Bioluminescence, the production of light by living organisms as the result of chemical changes. A wide variety of lower animals and some species of fungi, bacteria, and algae emit light. They contain luciferin, a substance that emits light in certain chemical reactions, and luciferase, an enzyme that controls these reactions. The light thus emitted is often called cold light because very little heat is produced. Luciferase and luciferin, when combined with adenosine triphosphate (ATP), the source of energy in human cells, form a substance, called the luciferase-luciferin-ATP complex, that emits a greenish glow. The intensity of the light produced is proportional to the amount of ATP present. The complex is used in a variety of medical tests.

Light produced by fungi and bacteria is continuous in intensity; animals and algae produce light in flashes. Photophores, light-emitting organs, are responsible for the glow seen in fireflies and certain other insects and in various deep-sea animals, including certain jellyfish, squid, and fish. Colonies of bacteria produce light in such fish as flashlight fish and anglerfish.