The Algae Inside a Coral

[A] A reef-building coral is not one organism but two. The animal itself is a small polyp that builds a limestone cup around its body. Living inside its tissues, in enormous numbers, are single-celled algae. [B] The algae photosynthesise, and most of the sugar they make passes to the polyp, which in return supplies them with shelter and with the waste compounds they need.

The arrangement explains a great deal about where reefs are found. Because the algae need light, reefs grow only in shallow water, and because they are inefficient in cloudy water, only where the sea is clear. [C] Clear tropical water is also famously poor in nutrients, which is why a reef in such a place has been called an oasis in a desert. The partnership is what makes the oasis possible. [D] It is also the point of failure: when the water warms by a degree or two above the usual summer maximum, the polyps expel the algae, lose their colour and, if the algae do not return within a few weeks, starve. A reef that has bleached in this way can recover, but only if the warm spell is short.