Continued acidification of the global ocean as a result of rising concentrations of atmospheric carbon dioxide reduces the formation of calcium carbonate in the oceans and could therefore impair the growth of carbonate coral skeletons. Corals that occur in naturally acidic waters near the Yucatan Peninsula show decreased levels of calcification, according to a morphological analysis of corals in the region.
Elizabeth Crook of the University of California, Santa Cruz, and colleagues used CT scanning to study the skeletal density and structure of Porites astreoides, a common species of Atlantic coral. The corals were collected from a region off the Caribbean coast in Mexico characterized by a natural gradient in pH. The annual rate of calcification — derived from measurements of coral density and structure — was up to 66% lower in areas with the lowest seawater calcium carbonate concentrations, relative to the nearby corals growing in more favourable conditions. The reduction in calcification matched that observed in laboratory experiments with the same coral species. The findings suggest that the studied corals are unable to acclimate to low calcium carbonate concentrations, despite lifelong exposure to such conditions.
The researchers also detected an increase in the degree of coral erosion and predation in the low-calcium-carbonate waters, which they suggest could further impair the reef ecosystem.