How does rising atmospheric CO2 affect marine organisms?

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Acclimatization of Coral Reefs
Gates, R.D. and Edmunds, P.J.  1999.  The physiological mechanisms of acclimatization in tropical reef corals.  American Zoologist 39: 30-43.

What was done
The authors reviewed the scientific literature pertaining to the potential for reef-building corals to acclimatize to various environmental challenges.

What was learned
Reef corals have been shown to exhibit various physiological responses to environmental stresses, including the induction of heat shock proteins to ameliorate temperature stress and the regulation of enzyme activity to ameliorate the deleterious effects of noxious oxygen radicals generated in coral tissues.  Rates of protein turnover were also noted by the authors to be related to a coral's ability to acclimatize.  Corals with high rates of protein turnover, for example, are generally more likely to acclimatize than are those with low rates of protein turnover.

In addition to the various acclimatization strategies of the coral host, acclimatization mechanisms in the coral symbiont were also reviewed.  The authors noted that there is growing evidence of physiological differences among the different types of symbionts, and that coral bleaching is an adaptive mechanism whereby less stress-tolerant symbionts are expelled and replaced by more tolerant ones.  Citing one study in particular, the authors noted that corals that bleach less readily have "high concentrations of symbionts, chlorophyll and soluble protein (per unit area), and contain dinoflagellates whose physiology is characterized by high photosynthetic efficiency and low saturation irradiance."  Conversely, they also noted that "corals that exhibit a greater propensity to bleach contain dinoflagellates with lower photosynthetic efficiency that require higher saturation irradiances."  Such symbiont differences may provide reef corals with the "opportunity to modify the physiological performance of an intact association by 'shuffling' symbiont genotypes" when facing changes in their environment.

What it means
The authors' review reveals a number of physiological mechanisms within the coral host, as well as the symbiont, that appear to aid coral acclimatization to environmental stress.  This evidence, in conjunction with historical evidence of reef stability, suggests that concern about coral reef demise due to CO2-induced global warming may be unwarranted.  What is more, as the air's CO2 concentration continues to rise, it may actually help to stimulate coral reef growth by increasing symbiont photosynthetic activity, which has been postulated to have the capacity to offset whatever decreases in coral calcification rates may result from water chemistry changes induced by higher atmospheric CO2 concentrations.

Reviewed 1 August 1999