How coral turns sunlight into food
A reef-building coral is an animal, but much of its energy comes from microscopic algae living inside its tissues. These algae—often called zooxanthellae—use sunlight to make sugars and other compounds. The coral uses some of that energy to grow and build its hard skeleton.
The partnership is a trade, not simply a case of algae being held captive. Coral tissue gives the algae shelter and access to carbon dioxide; coral waste also supplies nitrogen and phosphorus, nutrients the algae need. In return, the algae transfer much of the energy they make to the coral. This arrangement helps corals thrive in clear tropical waters, where dissolved nutrients are scarce and there is relatively little plankton for them to eat.
Why heat can make a reef turn white
Photosynthesis depends on a finely balanced process. When seawater stays unusually warm, that process can become disrupted, producing harmful molecules inside the partnership. The coral may then expel many of its algae or lose their pigments. With much less color hiding it, the coral’s white limestone skeleton shows through: the coral is bleached, not painted white.
Bleaching is a crisis, but it does not mean every coral dies at once. If temperatures return to normal soon enough, a coral can regain algae and recover. If heat stress continues, however, the coral is left with far less energy and may starve, become more vulnerable to disease, or die. That is why a brief-looking change in color can signal a much longer threat to the reef.
