History is a valuable teacher—to natural systems as well as humans, apparently. In a study that could critically shape marine conservation efforts for years to come, scientists have found that sea life can learn from the past just as human beings do. Researchers discovered that corals with heat stress on their health records—meaning periods of dangerously high ocean temperatures—are more likely to survive similar stress in the future.
As ocean temperatures escalate in tandem with the behemoth that is climate change, we often hear about coral bleaching: a phenomenon where heat causes coral to essentially kick out the pigmented algae living inside of – and feeding – them. But a team led by Jessica Carilli of the Australian Nuclear Science and Technology Organization (ANSTO) observed that corals from an atoll in the central Pacific island nation of Kiribati with negligible exposure to El Niño events actually experienced worse bleaching than what their neighbors that had previously weathered thermal storms went through. Meanwhile, those corals that suffered substantial thermal ups and downs in their earlier days showed a higher tolerance to bleaching later on, as measured by the accumulation of temperature stress in excess of bleaching thresholds. The study is published in the scientific journal PLoS ONE.
“The big question was ‘to what extent can corals learn from their past experiences?’” says University of British Columbia assistant professor and study coauthor Simon Donner. “What’s very clear from past research is that coral reefs are about as vulnerable to change as any ecosystem in the ocean.”
This demonstrated vulnerability led the team to pursue an experiment on the natural laboratory of Kiribati’s Gilbert Islands, in a part of the ocean that warms up during El Niño events because of the islands’ location in the middle of the Pacific. This means the region’s reefs regularly endure El Niño heat waves, which makes them ideal for investigating how increased temperature affects coral health.
The scientists examined coral skeletal growth rates and fat tissue stores to understand the impacts of bleaching events in 2004 and 2009. Attempting to isolate past heat stress as the primary contributor to the corals’ future susceptibility to such stress, they restricted comparisons to reefs equivalent to one other in terms of external disturbances such as nutrient runoff, sedimentation, overfishing, and ocean acidification.
What they found backed up in a natural system what others had seen in test tubes. “It was a nice confirmation that what people had seen in the lab was applicable to real life; a bit of a ray of hope,” says Carilli.
The jury’s still out on the specific mechanisms that bring about this acclimatization by thermal stress history in corals, though other work offers some explanations. Corals may be picking up more thermally-resistant algae, for example; they might also be working solo with their own physiological defenses to reduce bleaching susceptibility. Or it could be that those reefs that were more resistant in the first place simply extend their dominance after bleaching events.
But whatever the reason, these findings deliver some precious good news for corals and the millions who rely on them. It’s comforting to know that even as we continue to relentlessly warm our earth, some yet-unknown apparatus is helping to keep some of our corals well and happy. Donner also noted that while the team’s results should not be taken to work for “every coral on the planet,” a deeper understanding of those factors that affect bleaching thresholds could have important implications for conservation.
“We can figure out what characteristics can make one reef tougher than another,” he explains. “If we want to figure out which reefs we want to target for protection, maybe this is the sort of logic we should be using: target the ones that are naturally subject to frequent heat stress.” Hopefully Donner is right – it’s sort of ironic that fluctuating temperatures could help us insulate our natural world against the effects of global warming, but it’s nice to have the promise of pleasant news for a change.