Bioluminescence refers to the phenomenon of organisms producing light through chemical processes, and organisms that have organs capable of producing light themselves are called luminescent organisms. It is widely distributed across a wide range of organisms, including marine vertebrates and invertebrates, as well as fungi, bacteria, and terrestrial invertebrates.
Bioluminescence is a biochemical process in which the main pigment of bioluminescence, luciferin, is oxidized to produce light through the catalysis of luciferase. Because the luminous efficiency is as high as 80-90%, unlike artificial light, it generates very little heat, so the light from bioluminescence is also called cold light.
Origins and history
Before the development of safety lamps for use in coal mines, dried fish skins were used as a weak light source in England and Europe. This experimental form of lighting avoided the use of candles, which could cause explosions. Another safe form of lighting used in mines was jars containing fireflies.
In 1920, American zoologist Newton Harvey published a paper called “The Nature of Animal Light,” which summarized early research on biophotonics1). According to this paper, in about 410 BC, Aristotle wrote about the light produced by dead fish and wet wood. In addition, Robert Boyle experimented with these sources of light and showed that air was required to produce light. In 1753, Jay Baker discovered the first luminescent creature that could be seen with the naked eye: the noctuid worm.
In the late 19th century, French pharmacologist Raphaël Dubois studied beetles (Pyrophorus) and marine mollusks (Pholas dactylus), refuting the conventional view that bioluminescence was derived from phosphorus and proving that it was related to the oxidation of a specific compound called luciferin by an enzyme. Crystals of luciferin were first obtained from the sea-firefly (scientific name: Vargula hilgendorfii) by Japanese chemist Osamu Shimomura, who, along with Martin Chalfie and Roger Tsien, discovered and developed green fluorescent protein (GFP) as a tool for biological research in 1961 and was awarded the Nobel Prize in Chemistry in 2008.
Distribution
Bioluminescence is widely found in marine organisms, including saltwater fish, jellyfish, comb jellies (phylum Ctenophora), crustaceans and cephalopods, and mollusks (Figure 1). Bioluminescence is also observed in a variety of terrestrial invertebrates, including some fungi and bacteria (Allivibrio fisheri is a prime example), as well as insects. About 76% of the major taxonomic groups of deep-sea animals glow either on their own or by luminescent symbionts.
Most marine bioluminescence is in the blue and green light spectrum. However, some deep-sea organisms also emit infrared or yellow light. The most common bioluminescent organisms are dinoflagellates, which live on the surface of the ocean and can often be seen at night on waves. Although not as widely distributed as marine organisms, terrestrial invertebrates such as fireflies and glow worms also have the ability to bioluminesce.
How it works
Bioluminescence is a form of chemiluminescence in which light energy is released by a chemical reaction. Bioluminescent organisms produce the luminescent substance luciferin and the enzyme luciferase, and under the catalysis of luciferase, luciferin reacts with ATP and oxygen to produce carbon dioxide (CO2) and light.
The molecular structure of luciferin varies depending on the type of luminescent organism, but the basic principle of oxidation to produce light is the same regardless of the type of organism. Luciferases are known to catalyze reactions that can be mediated by cofactors such as calcium (Ca2+) or magnesium (Mg2+) ions.
There are two types of luminescence: intracellular luminescence, in which the luminescent substance is produced inside the cell, as in the case of nocturnal insects and fireflies, and extracellular luminescence, in which the luminescent substance is secreted outside the cell, as in the case of the midges and the woolly midges (Marphysa sanguinea).
Midges (Chaetopterus) and jellyfish (Aequorea Victoria) use a different photoprotein, aequorin, instead of luciferase for their luminescence. Calcium ions bind to the aequorin protein and induce a conformational change in the protein, and the luminescent substance coelenterazine, which has a similar structure to luciferase, oxidizes inside the protein and emits blue light.
Biological significance
The biological significance of luminescence varies somewhat from organism to organism and is not clear in all cases. In animals, luminescence helps attract prey, provide external defense, and communicate with the opposite sex during courtship. In particular, the role of luminescence in courtship behavior has been well studied in polychaetes and fireflies.
Deep-sea organisms use light for a variety of purposes. The main reasons are to catch prey, defend against predators, and communicate with each other. Bioluminescent bacteria live in symbiosis with the luminescent organs of animals, which has the ecological advantage of allowing them to grow rapidly, efficiently, and in high concentrations, using the organic matter ingested by the animal as nutrients, rather than expending the high energy required for luminescence.
Microbial bioluminescence
Bioluminescent bacteria are microorganisms that possess a set of genes called the lux operon, which is the luciferin-luciferase system of microorganisms. To date, luminescent bacteria have been found in the families Vibrionaceae, Enterobacteriaceae, and Shewanellaceae, which belong to the class Gammaproteobacteria, with most species belonging to the family Vibrionaceae. A representative luminescent bacterium is Aliivibrio fischeri.
A. fischeri is a rod-shaped, gram-negative proteobacterium commonly found in marine environments that has a bioluminescent organelle and forms a symbiotic relationship with marine animals such as the short-tailed cuttlefish. The light emitted by the symbiotic A. fischeri reduces their exposure to moonlight in the ocean and allows them to avoid major predators.
The bioluminescence of A. fischeri is triggered by gene transcription driven by quorum sensing. The microorganism’s luciferin-luciferase system is regulated by a set of genes called the ‘Lux operon’. The Lux operon in A. fischeri consists of five genes (luxCDABE) that have been shown to be involved in the synthesis of bioluminescent substances, and luxR and luxI, which are known to regulate the expression of these genes.
LuxI produces a derivative called N-(3-oxohexanoyl) homoserine lactone, which is exported from the cell. When the population density increases, the inducer produced by luxI enters the cell and binds to the transcriptional regulator LuxR to activate the lux operon. luxR, the transcriptional regulator of luxCDABE, has its N-terminus bound to the plasma membrane of the cell at low population density, which prevents its DNA-binding C-terminus from binding to the lux promoter, resulting in the synthesis of the luminescent substance.
However, as the population increases, the intracellular concentration of the inducer produced by luxI increases and this inducer binds to the N-terminus of LuxR, freeing the C-terminus of LuxR from the cytoplasmic membrane to bind to the lux promoter and increase the transcription of genes involved in luminescent substance synthesis. Thus, the luminescence of A. fischeri only appears when the population density reaches a certain level.
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