Bioluminescence is light produced by a chemical reaction. This reaction is a type chemiluminescence that runs inside a living organism. As all metabolic activities running in a cell, this process is precisely coordinated and controlled. Thus it is a cold light, this means that it’s more than 80 % effective and less than 20 % of the reaction generates thermal radiation.

Most bioluminescent organisms dwell in the ocean. On land luminous taxa include bacteria, some fungi, various terrestrial invertebrates, including insects. There are almost no bioluminescent organisms in freshwater habitats, with the exception of some insect larvae, a limpet and unsubstantiated reports from depths of Lake Baikal.

In most animals the light is autogenic, produced by the animals themselves, in some it is bacteriogenic, produced by symbiotic bacteria, such as those from the genus Vibrio.

Process explained

Bioluminescent light is energy released during a chemical reaction.

The reaction requires two chemicals:

• a light-emitting molecule (luciferin);
• an enzyme (luciferase or photoprotein – a luciferase variant);
  • luciferase: the enzyme that triggers oxidation of the luciferin;
  • photoprotein: catalysing protein + ion or cofactor (Ca²⁺ or Mg²⁺, also adenosine triphosphate (ATP))

Reaction:

1. oxidation of a light-emitting molecule (a luciferin).
2. Interaction of luciferase with oxidised (oxygen-added) luciferin creates a byproduct, oxyluciferin. This chemical reaction creates light and luciferin is the compound that actually produces light.

These are general names, luciferins and luciferases are distinguished by the species or group (e.g. firefly luciferin). In all cases, the enzyme catalyzes the oxidation of the luciferin.

In evolution luciferins vary little, while luciferases vary widely between different species, which is evidence that bioluminescence has occured over several 10 times in evolutionary history.

Generically, this reaction can be described as:

See chemical reaction here.

Because of the diversity of luciferin/luciferase combinations, there are few commonalities in the chemical mechanism. From systems studied so far, the only unifying part is the use of molecular oxygen, which provides chemical energy. Often there is also a release of carbon dioxide (CO₂).

Example: firefly luciferin/luciferase reaction requires magnesium and ATP. Waste products are CO₂, adenosine monophosphate (AMP) and pyrophosphate (PP). Other cofactors may be required, such as calcium (Ca²⁺) or magnesium (Mg²⁺) ions and ATP for the luciferase. 

The bioluminescent colour (yellow in fireflies, greenish in lanternfish, bluish-green in dinoflagellates) depends on the arrangement of luciferin molecules.

Function

• Defense (warning, startle, misdirection): when exuded or secreted, the light may act as a smoke screen—a cloud of glowing fluid makes it difficult for the predator to track the location of its escaping prey. This behavior is seen in many animals, including copepods, shrimp, fishes, ctenophores and siphonophores.
• Offense (lure prey, illuminate prey, attract host),
• mating (attraction, recognition),
• counterillumination (camouflage).

A list of all known bioluminiscent living organisms:

• Bacteria (e.g. Vibrio)
• Dinoflagellates (e.g. Noctiluca)
• Siphonophores (colonial hydrozoans)
• Ostracoda (class of the Crustacea)
• Radiolarians (ameboid protists)
• Ctenophores (e.g. comb jellies)
• Cnidarians (corals, anemones, hydroids, medusae and siphonophores)
• Annelids (marine polychaetes)
• Molluscs (gastropods, cephalopods)
• Crustaceans (e.g. shrimp)
• Echinoderms (e.g. sea stars, sea cucumbers, sea lilies)
• Tunicates (e.g. salps)
• Fish (e.g. sharks)
• terrestrial invertebrates (centipedes, millipedes, earthworms and a snail)
• insects (fireflies, other beetles, flies)
• fungi, about 80 known species

Bacteria

Bioluminescent bacteria are common in oceans, especially in temperate to warmer waters. Among prokaryotes, light production is known only from the eubacteria, specifically Gram-negative γ-proteobacteria. Best-studied symbiotic bacteria are in the genus Vibrio.

Almost all luminous bacteria have been classified into the three genera: Vibrio, Photobacterium, and Xenorhabdus, with most of the species being marine in nature. Only Xenorhabdus species infect terrestrial organisms.

Land dwelling

Fungi

Currently there are more than 100 known species of bioluminescent fungi, all of which are members of the order Agaricales (Basidiomycota) with one exception of ascomycete.  They grow largely in temperate and tropical climates. All bioluminescent fungi share the same bioluminescent enzymatic mechanism, suggesting that it arose early in the evolution of the mushroom-forming Agaricales (white rot fungi capable of breaking down lignin (in wood)).

A two-stage mechanism is required:

1. luciferin is reduced by a reductase enzyme, NAD(P)H is used,
2. reduced luciferin is oxidised by another luciferase that releases the energy in the form of light.

Conditions that affect the growth of fungi, such as pH, light and temperature also influence bioluminescence. Bioluminescent fungi emit a greenish light at a wavelength of 520–530 nm. The light emission is continuous and occurs only in living cells. You may have seen this if you walked through a temperate forest at night and came across some freshly cut pine, beech or oak trees. The light can be seen under the bark and on pieces of bark fallen off the trunk. I only had a chance to see this once, it was otherworldly. Unfortunately, I didn’t have a camera with me, I returned next night, but there was no bioluminescence anymore.

Possible benefits of bioluminescence for fungi:

• the glow may attract insects that benefit the fungus by dipersing its spores.
• Bioluminescence is an oxygen-dependent metabolic process and may thus provide antioxidant protection against the potentially damaging effects of reactive oxygen produced during wood decay.

Fireflies

Lampyridae are a family of insects in the order Coleoptera (beetles). The light is chemically produced by both larvae and adults. The process occurs in specialized light-emitting organs, usually on the lower abdomen. Their bioluminiscent activity during twilight is to attract mates (adults) and/or prey (larvae and adults). They produce cold light all within wavelengths from 510-670 nm (no ultraviolet or infrared frequencies). The light production isn’t strong in brightness, but its efficiency is 95 %. Meaning, only 5 % of energy is heat. The reason for such efficiency is in the reflective layer, which gathers light and directs it in one direction.

These insexts are poisonous and light is also a way of messaging their predators (bats) to keep away from them. This consequntly proves that bats don’t just hunt by echolocation, but also by sight.

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We can’t talk about fireflies without adding this beautiful song. 🙂

Marine dwelling

Marine organisms use bioluminescence for vital functions ranging from defense to reproduction.

Most bioluminescent animals do not get their luminescence from bacterial symbionts, light is produced by the organisms themselves, in most of the volume of the ocean bioluminescence is the primary source of light.

In marine coastal habitats about 2,5 % of organisms are estimated to be bioluminescent, whereas in pelagic habitats, about 76 % of the main taxa of deep-sea animals have been found to be producing light. Most marine light-emission is in the blue and green light spectrum.

Bioluminescence occurs across a broad range of groups of organisms, especially in the open sea, including bacteria, protists, squid, jellyfish, comb jellies, crustaceans, cephalopod molluscs and fish. The most frequently encountered bioluminescent organisms may be dinoflagellates in the surface layers of the sea, which are responsible for the sparkling sometimes seen at night in disturbed water. A different effect is when thousands of square km of the ocean shine with the light produced by bioluminescent bacteria, known as the milky seas effect.

Dinoflagellates

These protists can be autotrophic (photosynthetic) or heterotrophic, feeding on other phytoplankton. In large numbers, some species may form potentially toxic red tides. There are at least 18 luminous genera, including Noctiluca, Protoperidinium, and Pyrocystis. Dinoflagellates invest a lot in their ability to luminescence, they direct energy to bioluminescence before growth.

Bioluminescent dinoflagellates produce light using the described luciferin-luciferase reaction. The luciferase found in dinoflagellates is related to the chlorophyll found in plants. Bioluminescent dinoflagellate ecosystems are rare, mostly forming in warm-water lagoons with narrow openings to the open sea. Whole lagoons can be illuminated at night.

Dinoflagellates may use bioluminescence for defense against predators. They shine when they detect a predator, possibly making the predator itself more vulnerable by attracting the attention of predators from higher trophic levels.

Squid

Many cephalopods are bioluminescent, including at least 70 genera of squid. Some squid and small crustaceans use bioluminescent chemical mixtures or bacterial slurries in the same way as squid use ink. A cloud of luminescent material is expelled, distracting or repelling a potential predator, while the animal escapes.

In many animals of the deep sea, including several squid species, bacterial bioluminescence is used for camouflage by counterillumination in which the animal matches the environmental light as seen from below. Photoreceptors control the illumination to match the brightness of the background. These light organs are usually separate from the tissue containing the bioluminescent bacteria.

Mutualism

Some bioluminescent organisms do not synthesize luciferin. Instead, they absorb it through other organisms, either as food or in a symbiotic relationship. Many marine animals, such as squid, keep bioluminescent bacteria in their light organs.

Many luminous bacteria in marine environment are free-living, but a majority are found in symbiotic relationships with fish, squids, crustaceans etc. In the symbiotic relationship, bacteria benefit from having a source of nourishment and a refuge to grow. Hosts obtain these bacterial symbionts either from the environment, spawning, or the luminous bacterium is evolving with their host. 

The most known species to use bioluminescence may be the anglerfish, which uses it to lure prey. The anglerfish has a large head, sharp teeth and a long, thin, fleshy filament on top of its head. On the end of the filament is a ball (the esca) that the anglerfish can light up. The fish uses the light to attract prey and to signal potential mates. The bacteria receive nutrients from the fish.

Conclusion

It is much easier to detect that a species is able to produce light than to analyze the chemical mechanisms, or to prove what function the light serves.

Bioluminescence has several functions in different taxa, as we have learned. But we can just say, that to human eyes, it is utterly mesmerising.

See here:

Have you ever had the chance to see the sea or forest glow at night?

A whale shark gliding through bioluminescent algae appears to be swimming in space.

Credit: Mike Nultypic.twitter.com/JZSz9TJEWy

— Wonder of Science (@wonderofscience) September 5, 2022

Interesting fact: did you know that humans are bioluminescent too? Indeed we are, the human body glimmers. The intensity of the light emitted by the body is 1000 times lower than the sensitivity of our naked eyes, it is an ultraweak photon emission. Read the research here.

Sources:

• Campbell Biology 10th edition
• https://en.wikipedia.org/wiki/List_of_bioluminescent_fungus_species
• https://www.nationalgeographic.org/encyclopedia/bioluminescence/
• Haddock S., Moline M. A., Case J. F. Bioluminescence in the Sea Steven. 1998
• Meighen E. A. Molecular Biology of Bacterial Bioluminescence. 1991
• Kobayashi M., Kikuchi D., Okamura H. Imaging of Ultraweak Spontaneous Photon Emission from Human Body Displaying Diurnal Rhythm. 2009