Asking how to save coral reefs leads to better understanding carbon sequestration

Calcite, a solid form of carbon dioxide locked up in calcium crystals; image by scanning electron microscope
Promo image Adam Subhas/Caltech

Some of the best scientific discoveries were made by accident. Jess Adkins of Caltech reflects on what that feels like:

"This is one of those rare moments in the arc of one's career where you just go, 'I just discovered something no one ever knew.'"

Scientists have long known that carbon dioxide is naturally absorbed in the ocean's waters. In fact, the oceans hold approximately 50 times as much carbon dioxide as is in the atmosphere.

As with most things in nature, the cycle of carbon dioxide requires a delicate balance. Carbon dioxide is absorbed into (or released from) the oceans as part of a natural buffer system. Once dissolved in seawater, the carbon dioxide acts like an acid (which is why coral reefs are threatened).

After time, that acidic surface water circulates to deeper parts of the ocean, where calcium carbonate collects on the sea floor from the many plankton and other shelled organisms that have sunk to their watery grave. Here the calcium carbonate neutralizes the acid, forming bicarbonate ions. But this process can take tens of thousands of year.

So scientists were asking themselves: how long does it take for the calcium carbonate of a coral reef to dissolve into the acidic seawater? It turns out that the tools for measuring this were relatively primitive and as a consequence, the answers were unsatisfying.

The team decided to use a new method. They created calcium carbonate made entirely out of "tagged" atoms of carbon by using only a rare form of carbon known as C-13 (normal carbon has 6 protons + 6 neutrons = 12 atomic particles; but C-13 has an extra neutron for a total of 13 particles in its nucleus).

They could dissolve this calcium carbonate and carefully measure how much C-13 levels increased in the water as the dissolution proceeded. The technique performed 200 times better than the older method of measuring pH (a way of measuring hydrogen ions as the acid balance of water changes).

The added sensitivity of the method also helped them to detect the slow part of the process...something chemists like to call the "limiting step." It turns out that the slow step already has a very good solution. Because our bodies have to maintain our acid balance even more carefully than the oceans need to manage it, there is an enzyme called carbonic anhydrase that speeds up this slow reaction so that our body can respond quickly to keep the pH in our blood just right. When the team added the enzyme carbonic anhydrase the reaction sped up, confirming their suspicions.

While this is still in the early stages of scientific discoveries, it is easy to imagine that this knowledge could help to solve problems with the slowness and inefficiencies that make carbon capture and sequestration such a challenging technical solution to the use of fossil fuels in a world with rising carbon dioxide levels changing our environment.

Lead author Adam Subhas points out the potential: "While the new paper is about a basic chemical mechanism, the implication is that we might better mimic the natural process that stores carbon dioxide in the ocean."

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