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. 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.
All bleached corals in direct contact with healthy corals achieved complete recovery and symbiont recolonisation within eight weeks, they reported. 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.
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).
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. 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. 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.
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.
But now, there appears hope, with a little help from algae, suggested by two separate studies published earlier this month.
