Coral bleaching
Reef-building corals look like plants or rocks, but they are animals. Each coral colony is made of many tiny polyps, and inside their tissues live microscopic algae. The relationship benefits both partners: the algae receive shelter and nutrients, and in return they use sunlight to produce sugars, much of which they pass to the coral. The algae also give many corals their colour.
When the water becomes unusually warm for a sustained period, this partnership breaks down. The stressed algae can begin to produce substances that are harmful to the coral, and the coral expels them. Without the algae, the coral’s tissue becomes transparent, and the white limestone skeleton shows through. This is known as bleaching.
A bleached coral is not necessarily dead. If temperatures return to normal fairly quickly, algae can recolonise the coral and it may recover. However, a coral without its algae loses its main source of food, so a long period of bleaching often leads to starvation, disease and death.
Bleaching events have become more frequent as ocean temperatures have risen. Scientists are studying why some corals tolerate heat better than others, and whether more heat-tolerant corals could help reefs survive. Most researchers agree, though, that such efforts can only buy time unless the warming itself is reduced.