Here, two juvenile elkhorn corals only 26 days old have survived a sojourn in the open water but have successfully settled onto a surface to begin their life’s work of building a colony. Future endeavors may not be as successful. Credit: Rebecca Albright/RSMAS, Univ. of Miami

The oceans are changing. You can’t tell by standing on a beach and watching waves roll in, but experiments show that ocean water is becoming more acidic. This process is called “acidification,” and it may mean bad news for animals like the elkhorn coral, which is found throughout the Caribbean Sea.

Elkhorn coral used to be easy to find in shallow water, but now it’s an endangered species. In the last 30 years, many populations of elkhorn coral have collapsed, thanks to disease outbreaks, hurricanes and elevated temperatures. Scientists are working on ways to save the coral, but they have a long way to go. A new study suggests that coral may face yet another threat: In more acidic waters, elkhorn coral are less successful at reproducing sexually.

A substance may be an acid or a base. Acids taste tart and may be corrosive, like vinegar or even battery acid. Bases tend to be slippery. Water is neutral, which means it’s right between acids and bases. Right now, the oceans are slightly more basic than water. But oceans are beginning to move more to the acidic side.

As acidification worsens, these elkhorn coral may produce fewer offspring. This change could mean fewer or smaller coral reefs — which could be a problem for the many animals and plants that live on those reefs.

Acidification happens because oceans absorb a gas called carbon dioxide from the atmosphere, and carbon dioxide makes the water more acidic. Carbon dioxide, or CO₂, in the atmosphere comes from many sources, and human activities have added a significant amount. When we burn oil or gas to generate power (such as electricity or to fuel cars), we add CO₂ to the air.

Carbon dioxide is also a greenhouse gas, which means it traps heat in the air — which leads to warmer temperatures on Earth’s surface. This is called global warming, and CO₂ is one of many gases that drive global warming.

And since the beginning of the industrial revolution more than 200 years ago, the amount of CO₂ in the atmosphere has been increasing quickly. This change in the air has affected the oceans, forcing them to absorb even more carbon dioxide.

Scientists estimate that oceans have become about 30 percent more acidic in the last 200 years. Previous studies have shown that marine animals like corals, oysters and sea urchins have a hard time building their shells and skeletons in more acidic water.

The new study was led by Rebecca Albright, a graduate student at the University of Miami. She wanted to know how elkhorn coral would reproduce in acidic water. To find out, she and her team added bubbles of carbon dioxide to ocean water in a tank to make it more acidic — like it will be in the future. The scientists placed elkhorn coral in the water.

Coral usually grows in reefs, and coral reefs often look like giant, colorful rocks in shallow parts of the ocean. They provide a home to many kinds of plants and animals. But coral is neither a rock nor a plant — it’s an unusual animal. Like other animals, elkhorn coral reproduces sexually, which means a sperm cell and egg cell come together to create a new organism.

Albright and her team observed that for coral living in water with extra carbon dioxide, sperm and egg combined less often than they do in ordinary seawater. Then she observed another obstacle to coral reproduction: Even if a sperm and egg managed to join, they had a hard time getting settled on the reef to grow.

As oceans become more acidic, the elkhorn coral may face increasing problems producing offspring. And it’s just one species.

It’s possible that elkhorn coral could evolve and adapt to the changing climate. “One of the limits with this kind of study is that it doesn’t tell you whether there is any potential for evolutionary changes to deal with the new stress,” Steve Gaines told Science News. He is an ecologist at the University of California, Santa Barbara, and did not work on Albright’s study.

However, Gaines points out, the climate is changing unusually fast. The bottom line remains the same: More carbon dioxide means more acidification, which probably means bad news for the elkhorn coral.

POWER WORDS (adapted from the Yahoo! Kids Dictionary)

acid Any of a class of substances whose aqueous solutions are characterized by a sour taste, the ability to turn blue litmus red, and the ability to react with bases and certain metals to form salts.

evolution The process by which species adapt and change over time.

carbon dioxide A colorless, odorless, incombustible gas formed during respiration, combustion and organic decomposition and used in food refrigeration, carbonated beverages, inert atmospheres, fire extinguishers and aerosols.

coral Any of numerous, chiefly colonial marine polyps of the class Anthozoa.

Factories, cars, and other machines spit out lots of a gas called carbon dioxide. Carbon dioxide (CO₂) is known as a greenhouse gas because it traps heat in the atmosphere. More and more of the gas has been accumulating in the air in recent years.

CO₂ has also been dissolving in seawater, and that’s been changing the water’s chemical composition. As a result, seawater at the surface of the world’s oceans has become more acidic. That shift could eventually make life tougher for a type of snail called the common periwinkle, say researchers from the University of Plymouth in England.

Common periwinkles, like this one, are able to grow thicker shells when predators are near, unless the seawater becomes too acidic.

Simon Rundle

The common periwinkle lives along coastlines throughout much of Europe. One of its main predators is the common shore crab. The hungry crabs grab the snails “like ice cream cones,” says lead researcher Simon Rundle. Snails with thin shells are most likely to get crushed and eaten.

Scientists already knew that snails grow thicker shells to protect themselves when predators live nearby. The British researchers wanted to know if an increase in the acidity of the water would affect this thickening process.

The scientists grew more than 100 periwinkles in tanks. They put half of the snails in tanks filled with normal seawater. They added CO₂ bubbles to the water in the other snails’ tanks to make it acidic. The researchers then put a crab in some of the tanks with both types of water.

In the tanks with normal seawater, the periwinkle shells grew substantially thicker when a crab was living at the bottom. In the tanks with acidic water, the snail shells did not get thicker. These results suggest that snails living in acidic water have a harder time defending themselves from predators.

Scientists measure acidity on what’s called the pH scale. A liquid with a pH of 7, such as distilled water, is considered neutral. A pH measurement of less than 7 indicates acidity. Lemon juice and stomach acid are examples of acidic substances. A pH of greater than 7 is the opposite of acidic, often called basic or alkaline. Bleach is one example.

Overall, the oceans are slightly alkaline, with a pH of 8.2. But studies show that the pH of ocean water has dropped by about 0.1 unit in the past few hundred years. And computer models suggest that ocean pH could drop another 0.3 to 0.4 unit by 2100.

That change could be a problem for all sorts of underwater organisms. As seawater becomes more acidic, these creatures have an increasingly difficult time producing a mineral called calcium carbonate. This material makes up coral reefs, sea urchin teeth, and snail shells, among other structures.

Until now, studies of seawater acidity have mostly looked at its effects on individual species. The new study shows that changes in the oceans are influencing interactions between species, too.—Emily Sohn

Going Deeper:

Milius, Susan. 2007. Bad acid: Ocean’s pH drop threatens snail defense. Science News 172(Oct. 20):245-246. Available at http://www.sciencenews.org/articles/20071020/fob7.asp .