The experiment was carried out in the laboratory by researchers at Stanford University, in the United States, and published this Wednesday (14) in the scientific journal Science. This is the first demonstration that physical contact can completely reverse bleaching.
The donated “organ” is the microalgae that live inside the corals and give them color and energy. At the point of contact, the tissues of the two fragments fused and opened a passage for them. Once inside the diseased coral, the algae multiplied at a rate up to ten times faster than that recorded on reefs in nature.
Bleaching is a response to ocean warming. Corals live in partnership with these microalgae, which live in their tissues and produce food through photosynthesis. When the water gets too hot, this relationship breaks. The coral expels the algae, loses its color and begins to starve. If the heat continues, it dies.
The damage goes beyond corals. Reefs are home to a quarter of all marine life and act as a natural barrier against storms. They also support fishing and drive tourism.
In Brazil, the phenomenon is already repeating itself. The first major bleaching event in the country was recorded in 2019. The extreme heat of 2024 caused a new episode, and the El Niño of 2026 puts the reefs at risk again.
In the world, there have been four global events since 1998, and the most recent lasted from 2023 to 2025. Projections cited in the study indicate that mass bleaching could become annual by 2080.
“Although bleached corals may be dying, they are not dead yet,” said lead author Danny Collins, an oceans doctoral candidate at Stanford.
Two corals found in the aquarium began the study
The idea came about by chance. In 2025, biologist Steve Palumbi, co-author of the study, noticed something curious in one of the laboratory's tanks. Two pieces of coral from the same colony had fallen and rested against each other: one bleached and the other healthy.
At the point of contact, the white fragment began to regain color. It was a sign that the algae was returning.
At the time, Collins was studying the opposite path: how harmful bacteria attached to plastic debris reach corals through physical contact. The scene led the group to test whether touch could also transmit something beneficial.
The test was simple. The researchers used the cauliflower coral (Pocillopora damicornis), a species common on reefs in the Indian and Pacific Oceans.
First, they bleached branches in an aquarium, heating the water to about 34.5°C. Then, they formed ten pairs of fragments measuring 1 to 1.5 centimeters, always one bleached and one healthy, coming from the same colony.
In five pairs, the fragments rested together. In the other five, they remained at a distance of 5 to 7 millimeters, enough so that they did not touch each other even with the polyps extended. Polyps are the small units that form the coral, each with a mouth surrounded by tentacles.
The contrast appeared within a few weeks:
In contact: all fragments recovered completely within eight weeks, and one of them in just four. The color returned first at the meeting point and then spread to the rest of the choir.
Separated: the fragments remained white for weeks and were taken over by filamentous algae. After nine weeks, two had died and the other three had more than 87% of the surface covered.
The recovered corals ended the experiment with more polyps than at the beginning, a sign that they had started to grow again. In the 20th week, new branches were already forming. The isolated ones, at this point, were even worse off.
A week of contact saved the choir
To measure how much touching time was needed, the team separated the pairs after intervals ranging from one day to three weeks.
A week was enough, and in one case, five days. From then on, the bleached fragment completed the recovery alone, at the same pace as those that remained touching. After three days, there were signs of improvement, but the invasive algae ended up winning. With one day, nothing changed.
The healthy fragment then functions as a seed donor. He delivers the first batch of microalgae, and the rest of the work is up to them, which reproduce inside the newly colonized coral.
And they do it quickly. In the second week of contact, the algae population doubled every four days, on average. Previous studies cited by the authors show that the same process took three months in the Great Barrier Reef, in Australia, and 16 to 24 days in corals in Samoa.
"When beneficial algae enter bleached corals, they don't just spread. They invade and begin dividing at the fastest rates ever seen in corals," Palumbi said.
The team still does not know how to explain this speed. One hypothesis is that algae coming from healthy coral, without experiencing heat stress, arrive in better conditions to multiply.
And why does this happen?
Using a laser microscope, the researchers saw that the mass passage of algae began between 36 and 60 hours after contact, always at specific points. There, the tissues of the two fragments fused and formed small bridges.
When the pairs were separated, identical tears appeared on both sides, in the exact place where the algae had started to penetrate. From these bridges, they followed the coral's internal channels, a network of microscopic tubes that circulate fluids throughout the organism.
Fusion only happens because the two pieces are genetically identical. In most species, the coral multiplies by producing copies of itself, and these clones grow together to form the colony.
Palumbi compares the process to organ transplants in humans, which tend to be successful when the donor is an identical twin.
When scientists brought fragments from different colonies together, the meeting turned into a dispute. Within hours, the corals released filaments covered in stinging cells, a weapon used against neighbors.
Some algae came through, swallowed by the polyps' mouths, but in very small quantities. None of the bleached fragments recovered, and four of the six ended up completely covered by healthy coral.
Now, can you save the world's corals like this?
The research is very promising, but the reality is not that simple. The most effective solution is still to preserve the reefs.
The authors themselves emphasize that the result is still far from becoming a restoration tool. The test involved a single species, kept in an aquarium and probably better fed than it would be at sea.
It is not known whether the effect is repeated in other corals, with other types of algae or in colonies of different shapes.
There is also the issue of scale. Keeping pieces of coral close together for days at a time, in the middle of the ocean and along a reef, would be difficult to do.
Still, the study changes the view on what to do after a heat wave. Most research to date has sought to prevent bleaching, with selective breeding of heat-resistant corals, genetic engineering and microbiome transplantation.
Little research has been done on how to save those who have already lost their color, in part because they are often seen as lost cases.
This is not the first attempt to use healthy corals as donors. In a previous work cited by the authors, the transfer took place through water, without contact. After 55 days, only 5 of 45 fragments had received algae from the donor, and in minimal quantities.
The new experiment points to a natural process, which does not depend on genetic modification or sophisticated equipment. According to the authors, many reefs bleach in patches, with healthy colonies alongside those affected. These heat-resistant patches can act as a stockpile of algae ready to repopulate their neighbors, yet another argument for keeping the reefs protected.
The team's next step is to find ways to deliver the algae without relying on continuous contact. One idea is to collect them from healthy corals before heat waves and apply them in large quantities to bleached corals afterwards.
For Fiorenza Micheli, Collins' advisor, the expectation is that the find will inspire solutions capable of keeping corals alive for long enough. Thus, slower processes, such as growth and reproduction, would have a chance to complete recovery.
