In the Florida Keys, devastating marine heat waves can turn vibrant coral reefs into underwater graveyards. But according to two new studies published earlier this month, bleached and struggling corals may have another lifeline: help from healthy, genetically identical neighbors.
And the discovery happened by accident.
In his lab, Stanford University marine biologist Stephen Palumbi was experimenting with intentionally bleached corals – a process in which stressed corals expel the symbiotic algae living within them.
While not an immediate death sentence, bleaching renders a coral more vulnerable to disease and deprives it of critical energy normally provided by the algae – and, as the name implies, gives it a stark white color. Once its white skeleton is exposed, the clock is ticking on the coral; if water temperatures return to normal quickly enough, there’s a chance for recovery as the coral reabsorbs symbiotes from the water column. If not, it may starve to death.
In Palumbi’s lab, a piece of healthy coral accidentally rested against a bleached piece from the same genetically identical colony. He noticed the bleached coral began to rapidly regain its color as the symbiotic algae returned, and relayed his findings to another Stanford researcher, Ph.D. student Danny Collins.
Already at work to see if algal symbionts could be artificially delivered to stressed corals, Collins set up a series of experiments, intentionally bleaching pieces of cauliflower coral and putting them in contact with healthy pieces from the same or different colonies.
If the healthy corals weren’t identical to the bleached partners they touched, the approach failed, with the Stanford team comparing the response to “biological warfare” and an immune response as the polyps deployed stinging cells against their neighbors.
But for cases of identical corals, recovery rates skyrocketed to as much as 10 times faster than normal.
By week eight of the Stanford study, all fragments in contact with healthy corals showed 100% polyp recovery, with one group soaring to full recovery by week four.
Even given an extra week, no non-contact fragment reached more than 20%.
The fragments, researchers noted, didn’t need to stay in constant contact. Separating the bleached corals from healthy fragments after different periods of time, investigators found that bleached fragments usually saw accelerated recovery if given at least a week of contact with their healthy partners.
Only part of the picture
While the method provides an exciting avenue for further exploration, the Stanford study noted several hurdles before looking at the discovery as a widespread bleaching recovery tool. It’s not yet known if other coral species will show the same degree of recovery using the contact method, or whether it can be scaled to repopulate larger bleached corals.
A controlled lab environment can’t match the ever-changing environment of open water, and the variables that come with it. And if corals fill back up with symbiont strains that still can’t handle the heat or other stressors, it’s a matter of time before they end up in trouble again.
“Extending these findings into ecologically relevant field contexts will be essential for evaluating coral bleaching recovery at the scale of natural coral reef systems,” the Stanford team wrote.
A second paper, published by a team at the Australian Institute of Marine Science led by researcher Bede Johnson, appears to answer part of those troubles.
Taking the Stanford contact method one step further, investigators allowed octopus and wrinkle corals to absorb lab-grown heat-resistant algae from the water column before touching those healthy corals to their bleached counterparts.
After about 50 days, researchers detected the heat-tolerant algae in 15 of 19 fused octopus corals, though similar algae were also detected in unfused corals sharing the same water. Scientists therefore could not determine how much of the transfer occurred directly through fused tissue.
The specialized algae were also detected in unfused corals sharing the same water, leaving researchers unable to determine precisely how much transfer occurred through direct tissue contact. And while the recovered corals showed improved physiological function, the study did not establish whether they could better survive another heat wave.
Still, the research raises an intriguing possibility for the Florida Keys if developed into a scalable practice.
A real and present need
The new studies are by no means an instant solution for Keys reefs. But with rising ocean temperatures, the methods may be worth a look as investigators find new ways to bolster corals’ ability to weather heat stress events every summer.
In 2023, an unprecedented marine heat wave sent summer ocean temperatures soaring across South Florida, with some shallow-water monitoring stations recording temperatures above 100 degrees Fahrenheit. The prolonged heat triggered widespread bleaching across the Florida Keys reef tract, devastating coral nurseries, restoration sites and natural reefs.
At iconic locations like Sombrero Reef and Looe Key, corals that had survived decades of environmental challenges succumbed to extreme thermal stress. Coral restoration practitioners scrambled to move vulnerable coral colonies into temperature-controlled facilities, while scientists documented catastrophic losses among some of the region’s most important reef-building species.
The event highlighted a troubling reality: Even corals painstakingly raised in nurseries and transplanted onto reefs are vulnerable when ocean temperatures climb beyond their biological limits.
The summer of 2023, and years since, have seen a rise in collaborative efforts and new techniques among restoration practitioners to preserve existing corals and outplant new ones – everything from moving corals to deeper nurseries with cooler waters to installing shade structures and ramping up experiments with heat-resistant hybrids.
The new findings could eventually add another tool to that growing arsenal, offering a way to help damaged corals recover faster while potentially improving their resilience to future heat waves. And in a region where increasingly extreme summers can undo years of restoration work in a matter of weeks, any advantage could prove invaluable.
