Coral reefs derive most of their color and a substantial share of their energy from zooxanthellae, single-celled algae that live within coral tissue in a mutualistic partnership. The coral provides the algae with a protected, well-lit habitat and a steady supply of carbon dioxide and nitrogenous waste; the algae, through photosynthesis, supply the coral with sugars and amino acids that can meet up to ninety percent of its metabolic needs. This exchange allows reef-building corals to thrive in nutrient-poor tropical waters that would otherwise support little complex animal life. The partnership is delicately temperature-sensitive. When sea surface temperatures rise even one or two degrees above the local summer maximum for several consecutive weeks, the photosynthetic machinery inside the zooxanthellae begins to malfunction, generating reactive oxygen compounds that damage both the algae's cells and the surrounding coral tissue. Rather than tolerate this toxic byproduct, the coral host expels the algae en masse, a process that strips the tissue of its pigment and reveals the white calcium carbonate skeleton beneath, the event popularly known as bleaching. Bleaching is not itself death. A coral colony that expels its algae can survive for weeks on stored energy reserves and can, if temperatures fall again promptly, reacquire zooxanthellae and recover close to full health. But prolonged or repeated bleaching leaves little time for recovery between events, and a starved, algae-free coral becomes markedly more vulnerable to disease and to overgrowth by faster-growing competitors such as fleshy algae, which can permanently occupy the reef substrate once coral cover declines.