Introduction to Ocean
Acidification

Introduction to Acidification's Effects on Coral

Key Physiological Characteristics of Coral

Chemistry of Coral Calcification

How is Coral Calcification Affected by Acidification?

Acidification's Effects on Other Calcifying Organisms

How are calcification mechanisms affected by acidification?

A 2009 study by Silverman et al. found that by the time atmospheric levels of CO2 reach 560 ppm (current levels are approximately 390 ppm), all coral reef systems in the world will begin to dissolve.  Another study found that when CO2 reaches 450 ppm, only 8% of the world’s reefs will be able to calcify (Cao and Caldeira, 2008).

How, and why?

As stated previously, coral utilize calcium carbonate to build their skeletons.  Calcium carbonate is secreted in the mineral forms of aragonite or calcite, with coral utilizing the more soluble aragonite (Fabry et al., 2008, Erez et al., 2011).  Just as with calcium carbonate, as atmospheric CO2 levels increase, the saturation of aragonite in the ocean decreases (Fabry et al., 2008).  Aragonite saturation is also dependent upon levels of dissolved inorganic carbon and temperature.  Dissolved inorganic carbon is projected to decrease with rising CO2.  Temperatures are also increasing with rising CO2.  The consequences of these changes lead to further de-saturation of the oceans aragonite supplies (Fabry et al., 2008).  A representation of the relationship between CO2, dissolved inorganic carbon and aragonite saturation is seen below in figure 4.

Figure 4. Adapted from Fabry et al. 2008. Relationship between atmospheric CO2 and factors affecting coral calcification, including carbonic acid, calcium carbonate, dissolved inorganic carbon (DIC), seawater pH, saturation of calcite and satuation of aragonite.

To determine how decreased aragonite will affect reefs, Silverman et al. calculated coral calcification rates as a function of local aragonite saturation in 9,733 reef locations around the world.  They then factored in effects of bleaching (caused by temperature increase, and thus also caused by increased CO2) on coral calcification, and displayed the overall effects on calcification as a function of CO2.  Their data is shown below in figure 5.

Figure 5. Reduction in gross community calcification of coral reefs in percentage relative to preindustrial rates (PIR, 280 ppm) for different atmospheric pCO2 scenarios.
Reprinted from Erez et al. 2011 with permission from J. Erez. Originally printed in Silverman et al., 2009.

A 2011 study by Venn et al found that corals elevate the pH under their calcifying tissue relative to seawater.  This finding has raised concerns that as seawater pH decreases, coral will no longer be able to increase the pH under their calcifying tissue to the extent needed (Venn et al., 2011).

Though our understanding of coral calcification is still limited, and the direct impacts of decreased seawater pH (as opposed to decreased aragonite saturation, which is an indirect impact of decreased seawater pH) on coral calcification are unknown, there is significant evidence that calcification rates will drop dramatically as aragonite saturation decreases (Erez et al., 2011).  It is likely that we will see unprecedented and catastrophic decrease in coral calcification within the next 50 years (Erez et al., 2011, Silverman et al., 2009, Anthony et al., 2011).  This will have devastating effects on reef ecologies and human populations around the world (Erez et al, 2011, Cooley et al., 2009).