A staggering 80 per cent of the world’s vibrant coral reefs experienced extreme levels of heat stress from warming oceans between 2023 and 2025, exacerbated by the 2023 El Niño. Swathes of reefs were struck by bleaching and mortality, and India’s corals were no exception in this fourth global mass coral bleaching event in recorded history.
Lakshadweep was found to be the worst affected with 37 per cent of its coral cover “bleached or dead,” according to a paper published earlier this year in Springer Nature. In the Andamans’ Mahatma Gandhi Marine National Park, the figure was 11.5 per cent.
The coral species most affected were the small, stony, brown-green Acropora, the “cauliflower coral” Pocillopora, the purple-tinged Galaxea and Montipora.
In the natural environment, it is only sometimes that bleached corals recover once the heat stress subsides. But now, there appears hope, with a little help from algae, suggested by two separate studies published earlier this month. Algae supply most of the coral’s nutrients through photosynthesis, and when the ocean waters warm, corals expel the algae that causes bleaching, explains one study published in Science.
The scientists demonstrated, however, that even a brief period of contact between healthy corals and bleached ones can transfer the algae to the latter, leading to their full recovery. In fact, this process, that takes place through tissue fusion between the corals, could take place within a matter of days.
But there is a caveat: this algal transfer can only occur between corals from the same genetic background.
The scientists showed coral bleaching recovery through contact-mediated algae transfer from healthy corals, using small coral fragments. The transfer, they wrote, “is rapid, occurring over days to weeks, and highly efficient, with only brief physical contact with healthy corals required.”
Back to life in days
In fact, just a single week of contact, and sometimes just three days, “provides sufficient symbionts [organisms that lives in a close, long-term association with an organism of a different species] to initiate sustained and full self-recovery,” they wrote.
For the experiment, the researchers placed bleached coral fragments (1 to 1.5 cm in length) of P. damicornis in contact with healthy fragments of the same genet (genetic background). All bleached corals in direct contact with healthy corals achieved complete recovery and symbiont recolonisation within eight weeks, they reported.
This process could aid, what the authors term “an ongoing, urgent race for solutions to mitigate the catastrophic loss of coral due to bleaching.” Coral reefs, the “rainforests of the sea” that support a fourth all marine life cover less than 1 per cent of the ocean floor.
There have been previous efforts to save corals, but they have been designed to limit the effects of coral bleaching before or during thermal stress. These include: genetic engineering, selective breeding, microbiome transplants, shading, coral transplantation, assisted gene flow, larval seeding, and coral nurseries. “However, these solutions are largely preventative,” explained the paper.
As predictions suggest that mass coral bleaching will occur annually by 2080 with global warming, coral reefs “are approaching tipping points of collapse,” said the paper, adding that the new findings of the experiment reveal a previously unrecognised natural process for efficiently reintroducing symbionts to bleached corals.
This could provide “a foundation for research into potential recovery interventions,” hoped the authors.
Heat tolerance
With algae at centre-stage again, researchers of another study published in Environmental Microbiology Reports took their experiment further, and made them heat tolerant in the context of warming oceans. They used a lab-created heat-tolerant algal strain named SS8 and injected it into two Great Barrier Reef coral species Galaxea fascicularis and and Psammocora columna, which were first chemically bleached to remove their algae. Once they recovered, these inoculated coral fragments became donors, and were positioned in direct contact with recipient bleached corals in a water tank.
What the researchers found was that all donor-recipient pairs fused: it took just three days for Galaxea fascicularis and nine days for Psammocora columna to do so. “In both species, fusion was followed by increased pigmentation,” the paper added.
Significantly, 13 out of 18 unfused corals in the water tank showed the heat-tolerant SS8 strain, suggesting that SS8 can spread to corals through water too.
“Both these papers show, with eloquent simplicity, a mechanism that has been suspected was possible for a long time,” Rohan Arthur, a marine biologist with the Nature Conservation Foundation and co-author of the Springer Nature paper, told The Hindu.
Symbiont switching is something that researchers working on coral resistance have suggested may be a possible pathway by which corals could transfer adaptive potential through contact, “and it is heartening to see this being confirmed with experimental proof,” he added.
70,000 people in peril
However, these small experimental studies are still a very long way from large-scale inoculations of corals in the field, he noted. “Because bleaching resistance was only transferred between individuals of the same genet, it is difficult to conceive of how this can translate into reef-wide interventions.” What it does suggest, however, is that reefs have a greater flexibility in their bleaching response than earlier imagined, said Dr. Arthur.
He underscored the importance of continued efforts to enhance local resilience, such as low land-based pollution, high amounts of predatory and herbivory fish, and low levels of anchor damage and other physical disturbance. “It is a more systemically meaningful way to manage ecological integrity and works much better at ecologically relevant scales,” he said.
His team has tracked Lakshadweep’s reefs for nearly three decades, and have observed that as corals decline, the very architecture of the reef also becomes simpler each year, resulting in fewer nooks and crannies for fish and invertebrates to hide, and an increasingly impoverished reef.
“Even more worryingly, our work is showing that with declining coral, the ability of these reefs to produce calcium carbonate is reducing, which results in lowered protection of atoll islands from storms, high waves and sea level rise. This constitutes an existential threat for the 70,000 people who call Lakshadweep their home,” said Dr. Arthur.