New research from the University of Copenhagen has identified a previously unknown mechanism linking ocean warming to coral death — microscopic hair-like structures that help corals “breathe” break down under heat stress, cutting off their oxygen supply.
Coral reefs, which support more biodiversity than virtually any other ocean ecosystem, are in deeper trouble than scientists previously understood. A new study published in Science Advances has uncovered a hidden biological mechanism through which rising ocean temperatures can starve corals of oxygen — and potentially kill them before the more familiar warning sign of bleaching even appears.
Researchers at the University of Copenhagen, in collaboration with scientists from institutions across Europe, Australia and Saudi Arabia, used a combination of laboratory experiments and mathematical modeling to examine how tiny, hair-like cellular structures called cilia help corals absorb oxygen from surrounding seawater — and how heat disrupts that process.
How Corals Actually Breathe
Corals lack a dedicated respiratory system, and were long assumed to absorb oxygen passively from seawater. The new research challenges that assumption. The surface of a coral is blanketed by thousands of microscopic cilia, each about 10 to 15 micrometers long and roughly 500 times thinner than a human hair. When these cilia beat in a synchronized pattern, they generate small but critical water movements just above the coral surface. At night — when corals cannot rely on their photosynthesizing symbiotic algae for oxygen — this ciliary-driven circulation is their primary means of obtaining the oxygen they need to survive.
Under moderately elevated water temperatures, corals were found to ramp up their ciliary activity to compensate for higher metabolic oxygen demand — essentially breathing harder as conditions worsen.
“In this temperature range, corals can compensate for higher oxygen demand by effectively increasing their ‘breathing’. However, this compensatory mechanism does not persist at higher temperatures,” first author Cesar Pacherres, an assistant professor in the Department of Biology at the University of Copenhagen, said in a news release.
A Breaking Point at 37 Degrees
Beyond a certain thermal threshold, however, the system collapses. In laboratory experiments, ciliary motion began to falter as temperatures climbed, with the cilia slowing down, losing their coordinated rhythm and eventually ceasing to move altogether. In the experiments, this breakdown occurred at approximately 37 degrees Celsius. With cilia no longer functioning, the thin layer of water directly above the coral surface became progressively depleted of oxygen even as the coral’s tissue continued consuming more of it. The result was acute oxygen stress, tissue breakdown and, ultimately, death.
The researchers are careful to note that the 37-degree threshold is not a universal number. Depending on local ocean conditions, the evolutionary history of a particular reef and the specific coral species involved, this critical point could be reached at lower temperatures — making some reefs more vulnerable than the experiments alone might suggest.
A New Window Into Coral Bleaching
The findings also shed new light on coral bleaching, the widely documented phenomenon in which corals expel their symbiotic algae under thermal stress, losing both their color and a primary energy source. The study suggests that oxygen stress and bleaching are not separate events but deeply intertwined processes.
“As temperatures rise, the coral’s metabolism and oxygen demand increase. If the cilia’s ability to transport oxygen is impaired at the same time, the coral experiences oxygen stress precisely when it is under the greatest physiological pressure,” added senior author Michael Kühl, a professor in the Department of Biology at the University of Copenhagen.
In some cases, oxygen-driven tissue damage may occur and even cause death before bleaching becomes externally visible — meaning scientists and conservationists relying on bleaching as an early-warning signal could be missing coral distress that is already underway.
Why It Matters
“Marine heatwaves are becoming more frequent and intense as a result of global warming, affecting coral reefs worldwide. At the same time, oxygen levels in the oceans are declining. Both changes are critical for marine life, and our study identifies a mechanism that directly links ocean warming and oxygen loss, which in the worst case can lead to rapid coral death,” Kühl added.
Coral reefs cover less than 1% of the ocean floor but support an estimated 25% of all marine species. They also provide coastal protection and food security for hundreds of millions of people globally — making their decline an urgent concern far beyond the marine biology community.
The team’s mathematical model adds a predictive dimension to the findings. By simulating how different environmental conditions and coral metabolic rates interact, the model can identify which scenarios push corals toward dangerous oxygen deficits fastest — a tool that could help prioritize conservation and reef restoration efforts.
The implications extend beyond coral reefs as well. Sponges, sea anemones, sea squirts and many other marine organisms also use cilia to manage water flow and oxygen uptake. The mechanism identified in this study could be relevant across a broad range of species already under pressure from warming and deoxygenating seas.
As Pacherres noted, local interventions informed by this research — such as targeted reef restoration — can help, but the scale of the threat demands a broader response. Preventing widespread coral loss, the researchers emphasize, ultimately requires significant reductions in global greenhouse gas emissions.
The study was supported by the Gordon and Betty Moore Foundation’s Aquatic Symbiosis Initiative, with additional funding from the Novo Nordisk Foundation, the Leibniz Association, the Australian Research Council and an EU MSCA postdoctoral fellowship. Research was led by the University of Copenhagen in collaboration with the Leibniz Institute for Baltic Sea Research, the University of Melbourne and King Abdullah University of Science and Technology.
Source: University of Copenhagen