Reefs
Gattuso, J.-P., Allemand, D. and Frankignoulle, M.  1999.  Photosynthesis and calcification at cellular, organismal and community levels in coral reefs: A review on interactions and control by carbonate chemistry.  American Zoologist 39: 160-183.

What was done
The authors review what is known about the effects of global warming and global atmospheric CO2 concentration increases on calcification rates in coral reefs.

What was learned
In many corals, calcification rates moderately rise in response to an increase in water temperature; but they decline significantly in response to the decrease in calcium carbonate saturation state that is predicted to occur in response to the ongoing rise in the air's CO2 concentration.  On the other hand, the authors find that photosynthesis and calcification are tightly coupled in most corals and coral reef communities, and that metabolic CO2 derived from photosynthetic products seems to be a significant source of carbon for the calcification process.  This finding suggests that "hydrospheric CO2 enrichment" [our terminology] might possibly enhance coral calcification rates more than CO2-induced changes in aqueous carbonate chemistry reduce them, as many aquatic plants display such positive growth responses.  However, most coral symbionts are highly effective bicarbonate users that possess a carbon-concentrating mechanism and thus might not exhibit this response, although some studies have suggested that CO2 does play a major role in this regard.  The authors also point out that zooxanthellae display considerable ability to change their mechanism of carbon supply, depending on their environment; and that an increased concentration of dissolved CO2, together with an increase in uncatalyzed rate of CO2 generation by HCO3- dehydration, may end up favoring the diffusional carbon supply at the expense of the carbon-concentrating mechanism of carbon supply.  Clearly, the situation is so complex that the authors conclude that our "inadequate understanding of the mechanism of calcification and its interaction with photosynthesis severely limits the ability to provide an accurate prediction of future changes in the rate of [coral] calcification."

Viewed from another perspective, however, the authors note that "the signs and magnitudes of many environmental changes that are expected in the next decades are similar to the current range of climatic variation within which reefs exist, and are considerably less extreme than those experienced by reefs during geological history."  They thus note that this fact implies the "existence of efficient adaptative mechanisms in reef organisms and the potential for the persistence of reef ecosystems."

What it means
Although serious concerns have been raised about the future of earth's coral reefs in a global environment of increasing air temperatures and atmospheric CO2 concentrations, not enough is known about the many factors involved to make confident predictions about the future on the basis of our current understanding of the chemical and biochemical phenomena involved.  Nevertheless, our knowledge that reefs have persisted throughout geologic time and withstood environmental changes even more extreme than what is being predicted for coming decades (which predictions, of course, are highly debatable) suggests that they are not the helplessly fragile ecosystems they are often made out to be.

This conclusion should not to be construed to suggest that all is necessarily well with this sector of the biosphere; it merely suggests that we still have a lot of work to do before we can make a correct diagnosis of its health.  And unless a correct diagnosis is made, how can the proper treatment be prescribed?  Worse yet, a rush to judgment on this matter could lead us to treat a malady that existed only in the minds of men.  And that would indeed be a bitter pill to swallow, both for us and for the objects of our treatment.

Reviewed 1 August 1999