In coral skeletons, microscopic portraits of resilience?

Photo credit: V. Manichev and S. Von Euw/Rutgers

Coral reefs are sprawling, intricate ecosystems that house an estimated 25 percent of all marine life and can sometimes be seen from space. Yet they are formed by a process invisible to us.

A study published in Science on Wednesday now presents a microscopic picture of the biology that makes corals’ skeletons grow. The findings suggest that coral may be more robust in the face of human-driven ocean acidification than commonly thought.

Corals grow their armor by diligently secreting a chunk of hard skeleton smaller than the width of a human hair each day. This process is called calcification and scientists have debated which parts of it are most important for decades.

One view prioritizes chemical interactions with the seawater. Using ion pumps, corals can possibly decrease the acidity of seawater enough that calcium carbonate — the stuff of limestone and chalk and the basis of coral skeletons — forms spontaneously. Under these circumstances, if oceans become more acidic — a potential consequence of human-emitted carbon dioxide in the atmosphere being absorbed by the seas — coral may struggle to form a skeleton.

The alternative view contends that calcification is primarily a biological process, coordinated by proteins similar to the ones that help us make our teeth and bones. The new study provides evidence for this perspective and some hope for corals in a world with more carbon.

“Coral is not just a rock,” said Paul Falkowski, a professor of marine sciences at Rutgers University and senior author of the study. “And because of that, we’re pretty confident that they’ll be able to continuing making their skeletons even if the ocean becomes slightly more acidic.”

Not all scientists agree.

“The problem is, we have lots of data that show many coral species are very sensitive to environmental change,” said Alexander Venn, a senior scientist at the Scientific Center of Monaco, who was not involved in the study. “While this paper builds a strong model for the biological control of calcification, there are still pieces of the puzzle missing.”

Dr. Falkowski and his colleagues used ultrahigh-resolution microscopic imaging and techniques for observing the structure of molecules to study skeletal branches from smooth cauliflower coral, a well-studied species common in the Indo-Pacific.

The result is a model of coral calcification that starts with a malleable form of calcium carbonate, called amorphous calcium carbonate.

The researchers say they believe that amorphous calcium carbonate is initially formed by proteins. Through a process not yet fully understood, little balls of the material then give way to aragonite, the form of calcium carbonate that makes up a mature coral skeleton.

Similar transitions have been observed in sea urchins and shellfish, and some scientists even suspect amorphous calcium carbonate may be a common precursor for calcification across the tree of life.

“When we precipitate aragonite in the lab, just in a bucket of seawater, it forms this very characteristic pattern with very long, needle-shaped crystals,” said Nicola Allison, a lecturer in earth sciences at the University of St. Andrews, who did not participate in the research.

“This is the first report of amorphous calcium carbonate in coral, and it really does suggest the organism is able to control how solid material is deposited,” she added.

Alex Gagnon, an assistant professor of oceanography at the University of Washington who was not involved in the research, suggested it was an oversimplification to take seawater chemistry out of the equation. Acid dissolves calcium carbonate, so the more acidic the ocean is, the more difficult it is for corals to organize that first bit of skeleton.

“At the end of the day, the fundamental rules of chemistry and physics still apply,” he said.

It’s true that corals lose calcium carbonate in a more acidic environment — but they maintain the ability to grow back that skeleton, “which is good news,” Dr. Falkowski said.

Given current projections of ocean warming and acidification, he is more concerned about warming, which stresses the algae living inside corals and causes coral bleaching.

That said, Dr. Falkowski acknowledges that the cause of warming and acidification is one and the same: carbon emissions from fossil fuel burning. “For all intents and purposes, they’re linked,” he said.

Steph Yin, The New York Times, 1 June 2017. Article.

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