What causes coral reefs to bleach?

Coral reefs bleach when stressed by environmental changes, such as elevated sea temperatures, pollution, or increased solar radiation. These stressors cause corals to expel the symbiotic algae (zooxanthellae) that live within their tissues and provide nutrients and colour. Without the algae, the corals turn white and are more vulnerable to disease. Prolonged bleaching can lead to coral death, threatening the biodiversity of reef ecosystems that rely on them.