Coral reefs are among the main victims of climate change, due to warming waters. But a study published Wednesday by Stanford University found that planting healthy corals alongside dying ones can be enough to revitalize them.
“It’s astonishing,” raves the study’s lead author, Daniel Collins, to AFP, who still remembers the moment when he saw the first positive results and ran to his thesis director’s office to tell him the news. The restoration of coral reefs, barriers against erosion and reservoirs of biodiversity, is a vast field of scientific research. Efforts today are focused on the use of artificial nurseries or the development of species more resistant to high temperatures.
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Conversely, bleached corals have been largely dismissed because of their high mortality rate, explains Daniel Collins, who has on the contrary demonstrated with his work that they “really should not be considered a lost cause”. The results of the study, published in the journal Science Advances, could well pave the way for a concrete intervention if it can be reproduced on a large scale.
Healthy Algae
Corals live in symbiosis with algae which give them the nutrients they need, as well as coloring. In the event of thermal stress, these algae can produce toxins which cause the corals to expel these algae: this is bleaching. The coral then loses its ability to feed. The method used by the Stanford team in the United States involves transferring healthy algae to these bleached corals. The tests carried out with cauliflower corals were conclusive: after five days of contact between healthy specimens and those undergoing bleaching, the latter began a spectacular recovery. Two things gave rise to this idea in Daniel Collins.
First, a study which demonstrated that plastic waste could contaminate corals by transmitting harmful pathogens to them. He then wondered if it was possible to reverse the process by transferring beneficial elements. The second source was a happy accident observed by one of the study's co-authors, Stephen Palumbi: he realized that two corals, one healthy and one not, had fallen over and were touching, and the bleached one had begun to recover.
Reproduce on a large scale
A minimum of three days of contact between two specimens is necessary for the bleached coral to begin healing, and after five days, it is truly restored. The researchers observed the phenomenon under a microscope: after one to two days of contact, the coral tissues fuse, creating a “nutrient bridge”. But the study quickly demonstrated important limitations. For example, the corals had to be in direct contact. A gap of a few millimeters between the two specimens is enough to block any recovery.
Furthermore, the phenomenon only works between two corals which share the same genetic heritage. Otherwise, the specimens trigger stinging cells, a reaction similar to organ transplant rejection in humans. The main challenge will now be to reproduce this laboratory discovery on a large scale. “It is unlikely that we will put corals in contact with each other on reefs” in the open sea, underlines Daniel Collins. It is nevertheless possible to extract algae that live in symbiosis with corals and cultivate healthy ones. The team is now considering ways to reintroduce them to bleached corals. This solution could also solve the problem of genetic heritage, which only applies to corals and not to algae.
