The writer is a science commentator
A few days after Stephen Palumbi, a marine biologist at Stanford University, put a piece of bleached coral in a tank with healthy coral from the same colony, he noticed the bleached coral had fallen over and was resting on the healthy one.
But there was something else: the bleached fragment had regained some colour. As Palumbi and colleagues reported last week, the two coral neighbours had fused, allowing a flow of health-giving algae from one to the other.
The revelation of this healing process — which I am tempted to unofficially name polyp-pharmacy, because coral reefs are made up of individual animals called polyps — shows that serendipity still matters in science. But it offers other messages too: a flicker of hope, suggesting new ways of tackling coral bleaching and reef degradation caused by climate change. Given that this process was already suspected by amateur aquarists, it also affirms the important contributions of hobbyists to the wider scientific cause.
Corals are animals that have a symbiotic relationship with the microscopic algae that live in their tissues. These algal “symbionts” provide food and colour to their host but are expelled from the coral if the water becomes too warm (or cold). Chemical run-off, or exposure to air for long periods, can also cause such bleaching.
That symbiont exodus drains corals of colour and disrupts immunity and reproduction (depending on the species, corals can reproduce both asexually, by parent polyps producing genetically identical buds; and sexually, by releasing both male and female gametes into the water).
Bleaching damages more than just the corals: coral reefs bring an estimated $9.8tn of global benefits a year to society, including through fisheries, tourism and coastal protection. Affected corals can recover if their symbionts reproduce sufficiently or by absorbing symbionts floating in the water. But this can take months or years, often too long to stave off death.
As Palumbi and colleagues reported, the observed fusion of tissue triggered recovery over days or weeks, with even three days of contact with healthy coral being enough in some situations. The findings, the authors say, “reveal a previously unrecognised natural process for efficiently reintroducing symbionts to bleached corals, providing a foundation for research into potential recovery interventions”.
Michael Sweet, an aquatic biologist at the University of Derby, told me the result was unsurprising, with the hobbyist and aquarist community already aware that algal traffic flow was possible. Still, he welcomed the publication of a rigorous, peer-reviewed scientific paper on the topic: “Any tool or technique to help corals in their fight against climate change is great news . . . [and shows] that all hope is not lost.”
Still, this “symbiont transfer” technique has a drawback: it only seems to work between genetically identical members of the same colony. Placing fragments of genetically different colonies next to each other can spark an immune clash. In short, Sweet says, this is no global silver bullet, but it could enable low-cost reef restoration in some countries.
James Guest, a Newcastle University coral biologist who studies bleaching and diseases, said that while corals are known to be flexible and resilient — pieces can be broken off and fused back together — the paper sheds important light on how symbionts move within and between corals. That, he said, could open up avenues for infecting corals with new symbiont types, or building an experimental colony by fusing fragments with different symbionts.
While the Stanford research suggests that grafting healthy coral fragments on to a surviving bleached colony can speed up recovery, Guest added, “the issue with marine heatwaves is that the stress is so severe and prolonged that many corals just die . . . sometimes without even bleaching”. Plus, simply restoring a colony does not make it less vulnerable to future shocks.
Guest, who says he frequently collaborates with aquarists and hobbyists because of their expertise, is now exploring selective breeding for heat tolerance. A separate paper published this month raises the idea of using lab-raised, heat-resistant symbionts to improve wild coral survival.
Any and every method matters. The International Union for the Conservation of Nature said this month that 44 per cent of reef-building corals face a raised risk of extinction. As these marine treasures vanish, future generations will have another reason to thank the citizen scientists and hobbyists who strive to keep them.