Beneath the surface of the eastern tropical Pacific Ocean, a breathtaking scene unfolds where vibrant coral reefs, imbued with color by photosynthetic algae, flourish. These algae serve as the primary energy source for the corals. However, during the early 1980s, an intense El Niño-induced heat wave triggered a catastrophic bleaching event, decolorizing over 90 percent of these corals to a stark, ghostly white. The algae, long-time residents within the corals, could no longer survive in their hosts.
Subsequent strong El Niño occurrences in the late 1990s and again in 2015–2016 warmed these Pacific waters yet again. However, these later events did not devastate the reefs as severely as the earlier one did. Ana Palacio, a marine biologist from the University of Miami, observed post-event that some corals were either resisting or recuperating from the bleaching effects. This led her to speculate that these corals might be adapting to their changing environment.
Corals, anchored to the very reefs they construct, cannot escape to cooler waters, making them especially susceptible to climate change. Yet, they demonstrate remarkable resilience. Researchers are uncovering the various ways corals adapt: some switch their algal partners to species that can tolerate more heat, others utilize minuscule body hairs to dispel harmful oxygen produced by stressed algae, and certain juvenile corals adjust their metabolism to better cope with warmer waters. These natural adaptations are crucial as scientists strive to preserve these vital components of marine ecosystems.
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Following the 2015–2016 heat wave, Palacio and her team discovered that certain corals, specifically the Pocillopora species which are predominant reef-builders in the region, had expelled their usual algae and adopted other, more heat-resistant algal species. These corals increasingly associated with a thermotolerant algal symbiont known as Durusdinium glynnii—a name derived from the Latin durus, meaning rough or tough. Unlike most symbiotic algae which produce toxic levels of oxygen under heat stress, Durusdinium manages to maintain it at bearable levels.
Researchers have also found that corals can independently mitigate excessive oxygen. They use rows of cilia—small, hair-like structures—that act as a personal cooling system, dispersing the oxygen to prevent harmful build-up. In 2022, marine biologists Cesar O. Pacherres and Soeren Ahmerkamp demonstrated that these cilia create tiny whirlpools in the water, effectively circulating the oxygen. This system is present in all corals, although its use varies among species. The team is now exploring whether corals in areas like the Great Barrier Reef increase their cilia activity in response to rising temperatures.
Corals have another advantage; they are not entirely immobile. Their larvae drift freely in the ocean until they settle, providing opportunities for the species to move to cooler waters or spread heat-tolerant genes. Ariana Huffmyer, a marine biologist at the University of Washington, focuses on how juvenile corals adapt to elevated temperatures. Research conducted with the Hawai’i Institute of Marine Biology showed that coral larvae can alter their metabolism after just three days in warm lab waters to better handle heat stress and prevent bleaching.
Corals typically engage in a nutritional exchange with their resident algae, providing nitrogen in exchange for carbon, used as energy. Under stress, corals produce excess nitrogen, prompting the algae to overproduce and hoard carbon. Huffmyer found that heat-stressed juvenile corals learn to retain this excess nitrogen, maintaining a stable relationship with their algae.
Pacherres cautions that these adaptations have their limits. Just as humans can only withstand certain temperatures, corals too have their thresholds. However, understanding these natural defenses is essential for developing conservation strategies. Protecting larvae from heat-adapted reefs is particularly crucial as they are likely to carry genes beneficial for survival in warmer waters.
These natural coral adaptations could also support direct intervention efforts, like acclimatizing corals to stress in nurseries before reintroducing them to the ocean. Encouraging corals to host heat-resistant algae or activating specific genes could significantly enhance their survival prospects during marine heatwaves.
Huffmyer reflects on the motivation derived from witnessing both thriving and bleached reefs. The contrast between vibrant, lively reefs and their bleached counterparts fuels the passion to apply one’s skills and enthusiasm towards coral conservation.
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Cameron Aldridge combines a scientific mind with a knack for storytelling. Passionate about discoveries and breakthroughs, Cameron unravels complex scientific advancements in a way that’s both informative and entertaining.