Acidic oceans are forcing corals to build weaker skeletons

When scientists raised baby reef-building corals in acidic seawater pushed toward about pH 7.6, the youngsters still built skeletons.

The tiny skeletons were denser yet less stable, so the corals were more likely to snap when waves or animals pushed on them.

Across tropical oceans, reefs depend on countless such skeletons growing, thickening, and locking together.

As ocean acidification deepens, scientists are racing to understand whether coral skeletons can keep pace.

Inside a coral’s first skeleton

The work was led by Dr. Federica Scucchia, a postdoctoral associate at the University of Rhode Island (URI). Her research focuses on biomineralization, the way living organisms build hard mineral structures, in young reef building corals.

Each baby coral in this study was a primary polyp, a tiny animal that settles on rock and starts secreting mineral. It builds a cup shaped skeleton of calcium carbonate that will later support an entire colony.

That cup contains two main growth zones that coral scientists watch closely. The first are rapid accretion deposits, small centers where new coral skeleton first appears.

By contrast, thickening deposits are slender crystals that fill out and strengthen coral skeleton walls. Together, these zones form a stiff lattice that has to hold up under currents, storms, and hungry fish.

Coral skeletons in four dimensions

The team combined three-dimensional X-ray scanning with short growth intervals. They also used electron microscopes to see features smaller than a micrometer and to trace tiny crystals.

These tools let them map mineral density, crystal size, and growth zone shapes in Stylophora pistillata, a common Red Sea stony coral.

Under normal pH, the thickening deposits made up most of the skeleton and wrapped around a web of rapid accretion deposits.

Inside those fibers, much of the mineral turned out to be amorphous calcium carbonate, a disordered mineral form that later transforms into crystals. Only a smaller share had already organized into dense calcium carbonate crystals that pack tightly together.

In more acidic water, the pattern shifted in several important ways. Both growth zones became denser overall, and the crystals inside them grew larger, even though the total skeleton volume shrank.

Coral skeletons that are more fragile

To test how that new geometry might respond to stress, the team calculated the moment of inertia. This quantity measures how the shape of a structure resists bending forces.

The researchers focused on the thickening deposits, the zone that makes up most of the skeleton cross section. In the acid treated corals, that measure dropped near the base of the skeleton, where attachment to the seafloor is critical.

The reason came back to the rapid accretion deposits. Under acidic conditions these centers stayed smaller and appeared later along the growing skeleton, so they added less reinforcement where it mattered most.

That combination left the youngest skeletons especially vulnerable to breaking near their base. For a tiny polyp on the reef, losing that anchor point can mean losing its chance to become part of a mature colony.

How corals fight back

Corals are not passive in this story. They can adjust the chemistry of the calcifying fluid, the internal solution where corals control skeleton chemistry, to partly offset more acidic seawater.

Experiments using boron-based chemical tracers show that many species raise the pH in this internal space above the surrounding water. One study found that this internal pH buffering weakens under strong acidification, and calcification rates fall as a result.

Other work on long-lived reef builders indicates that corals in different regions tune this internal chemistry in distinct ways.

A global survey showed that maintaining high internal pH helps some corals keep aragonite saturation near optimal levels even as surface waters vary.

The new four-dimensional imaging adds a missing structural angle to that picture. It shows how shifts in mineral phases and crystal sizes accompany those chemical adjustments inside the calcifying space.

Crystals grow bigger under stress

In the acidified tanks, aragonite crystals in the thickening deposits were one third thicker than in normal water. That change ties into basic crystal growth physics.

Under lower supersaturation, a condition where water holds more dissolved minerals than usual, fewer new crystals start. This first step is called nucleation, the point when tiny mineral clusters begin forming.

Existing crystals then grab more ions and grow larger, which increases density but reduces the boundaries that can stop cracks.

Monte Carlo modeling of electron signals supported the idea that acid-treated skeletons contained a higher fraction of dense crystals and less amorphous material.

Those differences in phase mix and growth zone geometry help explain why the skeletons look denser but resist bending less well.

What this means for future reefs

Zoomed out to the reef scale, these tiny changes take on wider importance. Long-term coral core records show that acidification has already reduced growth in key genera on the Great Barrier Reef and other hotspots.

One broad study reported that mid-century carbon dioxide levels could cut global reef calcification rates by several tens of percent.

Broader projections suggest that if emissions stay high, many reefs could shift from slowly building up to slowly dissolving over the coming decades.

At the same time, the new study hints at a kind of structural flexibility. By adjusting how much of the skeleton forms as amorphous rather than crystalline calcium carbonate, young corals may be buying time as strength drops.

A narrow path to resilience

“The survival of the corals depends on the formation of a robust skeleton during these early stages of life, persisting into adulthood,” said Professor Tali Mass, a marine biologist at the University of Haifa (HU).

Still, the window for that strategy looks narrow. Taken together, the findings show that coral skeletons are not simply thinning in acidic water.

Their internal architecture is being rebuilt in ways that may help some polyps hang on but leave whole reefs more exposed to breakage.

The study is published in the journal Advanced Science.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates.

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–