A Symbiosis Under Stress
The New Caledonia Barrier Reef surrounds Grande Terre, the territory's largest island. It is the second-longest double-barrier reef system on Earth and was designated a UNESCO World Heritage site in 2008. This structure is built by billions of tiny coral polyps. These animals live in a partnership with microscopic algae called Symbiodiniaceae (or zooxanthellae) that live inside their tissues. The coral provides shelter, and the algae photosynthesize, producing up to 90% of the coral's food. This relationship is the basis of the reef ecosystem.
When sea surface temperatures rise, this partnership breaks down. During the 2016 bleaching event, the warmest season recorded in New Caledonia in 30 years, water temperatures rose 1–2°C above the climatic maximum. This heat stress causes the coral to expel its algal symbionts. Without the algae, the coral's tissues become transparent, revealing the white skeleton beneath—a phenomenon called bleaching. While reefs in New Caledonia have historically experienced less bleaching than other global systems, a major event in 2016 affected large areas, particularly on the east coast of Grande Terre.
Shuffling for Survival
After the 2016 bleaching, scientists observed an adaptive mechanism in some of Grande Terre's surviving corals. Instead of simply regaining their original symbionts, some corals changed their internal ecosystem in a process called "symbiont shuffling". Corals can host different types of algae, primarily from the genera Cladocopium and Durusdinium. Cladocopium species are effective photosynthesizers under normal conditions, supporting robust coral growth. Durusdinium species, however, are significantly more heat-tolerant.
Researchers found that after heat stress, some corals recovered by increasing the proportion of Durusdinium algae within their tissues. These tougher symbionts allow the coral to withstand higher temperatures without bleaching. This natural shuffling process is a positive sign for reef resilience. Studies suggest the transition can be cost-free for the coral host and may even be associated with lower stress susceptibility during subsequent heat waves. However, this resilience often comes with a trade-off, as Durusdinium may support slower coral growth compared to Cladocopium. This dynamic makes the reefs of New Caledonia a important natural site for understanding how corals might adapt to a warming ocean.