Bioluminescence is light produced by a chemical reaction within a living organism. Bioluminescence is a type of chemiluminescence, which is simply the term for a chemical reaction where light is produced. (Bioluminescence is chemiluminescence that takes place inside a living organism.)

Bioluminescence is a "cold light." Cold light means less than 20% of the light generates thermal radiation, or heat.

Most bioluminescent organisms are found in the ocean. These bioluminescent marine species include fish, bacteria, and jellies. Some bioluminescent organisms, including fireflies and fungi, are found on land. There are almost no bioluminescent organisms native to freshwater habitats.

Chemistry

The chemical reaction that results in bioluminescence requires two unique chemicals: luciferin and either luciferase or photoprotein. Luciferin is the compound that actually produces light. In a chemical reaction, luciferin is called the substrate. The bioluminescent color (yellow in fireflies, greenish in lanternfish) is a result of the arrangement of luciferin molecules.

Some bioluminescent organisms produce (synthesize) luciferin on their own. Dinoflagellates, for instance, bioluminesce in a bluish-green color. Bioluminescent dinoflagellates are a type of plankton—tiny marine organisms that can sometimes cause the surface of the ocean to sparkle at night.

Some bioluminescent organisms do not synthesize luciferin. Instead, they absorb it through other organisms, either as food or in a symbiotic relationship. Some species of midshipman fish, for instance, obtain luciferin through the "seed shrimp" they consume. Many marine animals, such as squid, house bioluminescent bacteria in their light organs. The bacteria and squid have a symbiotic relationship.

Luciferase is an enzyme. An enzyme is a chemical (called a catalyst) that interacts with a substrate to affect the rate of a chemical reaction. The interaction of the luciferase with oxidized (oxygen-added) luciferin creates a byproduct, called oxyluciferin. More importantly, the chemical reaction creates light. Bioluminescent dinoflagellates produce light using a luciferin-luciferase reaction. The luciferase found in dinoflagellates is related to the green chemical chlorophyll found in plants.

Bioluminescent dinoflagellate ecosystems are rare, mostly forming in warm-water lagoons with narrow openings to the open sea. Bioluminescent dinoflagellates gather in these lagoons or bays, and the narrow opening prevents them from escaping. The whole lagoon can be illuminated at night. Biologists identified a new bioluminescent dinoflagellate ecosystem in the Humacao Natural Reserve, Puerto Rico, in 2010.

Most bioluminescent reactions involve luciferin and luciferase. Some reactions, however, do not involve an enzyme (luciferase). These reactions involve a chemical called a photoprotein. Photoproteins combine with luciferins and oxygen, but need another agent, often an ion of the element calcium, to produce light.

Photoproteins were only recently identified, and biologists and chemists are still studying their unusual chemical properties. Photoproteins were first studied in bioluminescent crystal jellies found off the west coast of North America. The photoprotein in crystal jellies is called "green fluorescent protein" or GFP.

Bioluminescence is not the same thing as fluorescence, however. Florescence does not involve a chemical reaction. In fluorescence, a stimulating light is absorbed and re-emitted. The fluorescing light is only visible in the presence of the stimulating light. The ink used in highlighter pens is fluorescent. Phosphorescence is similar to florescence, except the phosphorescent light is able to re-emit light for much longer periods of time. Glow-in-the-dark stickers are phosphorescent.

Bioluminescent Light

The appearance of bioluminescent light varies greatly, depending on the habitat and organism in which it is found.

Most marine bioluminescence, for instance, is expressed in the blue-green part of the visible light spectrum. These colors are more easily visible in the deep ocean. Also, most marine organisms are sensitive only to blue-green colors. They are physically unable to process yellow, red, or violet colors.

Most land organisms also exhibit blue-green bioluminescence. However, many glow in the yellow spectrum, including fireflies and the only known land snail to bioluminesce, Quantula striata, native to the tropics of Southeast Asia.

Few organisms can glow in more than one color. The so-called railroad worm (actually the larva of a beetle) may be the most familiar. The head of the railroad worm glows red, while its body glows green. Different luciferases cause the bioluminescence to be expressed differently.

Some organisms emit light continuously. Some species of fungi present in decaying wood, for instance, emit a fairly consistent glow, called foxfire.

Most organisms, however, use their light organs to flash for periods of less than a second to about 10 seconds. These flashes can occur in specific spots, such as the dots on a squid. Other flashes can illuminate the organism's entire body.

Adaptations

Bioluminescence is used by living things to hunt prey, defend against predators, find mates, and execute other vital activities.

Defensive Adaptations

Some species luminesce to confuse attackers. Many species of squid, for instance, flash to startle predators, such as fish. With the startled fish caught off guard, the squid tries to quickly escape.

The vampire squid exhibits a variation of this defensive behavior. Like many deep-sea squid, the vampire squid lacks ink sacs. (Squid that live near the ocean surface eject dark ink to leave their predators in the dark.) Instead, the vampire squid ejects sticky bioluminescent mucus, which can startle, confuse, and delay predators, allowing the squid to escape.

Many marine species use a technique called counterillumination to protect themselves. Many predators, such as sharks, hunt from below. They look above, where sunlight creates shadows beneath prey. Counterillumination is a type of camouflage against this predatory behavior.

Hatchetfish use counterillumination. Hatchetfish have light-producing organs that point downward. They adjust the amount of light coming from their undersides to match the light coming from above. By adjusting their bioluminescence, they disguise their shadows and become virtually invisible to predators looking up.

Some bioluminescent animals, such as brittle stars, can detach body parts to distract predators. The predator follows the glowing arm of the brittle star, while the rest of the animal crawls away in the dark. (Brittle stars, like all sea stars, can re-grow their arms.)

When some animals detach body parts, they detach them on other animals. When threatened, some species of sea cucumber can break off the luminescent parts of their bodies onto nearby fish. The predator will follow the glow on the fish, while the sea cucumber crawls away.

Biologists think that some species of sharks and whales may take advantage of defensive bioluminescence, even though they are not bioluminescent themselves. A sperm whale, for instance, may seek out a habitat with large communities of bioluminescent plankton, which are not part of the whale's diet. As the plankton's predators (fish) approach the plankton, however, their glowing alerts the whale. The whale eats the fish. The plankton then turn out their lights.

Some insect larvae (nicknamed "glow worms") light up to warn predators that they are toxic. Toads, birds, and other predators know that consuming these larvae will result in illness and possible death.

Offensive Adaptations

Bioluminescence may be used to lure prey or search for prey.

The most famous predator to use bioluminescence may be the anglerfish, which uses bioluminescence to lure prey. The anglerfish has a huge head, sharp teeth, and a long, thin, fleshy growth (called a filament) on the top of its head. On the end of the filament is a ball (called the esca) that the anglerfish can light up. Smaller fish, curious about the spot of light, swim in for a closer look. By the time the prey sees the enormous, dark jaws of the anglerfish behind the bright esca, it may be too late.

Other fish, such as a type of dragonfish called loosejaws, use bioluminescence to search for prey. Loosejaws have adapted to emit red light; most fish can only see blue light, so loosejaws have an enormous advantage when they light up a surrounding area. They can see their prey, but their prey can't see them.

Attraction

Adult fireflies, also called lightning bugs, are bioluminescent. They light up to attract mates. Although both male and female fireflies can luminesce, in North America most flashing fireflies are male. The pattern of their flashes tells nearby females what species of firefly they are and that they're interested in mating.

Other Bioluminescence

Organisms can luminesce when they are disturbed. Changes in the environment, such as a drop in salinity, can force bioluminescent algae to glow, for instance. These living lanterns can be seen as spots of pink or green in the dark ocean.

"Milky seas" are another example of bioluminescence. Unlike bioluminescent algae, which flash when their environment is disturbed, milky seas are continuous glows, sometimes bright and large enough to be visible from satellites in orbit above the Earth.

Scientists think milky seas are produced by bioluminescent bacteria on the surface of the ocean. Millions of bacteria must be present for milky seas to form, and conditions must be right for the bacteria to have enough chemicals to light up. Satellite imagery of milky seas have been captured in tropical waters such as the Indian Ocean.

Bioluminescence and People

Biologists and engineers are studying the chemicals and circumstances involved in bioluminescence to understand how people can use the process to make life easier and safer.

Green fluorescent protein (GFP), for instance, is a valuable "reporter gene." Reporter genes are chemicals (genes) that biologists attach to other genes they are studying. GFP reporter genes are easily identified and measured, usually by their fluorescence. This allows scientists to trace and monitor the activity of the studied gene—its expression in a cell, or its interaction with other chemicals.

Other uses are more experimental. Bioluminescent trees, for instance, could help light city streets and highways. This would reduce the need for electricity. Bioluminescent crops and other plants could luminesce when they needed water or other nutrients, or when they were ready to be harvested. This would reduce costs for farmers and agribusiness.