flickr/ Dmitry Lyakhov

As the oceans soak up carbon dioxide from the atmosphere they are becoming more acidic. This process, known as ocean acidification, is harmful to marine life in many ways. For one, it weakens the shells and exoskeletons that animals rely on for protection from predators and extreme conditions. This dissolving of shells, which are made of calcium carbonate and rely on a specific oceanic pH, is already starting to happen in parts of the world.

A new paper in Scientific Reports shows that nature might be prepared to fight back against these human-driven changes. Researchers found that mussels grown under projected ocean acidification levels produce "more amorphous calcium carbonate"—i.e. they change their shell composition—as a mechanism to repair the damage created by changes to their environment. Amorphous calcium carbonate is the least stable formation of calcium carbonate.

Dr. Susan Fitzer, from the University of Glasgow and lead author on the paper, said that many marine organisms "need calcium carbonate to produce their shells and exoskeletons from calcium carbonate," in a statement. "But higher acidity reduces the concentration of carbonate ions available for shell formation and subsequently their shells are becoming more brittle which makes them more vulnerable."

Animals use a specific form of calcium carbonate, called aragonite, to build and maintain their shells. When concentrations of these minerals drop, shells can begin to dissolve, especially at early life stages. Corals, oysters, mussels, and small creatures  low on the food chain such as pteropods are all especially vulnerable to these impacts.

First evidence of marine snails from the natural environment along the U.S. West Coast with signs that shells are dissolving.
Steve Ringman/NPAA

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As carbon dioxide concentrations continue to increase, both in the atmosphere and the oceans, the waters inhabited by these shellfish will continue to acidify. According to the paper, global industrialization has already caused surface pH levels to decline from pH 8.1 to pH 8.0, meaning the water is less alkaline. By 2100, the pH level could drop to around 7.7.

Even if mussels do adapt to a more acidic environment, its unclear if their shells will be strong enough to protect them from predators or stormy environments, according to the researchers. This is because when the shells altered their material properties to adapt to projected future ocean conditions, their shells became harder and less elastic, making them more prone to fracture.

One recent study done in the waters around Alaska determined that these areas could reach life-threatening levels of acidity by 2030, thus imperiling the ability of animals to build their shells.

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"Our research shows that within 15 years, the chemistry of these waters may no longer be saturated with enough calcium carbonate for a number of animals from tiny sea snails to Alaska King crabs to construct and maintain their shells at certain times of the year," Jeremy Mathis, an oceanographer at NOAA's Pacific Marine Environmental Laboratory and the study's lead author, said in statement.

Of the 23 U.S. coastal regions identified by the Natural Resources Defense Council, 16 of them will face ocean acidification levels unfavorable for shellfish. Overlaying social factors, levels of agricultural runoff, local pollution and upwelling, a natural ocean process that brings more corrosive deep ocean water to the surface, helps tease out regional differences in vulnerability.

While the process of oceans absorbing carbon dioxide may be harmful to certain underwater creatures, those of us above ground should be thankful for the cycle. To date, the oceans have absorbed nearly a third of the carbon dioxide released into the atmosphere from human sources. When the oceans become fully saturated with carbon dioxide, then we'll all be in even more trouble.