Brain corals are quietly storing massive amounts of carbon

A sprawling colony of brain coral (Mussismilia hispida) rings the main island of the Alcatrazes Archipelago Wildlife Refuge off the northern coast of São Paulo. A new study reveals that this coral colony stores more than 20 tons of carbon every year.

Marine biologists from the Federal University of São Paulo (UNIFESP) conducted five years of painstaking work that combined underwater surveys, seabed sonar, and hospital-grade CT scans of coral skeletons.

These numbers are thought-provoking: the carbon held in these corals is equivalent to the emissions produced by burning roughly 85,600 gallons of gasoline.

Scanning coral for climate clues

“We wanted to understand the growth of brain coral, but it’s very difficult to cut the skeleton in a way that allows this measurement to be made,” said Luiz de Souza Oliveira, the first author of the study. “So we did CT scans, in which it’s possible to observe the so-called annual growth bands.”

By analyzing dozens of three-dimensional images captured between 2018 and 2023, Oliveira and his colleagues could count yearly bands – much like tree rings – and calculate how much calcium carbonate (CaCO₃) the colonies add each season.

Scaled up to the whole island, that translates into about 170 tons of new CaCO₃ a year. Because carbonate contains carbon, the team estimated how much is locked in minerals before it can become atmospheric CO₂.

Crucially, the researchers also needed to know how much of the island’s seafloor the coral actually covers. That task fell to co-author Mónica Andrade da Silva, who used side-scan sonar to map the benthic landscape and calculate the total footprint of Mussismilia around Alcatrazes.

Unexpected growth in cool waters

“The growth rate of the corals was higher than we expected, similar to that of corals in tropical regions,” said Guilherme Henrique Pereira Filho, who supervised the work and runs the Marine Ecology and Conservation Laboratory at UNIFESP.

Scientists often label subtropical reefs in the South Atlantic “marginal” because cooler water, lower light, and frequent storms push corals close to their physiological limits.

Yet the Alcatrazes colonies are piling on skeletal mass as fast as their tropical counterparts, which raises an intriguing puzzle: why do they not build towering reef frameworks like those in Abrolhos or Fernando de Noronha?

One idea is simply that they have not had time. Geological records suggest that Mussismilia may have spread into these latitudes only two or three millennia ago.

Another possibility is mechanical. The same storms that occasionally topple entire colonies might prevent long-term accretion of large reef mounds.

Corals as carbon storage

The Alcatrazes Refuge protects more than 40 at-risk species and helps reduce fishing pressure on overexploited stocks. The new study argues that it should also be valued as a blue carbon sink.

In tropical reef belts, intense biological respiration can make coral systems net emitters of CO₂. In subtropical waters, however, photosynthetic macroalgae cloak much of the rock while the corals themselves are smaller and more scattered. This balance may tip the whole ecosystem into negative-emission territory.

Moreover, corals sequester carbon in the most durable way imaginable: as mineral. Once locked into CaCO₃, carbon can stay out of circulation for centuries. That’s far longer than the organic carbon held by seaweeds or mangroves, which cycles back into the air as plant matter decomposes.

“Society tends to value an area like Alcatrazes mostly for what it’s protecting from fishing. However, this environment may be providing another essential service in a context where tons of carbon are emitted every day through the burning of fossil fuels,” Pereira Filho said.

Next steps in coral science

The team found nearby sediments rich in carbonate from broken coral skeletons and mollusk shells. Those grains settle onto the seabed and can persist there for millennia, further enhancing the refuge’s carbon vault.

With new funding from FAPESP, Pereira Filho’s group plans to investigate other carbonate-forming organisms along the São Paulo coast. They hope to assemble a comprehensive ledger of how much carbon Brazil’s subtropical shelf quietly stores each year.

The work, the researchers argue, could inform both conservation priorities and climate mitigation strategies – showing that, in the global effort to balance the carbon books, even a single species of brain coral can make a difference measured in tons.

The study is published in the journal Marine Environmental Research.

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