Why Coral Reefs Turn White

Coral reefs, often called the rainforests of the sea, support around a quarter of all marine species despite covering less than one per cent of the ocean floor. In recent decades, however, reefs around the world have suffered a phenomenon known as bleaching, in which corals lose their vivid colour and, if conditions do not improve quickly, eventually die. The colour of a healthy coral comes not from the coral animal itself but from microscopic algae called zooxanthellae that live inside its tissue in a mutually beneficial relationship. The coral provides the algae with a protected environment and the compounds needed for photosynthesis, while the algae supply the coral with the majority of its energy and, incidentally, its colour. When ocean temperatures rise even slightly above the seasonal average for a sustained period, this partnership breaks down. The stressed coral expels the algae, revealing the white, calcium-carbonate skeleton beneath the now-transparent tissue. Bleaching itself is not necessarily fatal. If temperatures return to normal within a matter of weeks, corals can reabsorb algae from the surrounding water and gradually recover their colour and health. The danger lies in prolonged or repeated heat stress, which leaves corals without their main energy source for too long, making them increasingly vulnerable to disease and, eventually, death. A single severe bleaching event can also weaken a reef's capacity to recover from other pressures, such as pollution or physical damage from storms. Scientists studying reef systems have observed that not all coral species respond to heat stress in the same way. Some, particularly slower-growing species with denser skeletons, appear to tolerate higher temperatures for longer periods than fast-growing branching corals. This has led researchers to investigate whether selectively breeding or transplanting more heat-tolerant coral varieties could help reefs adapt to warming seas, although such interventions remain experimental and are not yet used on a large scale. Conservation efforts today still focus primarily on reducing the local pressures that compound the effects of heat stress, such as overfishing and nutrient run-off from agriculture, in the hope of buying reefs more time to adjust.