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Rising ocean temperatures due to climate change may be disrupting the vital ventilation system among corals, leading to their total collapse.
Corals are often considered as static organisms, but their surfaces have hair-like cilia that work as a ventilation system. The structures beating in rhythm help corals “breathe” by moving oxygen-rich water across their surface.
This fundamental functioning is now under threat as new experiments reveal that heat stress disrupts ventilation system. The research, published in the journal Science Advances, found that warming seas initially make these cilia work faster.
But beyond a critical temperature they suddenly lose coordination, causing oxygen levels around coral tissues to collapse.
The authors studied the reef-building coral Porites lutea and its response to acute warming in darkness. The researchers exposed corals to gradually increasing temperatures from 27°C to 41°C in dark conditions to understand how the cilia responded.
“Moderate warming (~35°C) enhanced ciliary activity and advective transport yet paradoxically thickened the concentration boundary layer with O2- depleted water, exposing tissues to transient hypoxia (reduced supply of oxygen). At higher temperatures, ventilation failed to meet rising metabolic demands and anoxic regions expanded rapidly,” the authors noted.
They said that at moderate warming levels, the cilia became more active. Their beating frequency increased steadily from about 21 beats per second at 27°C to more than 30 beats per second at 37°C.
It means corals may begin experiencing oxygen stress even when surrounding seawater still contains normal oxygen levels.
Above ~37°C, ciliary coordination collapsed and vortical flows (swirling water currents) dissipated, shifting transport to a diffusion- limited regime accelerating coral mortality,” the authors noted adding these results identify ciliary beating as a key regulator of thermal tolerance and early indicator of critical physiological tipping points for reef-building corals in a warming, deoxygenating ocean.
“Deoxygenation events are already occurring with greater frequency and severity in reef systems globally,” Cesar Pacherres said in a press statement adding, “We reveal a hitherto unknown physiological tipping point in coral-environment interactions.”
The research identifies ciliary beating as a critical regulator of thermal tolerance. It demonstrates that the very mechanism corals use to buffer themselves against mild stress actually works against them as temperatures climb, trapping deoxygenated water against the tissue, the study explained.
This stronger motion initially improved water circulation around the coral surface. But the study uncovered a surprising problem — the intensified mixing also trapped oxygen-poor water close to coral tissues.
By 41°C, cilia movement had nearly stopped entirely, and coral mortality reached 100 per cent in the experiment.
The study suggests that the loss of ciliary rhythmicity is a definitive early indicator of acute physiological stress, preceding or amplifying mass bleaching and coral mortality in warming oceans and adding to the mortality already vulnerable to coral bleaching due to climate change.
“This abrupt loss of ciliary function coincides with thermal thresholds that commonly trigger bleaching and mortality in the field. While coral bleaching and mortality have classically been attributed to the breakdown of the coral-algal symbiosis under oxidative stress and/or nutrient imbalance, our findings suggest an additional pathway of vulnerability,” the study stated.
Researchers warn that warming oceans and falling oxygen levels may act together to intensify coral mortality during marine heatwaves.
