II.

How Light Is Produced

Bioluminescent light is produced when a substance called a luciferin is oxidized in a chemical reaction with molecular oxygen. The luciferin releases light and becomes an inactive oxyluciferin that must be replaced by the organism. The organism may replenish luciferin by synthesizing the chemical itself or by eating other organisms that contain luciferin. An enzyme called a luciferase acts as a catalyst and helps speed up the chemical reaction that releases light but the luciferase is not chemically changed itself. The chemical reaction that releases light produces virtually no heat.

The chemicals that occur as luciferins and luciferases are extremely diverse. Such variety suggests that bioluminescence has evolved independently many times among different types of living things. The chemical reactions may have first evolved as mechanisms to protect cells from oxidative damage (harmful chemical reactions with oxygen). Bioluminescence was a side effect that turned out to be useful to the organism in some way.

Bioluminescence may be blue, green, yellow, orange, or red. Land-living organisms tend to produce yellow or green light, and marine organisms blue or green light. In the deep sea most bioluminescence is blue, the wavelength of light transmitted best by ocean waters. Most deep-sea animals can only see blue light but a few fish, such as the black dragonfish, can see and produce both blue and red light. Because their red bioluminescence is invisible to most deep-sea animals, such fishes may use their red light for communication or predation. In shallow waters bioluminescence tends to be blue-green.

Bioluminescence should not be confused with phosphorescence or fluorescence, which occurs when certain materials absorb energy and give off light (see Luminescence). Bioluminescence is also distinct from iridescence in which colors are produced by interference patterns that break up white light.