Bioluminescence

Bioluminescence

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Bioluminescence is the emission of visible light by biological systems, which arises from enzyme-catalyzed chemical reactions. Bioluminescence can be distinguished from chemiluminescence in that it occurs in living organisms and requires an enzyme catalyst . These chemical-dependent emissions of light differ from fluorescence and phosphorescence, which involve the absorption of light by a compound followed by emission of light at a lower energy (higher wavelength) from the excited state of the molecule. The excited molecule produced during bioluminescence reactions, however, is

analogous to that produced during fluorescence, and consequently the luminescence emission spectrum can often be related to the fluorescence emission spectrum. It should also be noted that the processes of fluorescence and phosphorescence also occur in living organisms and should not be confused with bioluminescence.

Bioluminescence has been observed in many organisms and phyla throughout the terrestrial and aquatic worlds, with the majority of luminescent organisms being found in the ocean. Because of the ease with which light can be detected, recorded observations of bioluminescence extend back several thousand years. Both the ancient Chinese and the ancient Greeks recorded luminescence sightings. Aristotle, in the fourth century b.c.e., wrote that "some things, though they are not in their nature fire, nor any species of fire, yet seem to produce light."

Luminescent species are found among marine and terrestrial bacteria, annelids or segmented worms (e.g., fireworms), beetles (e.g., fireflies, click beetles, railroad worms), algae (e.g., dinoflagellates), crustaceans (e.g., shrimp, ostracod), mollusks (e.g., squid, clams, limpets), coelenterates (e.g., jellyfish, sea pansies, hydroids), bony fish (e.g., hatchet fish, flashlight fish, pony fish), and cartilaginous fish (e.g., sharks). Luminescent vertebrates (except for certain fish), mammals, higher plants, and viruses do not exist—except for those versions created by recombinant technology.

Most, if not all, bioluminescence reactions have oxygen as a common reactant and a conjugated system as part of one of the substrates—both needed to generate molecules in an excited state, leading to the emission of light in the visible region. However, the bioluminescence reactions in some organisms are quite different from those in other organisms, and consequently the enzymes catalyzing the reactions (luciferases) and the substrates (often but not always referred to as luciferins) are also quite

distinct. Four bioluminescence systems (fireflies, dinoflagellates, bacteria, and imidazolopyrazine-based e. g., coelenterates) have been studied in greatest detail, and their chemical reactions reflect both their differences and their common features.

Beetles/Fireflies

Luciferases from click beetles, fireflies, and railway worms catalyze the ATP-dependent decarboxylation of luciferin (Figure 1). An AMP derivative of luciferin is formed, which subsequently reacts with O₂. Cleavage of this dioxy derivative results in the emission of light characterized by wavelengths ranging from 550 nanometers (2.17 × 10⁻⁵ inches; green) to 630 nanometers (2.48 × 10⁻⁵ red, depending on the particular luciferase), and the release of CO₂. Fireflies generally emit in the yellow to green range, as part of a courtship process; click beetles emit green to orange light; whereas railway worms emit red light, with green light being emitted on movement.

Dinoflagellates

Much of the brightness that is observed on the surface of the oceans is due to the bioluminescence of certain species of dinoflagellates, or unicellular algae, and this bioluminescence accounts for many of the recorded observations that have described the apparent "phosphorescence" of the sea. Dinoflagellates are very sensitive to motion induced by ships or fish, and respond with rapid and brilliant flashes, thus causing the glow that is sometimes seen in the wake of a ship. The luciferin in these instances is a tetrapyrrole containing four five-member rings of one nitrogen and four carbons, and its oxidation , catalyzed by dinoflagellate luciferase, results in blue-green light centered at about 470 nanometers (1.85 × 10⁻⁵ inches; Figure 2).

Bacteria

Bacterial luciferase catalyzes the reaction of reduced flavin mononucleotide (FMNH₂) with O₂ to form a 4a-peroxyflavin derivative that reacts with a

long chain aldehyde leading to the emission of blue-green light (490 nanometers, or 1.93 × 10⁻⁵ inches) and the formation of riboflavin phosphate (FMN; the phosphorylated form of vitamin B₂), H₂O, and the corresponding fatty acid (Figure 3). Luminescent bacteria are found throughout the marine environment, living free, in symbiosis, or in the gut of marine organisms (including many fish and squid), as well as in the terrestrial environment as symbionts of nematodes.

The luciferins believed to be the most widespread among phyla living in the ocean have structures based on imidazolopyrazine, for example, coelenterazine, found in luminescent coelenterates contains imidazolopyrazine as its central bicyclic ring (Figure 4). The typical reaction involves the oxidation of the imidazolopyrazine ring with the emission of blue light (460–480 nanometers, or 1.81 × 10⁻⁵–1.89 × 10 ⁻⁵ inches), and proceeds according to a mechanism that is very similar to that of the oxidation of firefly luciferin. Among the most commonly studied imidazolopyrazine-utilizing organisms are species of Renilla (sea pansy) and Aequorea (jellyfish) both of which utilize coelenterazine. The luciferin of a crustacean (Cypridina or

Vargula ) also is an imidazolopyrazine-based compound related to coelenterazine. The luciferases of the luminescent species, however, vary widely. Recent evidence suggests that some, and possibly many, marine luminescent organisms (including the jellyfish) acquire luciferins via the ingestion of other luminescent organisms, which would account for the widespread distribution of imidazolopyrazine-based luciferins. Many luminescent species also have a binding protein that releases the luciferin upon Ca⁺⁺ uptake, while some have a fluorescence protein that absorbs and then emits light at a higher wavelength.

Although other luminescent systems have been studied (including those of the fireworm and the limpet, both of which use aldehydes as luciferins), bioluminescence remains somewhat mysterious. Elucidation of the chemical and biological bases for luminescence systems in other organisms should improve understanding of why the remarkable and beautiful phenomenon of bioluminescence appears in so many species.

see also Chemiluminescence.

Edward A. Meighen

Bibliography

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