Voyager: At what level of hydrogen ion concentration in the ocean will the coral reefs and other calcifiers begin to show signs of dissolution?

The hydrogen ion concentration is a measure of the acidity of seawater and is commonly expressed on the pH scale; the lower the pH, the higher the hydrogen ion concentration and the acidity of seawater. Increasing acidity results in a decrease in the concentration of the carbonate ion, which in chemical terms is known as CO₃^2-. It is one of the building blocks for marine organisms that make shells or skeletons out of calcium carbonate (CaCO₃). Less CO₃^2- means there is less building material to produce calcium carbonate, but also that the organisms face an increased risk of dissolving.

Dissolution of calcium carbonate is actually already taking place in many environments as a result of microbes eating away at organic material and releasing acids that dissolve CaCO₃ just like bacteria can cause cavities in teeth. Dissolution is also facilitated by bioeroders such as sea urchins and parrotfish that actively rasp or chew on CaCO₃ while feeding on algae that grow on its surface. This creates the fine sand found on many tropical beaches. However, the rate at which corals, marine snails, and other marine calcifiers build calcium carbonate today is typically faster than the rate at which it dissolves.

One of the concerns with ocean acidification and increasing hydrogen ion concentration is that dissolution of coral reefs and also individual living organisms eventually may exceed the rate of calcification. When this will take place and how acidic the seawater needs to be varies due to a range of factors. Those factors include environmental and chemical properties of different environments, such as temperature and whether they are near the poles or closer to the equator (cold environments are in general closer to a condition where dissolution will exceed calcification). Other factors include the mineral composition and micro-architecture of the calcium carbonate structure; and the organism’s ability to control the calcification process. If an organism has sufficient energy in terms of food and nutrition it may be able to compensate for changes in seawater chemistry, but potentially at a cost of other life-essential processes.

So there is no simple answer to this question, but predictions for when most coral reefs will lose more CaCO₃ than they are able to produce currently range anywhere from the middle to the end of this century, which corresponds to an average open-ocean surface seawater pH of approximately 7.95 to 7.75

-- Andreas Andersson, assistant professor, Geosciences Research Division