How ocean acidification weakens coral skeletons

"Our next step is to expand our model to incorporate the effects of multiple stressors on corals' skeletal growth," said lead researcher Weifu Guo.
By Brooks Hays  |  Jan. 29, 2018 at 4:49 PM
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Jan. 29 (UPI) -- The ongoing acidification of Earth's oceans is a threat to a variety of marine organisms, including coral, the backbone of many diverse ecosystems.

According to a new study published this week in the journal PNAS, acidification is curbing coral's ability to reinforce the strength of its skeleton through thickening.

"Our research incorporates the nuances of coral skeletal growth, enabling more precise projections of how, where, and by how much, ocean acidification will affect tropical reef-building corals," Nathaniel Mollica, an oceanography at the Woods Hole Institute in Massachusetts, said in a news release.

Lab experiments attempting to measure the effects of acidification on corals have offered varied results, but the latest research suggests such confusion is a product of the complex nature of coral skeleton construction.

Coral polyps, the soft bodied organisms living inside the coral skeleton, pump in bicarbonate, carbonate and calcium ions from the surrounding seawater, while pumping out hydrogen ions. The exchange facilitates the creation of aragonite, a form of calcium carbonate used to construct the coral skeleton.

The upward trajectory of coral, as it grows toward the sunlight, is made possible by the vertical stacking of aragonite crystals. The addition of aragonite crystals to the sides help thicken the skeleton. But as the ocean becomes more acidic, there are fewer bicarbonate and carbonate ions.

To determine how a lack of bicarbonate and carbonate ions affects coral skeleton growth, researchers collected coral samples from a variety of reef sites, each with different pH levels.

Researchers used CT scans to measure rates of coral growth in the different samples. The analysis revealed no link between pH levels and vertical growth but did show a correlation between acidity and diminished thickening.

"This very important study determined the specific way a coral species is affected by ocean acidification and modeled the effect of future environmental conditions," said David Garrison, director of the National Science Foundation's Biological Oceanography program, which helped fund the research.

An analysis of the data involved in previous coral studies showed a similar relationship between acidity and reduced thickening.

Researchers did acknowledge that warming ocean temperatures, pollution and other factors can also impact coral growth rates.

"Our next step is to expand our model to incorporate the effects of multiple stressors on corals' skeletal growth," said lead researcher Weifu Guo, a geochemist at WHOI. "Knowing the details of how the different facets of ocean change will affect corals will enable us to quantitatively project the trajectory of reef-building corals under 21st century climate change."

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