AAAS: "Coral can heal rapidly from bleaching—if it touches a clone," by Warren Cornwall.

"When coral bleaches during a heat wave + ejects its symbiotic algae, it usually means either a death sentence or a slow recovery over months or years." Some corals have a reproductive strategy that involves passing symbionts from parent to offspring, which might make that species more responsive to a direct 'transfusion' method than others, but in most species, young corals capture algae from the surrounding water.

However, a serendipitous discovery led to a 'deceptively simple' approach reported in a 2 October paper in Environmental Microbiology Reports + another published more recently in Science Advances. "By touching a piece of bleached coral to a healthy piece, a kind of algal transfusion can occur...symbionts flow through a bridge of tissue + then rapidly reproduce, repopulating the bleached piece in as little as 4 weeks." 

But the method didn’t work when the healthy coral came from a genetically different colony. In those situations, the coral polyps sent out tiny stinging appendages as if attacking an intruder, comparable to our immune system rejecting an organ transplant from a stranger. 

"When corals from the same colony were physically touching, tissue from polyps in the two chunks merged to create a kind of algal highway." If the bits stayed in contact for a week, symbionts poured in + began to replicate, doubling approximately every 4 days, as much as 10 times faster than what has been witnessed on bleached reefs, which normally recover by reabsorbing free-floating algae.

Researcher Bede Johnston, a coral researcher at the Australian Institute of Marine Sciences, also addressed the puzzle of heat tolerance in 2 species of coral, Galaxea fascicularis + Psammocora columna. "They let polyps in some of the fragments absorb lab-raised symbionts bred for tolerance to extreme temperatures...when the inoculated fragments were joined to bleached fragments, some of the heat-tolerant symbionts found their way into the neighboring polyps."

Despite the practical challenges of inoculating thousands of square km of coral reef, I love seeing this steady advance in marine science.