The emission of light by living organisms that is visible to other
organisms. The enzymes and other proteins associated with
bioluminescence have been developed and exploited as markers or
reporters of other biochemical processes in biomedical research.
Bioluminescence provides a unique tool for investigating and
understanding numerous basic physiological processes, both cellular and
organismic.
Although rare in terms of the total number of luminous species, bioluminescence is phylogenetically diverse, occurring in many different groups (see table). Luminescence is unknown in higher plants and in vertebrates above the fishes, and is also absent in several invertebrate phyla. In some phyla or taxa, a substantial proportion of the genera are luminous (for example, ctenophores, about 50%; cephalopods, greater than 50%). Commonly, all members of a luminous genus emit light, but in some cases there are both luminous and nonluminous species.
Bioluminescence
is most prevalent in the marine environment; it is greatest at midocean
depths, where some daytime illumination penetrates. In these locations,
bioluminescence may occur in over 95% of the individuals. Where high
densities of luminous organisms occur, their emissions can exert a
significant influence on the communities and may represent an important
component in the ecology, behavior, and physiology of the latter. Above
and below midocean depths, luminescence decreases to less than 10% of
all individuals and species; among coastal species, less than 2% are
bioluminescent. Firefly displays of bioluminescence are among the most
spectacular, but bioluminescence is rare in the terrestrial environment.
Other terrestrial luminous forms include millipedes, centipedes,
earthworms, and snails, but the display in these is not very bright.
While not metabolically essential, light emission can confer an advantage on the organism. The light can be used in diverse ways. Most of the perceived functions of bioluminescence fall into four categories: defense, offense, communication, and dispersal to enhance propagation.
Bioluminescence does not come from or depend on light absorbed by the organism. It derives from an enzymatically catalyzed chemiluminescence, a reaction in which the energy released is transformed into light energy. One of the reaction intermediates or products is formed in an electronically excited state, which then emits a photon.
Bioluminescence originated and evolved independently many times, and is thus not an evolutionarily conserved function. It has been estimated that present-day luminous organisms come from as many as 30 different evolutionarily distinct origins. In the different groups of organisms, the genes and proteins involved are unrelated, and it may be confusing that the substrates and enzymes, though chemically different, are all referred to as luciferin and luciferase, respectively. To be correct and specific, each should be identified with the organism.
Luminous bacteria typically emit a continuous light, usually blue-green. When strongly expressed, a single bacterium may emit 104 or 105 photons per second. A primary habitat where most species abound is in association with another (higher) organism, dead or alive, where growth and propagation occur. Luminous bacteria are ubiquitous in the oceans and can be isolated from most seawater samples. The most exotic specific associations involve specialized light organs (for example, in fish and squid) in which a pure dense culture of luminous bacteria is maintained. In teleost fishes, 11 different groups carrying such bacteria are known, an exotic example being the flashlight fish.
Of the approximately 70,000 insect genera, only about 100 are classed as luminous. But their luminescence is impressive, especially in the fireflies and their relatives. Fireflies possess ventral light organs on posterior segments; the South American railroad worm, Phrixothrix, has paired green lights on the abdominal segments and red head lights; while the click and fire beetles, Pyrophorini, have both running lights (dorsal) and landing lights (ventral). The dipteran cave glow worm, in a different group and probably different biochemically, exudes beaded strings of slime from its ceiling perch, serving to entrap minute flying prey, which are attracted by the light emitted by the animal. The major function of light emission in fireflies is for communication during courtship, typically involving the emission of a flash by one sex as a signal, to which the other sex responds, usually in a species-specific pattern. The time delay between the two may be a signaling feature; for example, it is precisely 2 s in some North America species. But the flashing pattern is also important in some cases, as is the kinetic character of the individual flash (duration; onset and decay kinetics).
The firefly system was the first in which the biochemistry was characterized. In 1947 it was discovered that adenosine triphosphate (ATP) functions to form a luciferyl adenylate intermediate from firefly luciferin. This then reacts with oxygen to form a cyclic luciferyl peroxy species, which breaks down to yield CO2 and an excited state of the carbonyl product (thus emitting a photon). Luciferase catalyzes both the reaction of luciferin with ATP and the subsequent steps leading to the excited product.
Bioluminescence and chemiluminescence have come into widespread use for quantitative determinations of specific substances in biology and medicine. Luminescent tags have been developed that are as sensitive as radioactivity, and now replace radioactivity in many assays. The biochemistry of different luciferase systems is different, so many different substances can be detected. One of the first, and still widely used, assays involves the use of firefly luciferase for the detection of ATP. The amount of oxygen required for bioluminescence in luminescent bacteria is small, and therefore the reaction readily occurs. Luminous bacteria can be used as a very sensitive test for oxygen, sometimes in situations where no other method is applicable. An oxygen electrode incorporating luminous bacteria has been developed.
Luciferases have also been exploited as reporter genes for many different purposes. Analytically, such systems are virtually unique in that they are noninvasive and nondestructive: the relevant activity can be measured as light emission in the intact cell and in the same cell over the course of time. Examples of the use of luciferase genes are the expression of firely and bacterial luciferases under the control of circadian promoters; and the use of coelenterate luciferase expressed transgenically (in other organisms) to monitor calcium changes in living cells over time. Green fluorescent protein is widely used as a reporter gene for monitoring the expression of some other gene under study, and for how the expression may differ, for example at different stages of development or as the consequence of some experimental procedure.
Although rare in terms of the total number of luminous species, bioluminescence is phylogenetically diverse, occurring in many different groups (see table). Luminescence is unknown in higher plants and in vertebrates above the fishes, and is also absent in several invertebrate phyla. In some phyla or taxa, a substantial proportion of the genera are luminous (for example, ctenophores, about 50%; cephalopods, greater than 50%). Commonly, all members of a luminous genus emit light, but in some cases there are both luminous and nonluminous species.
| Group | Features of luminous displays |
|---|---|
| Bacteria | Organisms glow constantly; system is autoinduced |
| Fungi | Mushrooms and mycelia produce constant dim glow |
| Dinoflagellates | Flagellated algae flash when disturbed |
| Coelenterates | Jellyfish, sea pansies, and comb jellies emit flashes |
| Annelids | Marine worms and earthworms exude luminescence |
| Mollusks | Squid and clams exude luminous clouds; also have |
| photophores | |
| Crustacea | Shrimp, copepods, ostracodes; exude luminescence; |
| also have photophores | |
| Insects | Fireflies (beetles) emit flashes; flies (Diptera) glow |
| Echinoderms | Brittle stars emit trains of rapid flashes |
| Fish | Many bony and cartilaginous fish are luminous; |
| some use symbiotic bacteria; others are self- | |
| luminous; some have photophores |
While not metabolically essential, light emission can confer an advantage on the organism. The light can be used in diverse ways. Most of the perceived functions of bioluminescence fall into four categories: defense, offense, communication, and dispersal to enhance propagation.
Bioluminescence does not come from or depend on light absorbed by the organism. It derives from an enzymatically catalyzed chemiluminescence, a reaction in which the energy released is transformed into light energy. One of the reaction intermediates or products is formed in an electronically excited state, which then emits a photon.
Bioluminescence originated and evolved independently many times, and is thus not an evolutionarily conserved function. It has been estimated that present-day luminous organisms come from as many as 30 different evolutionarily distinct origins. In the different groups of organisms, the genes and proteins involved are unrelated, and it may be confusing that the substrates and enzymes, though chemically different, are all referred to as luciferin and luciferase, respectively. To be correct and specific, each should be identified with the organism.
Luminous bacteria typically emit a continuous light, usually blue-green. When strongly expressed, a single bacterium may emit 104 or 105 photons per second. A primary habitat where most species abound is in association with another (higher) organism, dead or alive, where growth and propagation occur. Luminous bacteria are ubiquitous in the oceans and can be isolated from most seawater samples. The most exotic specific associations involve specialized light organs (for example, in fish and squid) in which a pure dense culture of luminous bacteria is maintained. In teleost fishes, 11 different groups carrying such bacteria are known, an exotic example being the flashlight fish.
Of the approximately 70,000 insect genera, only about 100 are classed as luminous. But their luminescence is impressive, especially in the fireflies and their relatives. Fireflies possess ventral light organs on posterior segments; the South American railroad worm, Phrixothrix, has paired green lights on the abdominal segments and red head lights; while the click and fire beetles, Pyrophorini, have both running lights (dorsal) and landing lights (ventral). The dipteran cave glow worm, in a different group and probably different biochemically, exudes beaded strings of slime from its ceiling perch, serving to entrap minute flying prey, which are attracted by the light emitted by the animal. The major function of light emission in fireflies is for communication during courtship, typically involving the emission of a flash by one sex as a signal, to which the other sex responds, usually in a species-specific pattern. The time delay between the two may be a signaling feature; for example, it is precisely 2 s in some North America species. But the flashing pattern is also important in some cases, as is the kinetic character of the individual flash (duration; onset and decay kinetics).
The firefly system was the first in which the biochemistry was characterized. In 1947 it was discovered that adenosine triphosphate (ATP) functions to form a luciferyl adenylate intermediate from firefly luciferin. This then reacts with oxygen to form a cyclic luciferyl peroxy species, which breaks down to yield CO2 and an excited state of the carbonyl product (thus emitting a photon). Luciferase catalyzes both the reaction of luciferin with ATP and the subsequent steps leading to the excited product.
Bioluminescence and chemiluminescence have come into widespread use for quantitative determinations of specific substances in biology and medicine. Luminescent tags have been developed that are as sensitive as radioactivity, and now replace radioactivity in many assays. The biochemistry of different luciferase systems is different, so many different substances can be detected. One of the first, and still widely used, assays involves the use of firefly luciferase for the detection of ATP. The amount of oxygen required for bioluminescence in luminescent bacteria is small, and therefore the reaction readily occurs. Luminous bacteria can be used as a very sensitive test for oxygen, sometimes in situations where no other method is applicable. An oxygen electrode incorporating luminous bacteria has been developed.
Luciferases have also been exploited as reporter genes for many different purposes. Analytically, such systems are virtually unique in that they are noninvasive and nondestructive: the relevant activity can be measured as light emission in the intact cell and in the same cell over the course of time. Examples of the use of luciferase genes are the expression of firely and bacterial luciferases under the control of circadian promoters; and the use of coelenterate luciferase expressed transgenically (in other organisms) to monitor calcium changes in living cells over time. Green fluorescent protein is widely used as a reporter gene for monitoring the expression of some other gene under study, and for how the expression may differ, for example at different stages of development or as the consequence of some experimental procedure.
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