Dying corals may have found an unexpected lifeline: their neighbours.
Scientists at Stanford University have discovered a way to bring corals back from the brink of death by placing them in contact with healthy corals for just a few days.
Published in Science Advances, the study presents a promising new approach to restoring coral reefs as rising sea temperatures cause more frequent and severe coral bleaching events.
When water becomes too hot, corals expel the microscopic algae that provide much of their energy, an event known as bleaching, which leaves them vulnerable to starvation.
After bleaching events, it can take months or years for surviving corals to recover – but the scientists contend hope is not lost for them.
“While bleached corals may be dying, they are not dead yet,” says lead author Danny Collins, a PhD student at Stanford. “The question we wanted to answer is, once a coral has bleached, what can we do?”
How scientists brought bleached corals back to full strength
The discovery happened by accident in 2025, when Steve Palumbi, a professor of oceans and marine biologist at Stanford’s Doerr School of Sustainability, noticed two pieces of coral that had fallen against each other in a laboratory tank. One was healthy while the other was bleached. But where the two touched, the bleached coral was beginning to regain its colour.
Collins then arranged 10 pairs of coral fragments, each containing a healthy and bleached piece from the same colony. Five pairs were placed in contact with one another, and the others were kept a few millimetres apart.
Within seven weeks, all five bleached fragments put in contact with their healthy clones had recovered. The ones that were kept apart had died.
Under a microscope, the scientists discovered the corals had fused together, creating tiny channels that allowed algae and nutrients to pass from one to the other. Within three days, beneficial algae had begun moving from the healthy corals into the bleached ones. After about a week, some had received enough algae to recover on their own.
Once inside the damaged corals, the beneficial algae began multiplying rapidly. “They flood in and start dividing at the fastest rates seen in corals,” says Palumbi.
During the second week, the algae doubled in number every four days – about 10 times faster than in wild corals – allowing the damaged corals to continue recovering even after they were separated from the healthy ones.
The finding marked a breakthrough in the lab. “Bleached corals are often considered a lost cause because recovery after a heatwave is very slow, if it happens at all,” says Fiorenza Micheli, a Stanford professor and one of Collins’ faculty advisors.
But the scientists say the technique isn’t ready for wide-scale application in the oceans.
The corals must be genetically identical for their tissues to fuse. In other words, every damaged coral would need a healthy copy of itself – an impractical solution for restoring whole reefs. But the researchers hope to find ways to deliver the beneficial algae without requiring direct contact or identical twins.
Coral reefs may be running out of recovery time
The revelation comes at a crucial time for coral reefs.
According to the Global Coral Reef Monitoring Network’s latest assessment, the world's reefs have lost nearly 10 per cent of their living coral over the past four decades. Between 2023 and 2024 alone, it fell almost 9 per cent during a global bleaching event – the largest single-year decline ever recorded.
Global bleaching now occurs roughly every five to six years, leaving reefs less time to recover between bouts of extreme heat. The report warns that reefs will continue declining unless man-made emissions fall and threats such as overfishing, water pollution and coastal developments are curbed.
And while combining coral to help them recover from bleaching might save some of them, Palumbi stresses that it can’t protect them from rising sea temperatures.
“This is a recovery story. It doesn’t blunt heatwaves; it just helps the aftermath. There is still a huge need to reduce heatwaves and protect all of Earth’s life from rapid climate stress,” he says.
“Eventually, it can hopefully help the huge effort being put into restoration succeed better by helping those coral gardens survive heatwaves more completely.”