Corals are living animals that construct coral reefs, diverse underwater ecosystems. They are home to countless marine species, providing shelter and sustenance. Tiny algae exist in close association with corals. This connection supports the health of these underwater habitats.
The Symbiotic Partnership
Coral polyps engage in a mutually beneficial relationship with microscopic algae called zooxanthellae. These single-celled dinoflagellates live within coral polyp tissues. This symbiotic arrangement benefits both organisms.
The coral provides the zooxanthellae with a protected environment, shielding them from predators and harshness. The coral also supplies the algae with compounds needed for photosynthesis, such as carbon dioxide and metabolic waste products (e.g., nitrogen and phosphorus). This nutrient recycling is effective in the often nutrient-poor waters where coral reefs thrive.
The Algae’s Role in Coral Health
Through photosynthesis, the zooxanthellae convert sunlight into energy, producing sugars, oxygen, and other organic compounds. A significant portion (up to 90%) of these photosynthetically produced nutrients transfers directly to the coral host. These energy-rich compounds (e.g., glucose, glycerol, amino acids) are used by the coral for metabolic processes, growth, and reproduction.
Algal nutrients are also important for the coral’s ability to build its calcium carbonate skeleton, forming the reef’s physical structure. Zooxanthellae also provide the vibrant colors seen in many stony corals. Without this consistent food supply, most reef-building corals cannot survive and would eventually starve.
When the Partnership Breaks Down
Coral bleaching occurs when corals expel the zooxanthellae from their tissues, causing the coral to turn stark white as its underlying calcium carbonate skeleton becomes visible. This expulsion is a stress response, primarily triggered by elevated ocean temperatures. Even a slight increase (as little as one degree Celsius) can induce bleaching.
Other stressors can also contribute to this breakdown, including ocean acidification, pollution from land-based runoff, and diseases. Without their primary food source, corals become vulnerable to starvation and increased susceptibility to diseases. While a bleached coral may not die immediately, prolonged stressful conditions often lead to mortality.
Consequences of Coral Bleaching
Widespread coral bleaching has significant ecological impacts on the entire reef ecosystem. The loss of living corals directly impacts thousands of species of fish, invertebrates, and other marine life that depend on reefs for shelter, food, and breeding grounds. This leads to a decline in biodiversity and can cause a shift from complex coral-dominated reefs to less diverse, algae-dominated environments.
Beyond ecological impacts, coral bleaching also affects human livelihoods and coastal protection. Reefs provide natural barriers that reduce wave energy and protect coastlines from erosion and storm surges. The degradation of reef structures due to bleaching can increase coastal flood risk, potentially costing millions or billions annually in damages and protection. Additionally, the decline in fish populations impacts fisheries, affecting food security and income for many communities globally relying on reef-associated seafood and tourism.