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An experiment carried out at One Tree Island in Australia's Great Barrier Reef offers a stark warning of the growing risks that corals face as the oceans become more acidic with time. Ocean acidification due to the burning of fossil fuels could pose a severe risk to the integrity of these marine structures, according to a study published Wednesday in the journal Nature.

For the study, marine scientists pumped carbon dioxide-infused seawater across a patch of Australia's Great Barrier Reef to simulate how acidic the oceans will likely be in the next few decades and almost certainly before the end of the century.

The experiment allowed scientists, for the first time, to step outside of their laboratory and directly observe how an acidified ocean can eat away at coral skeletons. In this setting, the research team found that the heightened acidity suppressed coral reef growth by around a third, compared to average.

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"If what we’re seeing at the community scale is true, that's really concerning," said Rebecca Albright, the study's lead author and a coral reef researcher at the California Academy of Sciences, in an interview. "If we see a 30 percent decline over the next 30 to 40 years, that’s significant."

No long-term or irreversible harm was done to the vivid coral in this 400 meter-square area -- as the experiment was temporary, a small amount of acidified water was used, and normal seawater has since rinsed the reef clean.

Dyed seawater flowing over the reef on One Tree Island in The Great Barrier Reef.Credit: Aaron Takeo Ninokawa

The oceans are gradually growing more acidic because there's now significantly more carbon dioxide in Earth's atmosphere today than at any point in the last 800,000 years. This gas naturally dissolves into the ocean, and once there, carbon dioxide reacts with water to form carbonic acid, increasing the water's acidity (or lowering the pH).

Coral reefs have made big news in recent years, specifically because vast swathes of The Great Barrier Reef, along with many others worldwide, were bleached by historically high ocean temperatures in 2015 and 2016 -- forcing corals to expel the symbiotic algae that provide them with their vibrant colors and crucial nutrients.

Compared to coral bleaching, gradually acidifying oceans are a less visible, though insidious threat.

"It's a silent killer," said Kim Cobb, a paleoclimatologist at the Georgia Institute of Technology's School of Earth and Atmospheric Sciences, in an interview. Cobb was not involved in the study.

Acidification is already affecting coral reefs, and while this effect has been proven in labs, Cobb notes that it's critical to study in wild environments.

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"They did this very bold, difficult thing which is to try and alter the carbonate chemistry of a real reef ecosystem," Cobb said.

"That's extremely hard to do."

In the fall of 2016, Albright and her research team spent 30 days experimenting on a 400-meter square portion of flat, shallow reef. On 20 of these mornings, during low-tide, the team bubbled carbon-dioxide gas into a tank of seawater, lowering the pH to levels projected for some 30 years into the future. Vivid dye was added, and then the team pumped the plume of carbon dioxide-enriched seawater over the reef.

Researchers deploying their experiments on the reef.Credit: Aaron Takeo Ninokawa

On 10 of the days, dyed water without carbon dioxide enrichment was pumped over the reef. This served as a control, meaning scientists could compare their experimental conditions to normal conditions.

This increased acidity slowed the coral's ability to absorb an important mineral, aragonite, that makes up most of their skeletons. In previous laboratory settings, these higher acidity levels suppressed coral growth by about 15 percent. But it was measured at about 30 percent in the wild.

While an important finding, "I wouldn’t say that coral dissolution is now twice than what we though it was before," Kristen Davis, an assistant professor of Earth systems science at the University of California Irvine who was not involved in the study, told Mashable. "A lot of things are different in the natural environment than in the lab."

Also, notes Cobb, "No two reefs are the same." So this dramatic effect might not necessarily occur in other reefs with different species of coral, algae, fish, and other environmental conditions.

Even so, the results are troubling, since corals face threats from warming and acidifying oceans at the same time.

Healthy coral on One Tree Island.Credit: Aaron Takeo Ninokawa

"It's not good news -- coral are getting hit from a lot of angles," said Davis.

Acidification, for instance, eats away at the actual structure of the reef, allowing abundant boring creatures (like brightly colored coral worms) to have an easier time munching through the corals, Davis said.

Maintaining healthy coral reef ecosystems isn't just a priority for nature lovers and tourists, but an imperative for human survival. Reefs play host to a staggering array of diverse species of fish, and provide food for 500 million people, many of which live on low-lying Pacific Islands.

Although reefs comprise one percent of the oceans, they are home to 25 percent of all marine species.

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