Corals bleach when stress disrupts their relationship with microscopic algae living in their tissues. The algae provide much of a coral’s energy and help give it color. When the algae are expelled or lose their pigments, the coral’s pale skeleton shows through. A bleached coral is stressed, but it is not necessarily dead.
A coral and its algae live together
Many reef-building corals host tiny photosynthetic algae in their tissues. The algae use sunlight to make food, and the coral receives some of that energy. In return, the algae gain shelter and access to nutrients. The partnership helps coral build reefs in clear, sunlit tropical waters. [1]
Stress can disrupt the partnership
When conditions change—often when water remains unusually warm—corals can expel the algae or the algae can lose pigments. The coral then looks white because its translucent tissue no longer masks the pale calcium-carbonate skeleton. Temperature is a leading cause, but changes in light, nutrients, pollution, or unusually cold conditions can also contribute. [1]
Bleached is not the same as dead
A bleached coral is under stress and has lost an important energy source. If conditions return to a suitable range and the stress is not prolonged, some corals can regain algae and recover. If stress continues, the coral is more vulnerable to disease and may die. NOAA’s explanation is careful about the distinction: bleaching is a warning sign, not a final diagnosis for every coral. [2]
Frequently asked questions
Does coral bleaching mean the coral is dead?
No. Bleached coral is stressed and pale, but it may survive if conditions improve.
Is warm water the only cause?
No. Heat is a major cause, but other stresses such as unusual cold, intense light, or pollution can also contribute.
Are corals plants?
No. Corals are animals. Many rely on photosynthetic algae living inside their tissues.
Sources and evidence
- NOAA Ocean Service — What Is Coral Bleaching? ↗
Defines bleaching, describes coral–algae symbiosis, and distinguishes bleaching from death.
- NOAA Coral Reef Conservation Program ↗
Educational background on stress and bleaching in corals.
- NOAA — Coral Reef Ecosystems ↗
Context on coral reef ecosystems and their importance.
Sources consulted: 4 November 2026. Written with AI assistance and checked against the linked references; not independently reviewed by a subject specialist. Sources do not endorse this article. See our editorial policy.
Revision: New explainer; makes clear that bleaching does not automatically mean coral death.