Researchers have identified a previously unknown mechanism that shows how extreme ocean heat can disrupt the microscopic structures corals use to breathe.
Coral tentacles close up. The cilia are microscopic and too small to be seen with the naked eye.
A new international study led by researchers at the University of Copenhagen suggests that rising ocean temperatures may directly interfere with the microscopic cilia that corals use to circulate oxygen-rich water across their surfaces. The findings add a new layer to scientific understanding of why corals become stressed and die during marine heatwaves.
The study, published in the journal Science Advances, examined the tropical stony coral Porites lutea under controlled laboratory conditions. Researchers gradually increased the water temperature from 27°C to 41°C while monitoring the coral cilia.
Coral “breathing”
Corals rely on thousands of microscopic hair-like structures called cilia to move oxygenated seawater across their tissues. During daylight hours, corals receive oxygen from symbiotic algae through photosynthesis, but at night they depend on dissolved oxygen in the surrounding seawater.
Initially, the coral’s cilia beat faster as temperatures rose, apparently responding to increased metabolic demand. However, once temperatures exceeded around 37°C, the coordinated movement began to break down. At 41°C, the cilia stopped functioning entirely.
According to Professor Michael Kühl from the University of Copenhagen, the corals effectively “lose their breath” as warmer water disrupts the mechanisms that help them absorb oxygen.
A new mechanism
The researchers describe the findings as the first evidence that heat stress can directly impair the coral’s own ventilation system, potentially leading to oxygen deprivation at the coral surface.
This mechanism appears separate from the better-known process of coral bleaching, in which heat stress causes corals to expel the symbiotic algae that provide them with energy.
Australian coral scientist Ove Hoegh-Guldberg described the study as significant because it shifts attention toward the coral animal itself and its “cilia-based ventilation system”.
Corals under pressure
Scientists estimate that around half of the world’s coral reefs have already been lost since the 1950s, largely due to warming seas linked to climate change.
The new findings suggest that marine heatwaves may stress corals in ways beyond previously understood bleaching effects, combining bleaching effects with direct disruption of respiration and oxygen transport.
At the same time, some researchers caution that alternative interpretations may exist. Professor emeritus Hans Ulrik Riisgård of the University of Southern Denmark noted that warmer water also changes seawater viscosity, which, in turn, can influence cilia movement.
Even so, the study adds to growing evidence that prolonged marine heatwaves are affecting coral physiology in increasingly complex ways.
