A devastating combination of extreme ocean heat and a rare coral disease has killed seventy-five per cent of Goniopora corals at One Tree Reef on the Great Barrier Reef, marking one of the most alarming coral losses recorded in recent years. Scientists say the event shows how rapidly climate change is weakening even the hardiest coral species.
Fourth global bleaching event hits 84 percent of reefs
The die-off occurred during the world’s fourth global mass bleaching event, which has already affected eighty-four per cent of reefs worldwide. Researchers examining one hundred and twelve coral colonies at One Tree Reef found that severe bleaching caused by record-breaking temperatures was followed by an aggressive outbreak of black band disease, a fast-spreading bacterial infection that destroys coral tissue.
The study, published on December 10, 2025, in Proceedings of the Royal Society B, notes that One Tree Reef has not seen bleaching of this scale for decades. Goniopora corals, often found in lagoons and turbid waters, are typically considered thermally tolerant, making the sudden mortality particularly concerning.
Heat stress triggers vulnerability and rapid disease spread
Scientists explained that bleaching left the corals in a weakened condition, severely reducing their ability to fight infections. Warm water enhanced the strength of harmful pathogens, allowing black band disease to spread at an unprecedented rate.
Black band disease, which is common in the Caribbean but historically rare in the southern Great Barrier Reef, first appeared in a few colonies in February 2024. By April, sixty-one per cent of bleached colonies were infected, spreading rapidly across the reef in an epizootic pattern. Surveys of more than seven hundred Goniopora colonies revealed widespread bleaching, fast disease progression and high mortality. Notably, the disease was found only on corals that had bleached.
Record-breaking temperatures left reefs with little chance to recover
Between December 2023 and February 2025, sea temperatures at One Tree Reef remained extremely high, with average readings above twenty-eight degrees Celsius, minimum temperatures above twenty-nine degrees and maximum temperatures above thirty degrees. During the peak bleaching period, temperatures exceeded twenty-eight degrees for seventy-four consecutive days. Water temperatures rose even higher in early 2024, reaching thirty-four to thirty-five degrees.
Although black band disease is often linked to pollution and nutrient-rich waters, researchers said One Tree Reef’s offshore location shields it from most coastal stressors. Goniopora corals are adapted to slightly turbid conditions, making poor water quality an unlikely cause. Instead, scientists concluded that weakened coral health from prolonged heat stress allowed the disease to overpower the colonies.
Why the disease targeted only Goniopora corals remains unclear
The outbreak raises questions about why this rare disease appeared so aggressively in Goniopora colonies while other coral genera that bleached at the site did not develop the infection. Surveys from the northern Great Barrier Reef in 2024 reported very low black band disease levels, affecting only one to two per cent of Acropora and Montipora corals.
The researchers suggested that further investigation is needed to understand why Goniopora corals at One Tree Reef were uniquely vulnerable.
Scientists warn of accelerating climate threats to reefs
Experts say the event represents a serious global warning. With mass bleaching events becoming more frequent, corals are losing the time they need to recover. The study’s authors stressed that climate change is advancing faster than corals can adapt, pushing these ecosystems towards collapse.
Maria Byrne, professor of marine biology at the University of Sydney, said the loss of large, structure-forming corals would have long-term consequences for biodiversity, coastal safety and food security.
She added that coral reefs support more than one billion people globally, and that only strong and immediate action to reduce greenhouse gas emissions can prevent further destruction.