A Natural Climate Change Experiment
Coral bleaching occurs when rising sea temperatures cause coral polyps to expel the symbiotic algae living in their tissues. These algae, known as zooxanthellae, provide the coral with most of its food and its color. Without them, the coral turns white and can starve. In 2010, a severe marine heatwave swept through the Andaman Sea, causing widespread coral bleaching. Sea temperatures hovered around 30 degrees Celsius (86 Fahrenheit), leading to mortality rates of up to 90% in some reefs. The reefs in Mu Ko Surin National Park were hit hard, with over 50% of corals dying in all surveyed sites.
Yet, amidst the devastation, scientists noticed something unexpected. Certain coral colonies, particularly species of branching Acropora and Pocillopora, survived. This resilience sparked investigations into what made these specific corals different. The answer lies in a microscopic partnership with a particular type of heat-tolerant algae.
The Super-Symbiont
The reason for the survival of these corals is their partnership with a specific genus of dinoflagellate algae called Durusdinium (formerly known as Symbiodinium Clade D). While most corals partner with other algal types, like Cladocopium, some can host Durusdinium. This alga is exceptionally hardy, allowing the coral host to withstand higher water temperatures that would otherwise trigger bleaching.
Corals in the Persian Gulf, the world's hottest sea, rely on a similar relationship with a species named Symbiodinium thermophilum to survive water temperatures up to 36°C (97°F). The algae in Mu Ko Surin seem to provide a similar advantage. Corals that host Durusdinium show reduced stress during heatwaves. This ability to "shuffle" symbionts, favoring the hardier Durusdinium when temperatures rise, is an important adaptation mechanism. It suggests that some coral populations have a built-in defense system that may help them endure in a warming ocean. The Mu Ko Surin reefs benefit from a yearly influx of coral larvae from the Mergui Archipelago in Myanmar, which may help deliver these resilient algal strains.