Ocean acidification is only a tiny piece in the puzzle that is the ocean carbonate system. For simplicity ocean acidification is often said to dissolve calcium carbonate structures, such as shells, coral skeletons, coralline algal cells, urchin tests, plankton armor and even fish ear bones. It is more correct to say that protons released when carbon dioxide dissolves in water bind to carbonate ions that are needed to make calcium carbonate and therefore make it harder to build these structures.

To break this down let’s have a better look at the ocean carbonate system. The first fact to note that it is a buffer system, akin to that in our blood, meaning that it can ameliorate fluctuations in carbon dioxide. Ions that constitute seawater’s buffer system are referred to as its total alkalinity, the main players being bicarbonate – a part of baking soda – carbonate and hydroxide. They can each scavenge one or more protons depending on their charge, which creates the buffering effect. Adding one of these ions therefore produces the opposite effect of ocean acidification.

The second fact to note is that there are three rather than one inorganic carbon species in the ocean. In the atmosphere there is just carbon dioxide, but in the ocean there is carbonic acid, bicarbonate, and carbonate, the last two of which you already know. The three carbon species are in equilibrium, with bicarbonate contributing 90%. When more carbon dioxide is dissolved, the equilibrium shifts towards carbonic acid, when it is removed, the equilibrium shifts towards carbonate.

Interestingly, life meddles with the carbonate system in various ways. Respiration and photosynthesis add and remove carbon dioxide respectively, thus shifting the equilibrium towards or away from carbonic acid. But calcification, the process of biological calcium carbonate precipitation, interferes more drastically. It locks away carbonate, lowering total alkalinity and thereby the buffering capacity. This means that calcification effectively reduces the amount of carbon dioxide the ocean can hold, leading to outgassing. Calcifying organisms therefore play an active role in the global carbon cycle and are not just the victims of ocean acidification.

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