Matt Ridley (author of Genome and the Red Queen) on his blog The Rational Optimist clarifies recent thinking about ocean acidification and its predicted impact on coral reefs. It is a strong defense of recent observations and experiments that indicate that the degree of ocean acidification expected through global warming does not pose a catastrophic threat to corals.
Some additional thoughts I had.
Many coral species in experimental conditions show enhanced calcification rates at higher pCO2. That may happen because dissolving more CO2 in water increases the bicarbonate content of sea-water among other dissolved inorganic carbon species providing a source for CO3 to bond with Ca to form the skeletal material.
This is the representation:
CO2 (aq) + H2O H2CO3 HCO3- + H+ CO32- + 2 H+
So as pH increases so does the bicarbonate (HCO3) content of sea-water.
Just a reminder that decreasing ocean pH and calling it acidification does not mean that the oceans will becoming technically an acid, that is having pH below 7. In fact they will becoming less alkaline from the current pH of 8.1 to about 7.9 or so projected several decades on. This is within the range of variation in pH of seawater inside the coral body cavity and at the interface of the tissue and calcification sites. In other words organisms have already built in mechanisms to handle fluctuations in pH within certain limits.
Off course if pH keeps dropping, at some point the dynamics will change and calcification will be affected. That is because at the site of calcification bicarbonate (HCO3) is stripped of its proton (H+) and the carbonate used to bond with Ca. The proton has to be transported away from this site of lower pH (higher H+ activity) into the coral body cavity and beyond which is at relatively higher pH (lower H+ activity). If sea-water pH is lowered so will eventually the pH in the coral body cavity as there is an easy exchange between the two. The gradient in pH between the body cavity and the calcification site will decrease and that will hinder the transport of H+ away from the calcification site. With H+ building up at that site it becomes harder to strip more protons from HCO3 to form CO3. What that pH range is and whether such pH conditions will ever be reached in the natural oceans due to global warming is not very well understood.
Still there are other dangers to corals from global warming and the most important seems to be coral bleaching caused by the expulsion of symbiotic algae. This may happen as the temperature of water rises causing some internal mechanism within the coral to expel algae or via invasion of the coral by other parasitic microbes that may expel the algae.
Coral metabolism is tightly coupled to the health of this photosynthetic algae and CO2 released through coral metabolism contributes according to some estimates about 70% of the carbon for building skeletons. The other 30% comes from the sea-water HCO3 I mentioned in the earlier para. So removing the algae and degrading the metabolism of the coral would mean cutting off a significant supply of carbon for skeletons.
Again the effect of all these parameters have been demonstrated in experiments that have run from a few tens of hours to a few weeks. For example how important this metabolic source of carbon is in the long run and whether different coral species may be able to harvest more and more carbon directly from sea-water bicarbonate in case the supply of metabolic carbon is shut off is an interesting question.
Likely there will be coral species who will have flexible mechanisms to harvesting carbon. They will flourish and others who have no physiological flexibility might die off.
Nature is resilient due to its variability. The community structure of coral reefs may change over the next few hundred years. But corals as a group will live on.
Meanwhile there are many local dangers to coral reefs in the form of overfishing and destructive fishing methods, turbidity due to increased coastal runoff, nutrient overdose leading to algal blooms that have choke off the corals. As Matt Ridley's article reminds us, these have the potential of wiping out many coral reefs faster than global warming might.