Off the coast of Florida, in the United States, a group of divers descends to the ocean floor to plant young corals on a reef. They are scientists and volunteers who also assess how other specimens are developing in increasingly warm waters.
The mission is part of a long-term program called "Rescue a Reef," coordinated by the University of Miami. The initiative seeks to promote marine conservation and raise public awareness.
Part of the work involves investigating which coral species are best able to withstand the rapid ocean warming caused by climate change driven by human activities.
"Certain species may be more resilient, as they are better able to survive and recover from episodes of thermal stress," explained the program coordinator, Devon Ledbetter, during a recent dive at Rainbow Reef. The site is located about three kilometers off the coast of Key Biscayne, near Miami.
Coral reefs support ecosystems rich in biodiversity, serving as the marine equivalent of tropical rainforests. They also function as natural barriers, protecting coastlines from erosion and storm-induced flooding. Their situation, however, grows increasingly dire as the planet warms.
On the Florida reef, an unprecedented marine heat wave that began in 2023 caused the "functional extinction" of two ecologically important species: elkhorn coral and staghorn coral.
The National Oceanic and Atmospheric Administration (NOAA) believes the fourth global coral bleaching event likely came to an end in mid-2025. The arrival of a very intense El Niño, however, could trigger a new and larger-scale bleaching episode, including in Florida's waters.
Moreover, sea surface temperatures today remain higher than those recorded 25 or 30 years ago, when the first coral bleaching episode occurred, according to a recent study.
Thermal stress monitoring
During the expedition, participants work meticulously. Some fast-growing branching coral species are placed on "trees" — structures that resemble old television antennas and are suspended in the water with the help of buoys.
Others are placed on table-like structures anchored to the ocean floor. A third group is attached to supports screwed into small bases embedded in the reef.
These transplanted corals are fitted with small protective covers to shield them from predators and prevent fish from feeding on them.
Researchers position color reference charts around the newly planted corals to record their pigmentation. The method makes it possible to monitor their health and identify early signs of thermal stress.
Corals are formed by tiny animals called polyps, which maintain a symbiotic relationship with colorful microscopic algae that live within their tissues. But when these algae are affected by heat or pollution, they abandon the polyps.
This exposes the corals' calcium carbonate skeletons — a phenomenon known as bleaching. A bleached coral is not dead, but sick and more vulnerable.
"Based on what we have observed, mountainous star coral tends to be the most resistant to bleaching and thermal stress," Ledbetter explained.
This season, the program has already planted 615 corals, of which 228 are part of the bleaching study. The results are subsequently sent to state authorities responsible for the environment and wildlife.
Created in 2015, the program seeks to contribute to science, educate the public and rebuild the ecosystem. "One coral at a time," said the researcher.
