What happened
When the water gets too warm, corals expel the microscopic algae that live inside them and give them their color and most of their food. This is bleaching: the coral turns white and, if the algae do not return, it dies. Recovery, when it happens, is usually very slow.
A Stanford University team published a way to speed it up dramatically in Science Advances on October 14. The discovery started by chance: marine biologist Steve Palumbi noticed some coral fragments that had tipped over and ended up touching. The study is led by PhD student Danny Collins, with Palumbi and Fiorenza Micheli.
The experiment
The team set up 10 pairs of fragments. Both fragments in each pair came from the same coral, so they were clones: one bleached and one healthy. Five pairs were placed touching and the other five a few millimeters apart.
- Algae moved from the healthy fragment to the bleached one within three days of contact.
- Bleached fragments touching their twin fully recovered within seven weeks, the fastest recovery documented so far. Those that were not touching died.
- One week of contact, and in some cases five days, was enough for the coral to finish recovering on its own after being separated.
- In the second week, algae in the bleached coral doubled every four days, about ten times faster than in wild corals.
How it works
Genetically identical corals recognize each other and their tissues fuse on contact. Microscopic channels line up and act as a pathway for algae and nutrients. Once inside, what drives recovery is not a steady flow of new algae but the very fast growth of the ones that got in. "While bleached corals may be dying, they are not dead yet," says Collins.
What we don't know yet
- It is a lab experiment, in tanks, with a small number of pairs.
- It only works between clones. With corals of a different genetic background, or without contact, the effect disappears.
- Needing a healthy copy of every bleached coral is not feasible at reef scale, as the authors themselves point out.
- The next step is to find other ways to deliver algae to bleached corals, for example by collecting them from healthy corals before a heat wave and applying them afterwards.
Why it is good news
A bleached coral is usually written off. This study shows that, if it gets its algae back in time, it can be healthy again within weeks, when recovery usually takes much longer or never comes. It will not save reefs on its own, but it explains the mechanism and opens a concrete path to helping corals after each marine heat wave.
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Sources
- Rapid coral bleaching recovery via symbiont transfer from healthy corals. Science Advances (DOI 10.1126/sciadv.aeh1580), .
- Stanford scientists discover a way to reverse coral bleaching at record speed. Universidad de Stanford, vía EurekAlert!, .
- Scientists discover a way to reverse coral bleaching at record speed. Phys.org, .
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