Bioluminescence is the phenomenon in which living organisms produce light as a result of a chemical reaction. It is a form of chemiluminescence, in which two or more chemicals react to form an excited (high-energy) intermediate, which then breaks down, releasing some of its energy as photons of light, to reach its ground state. The electromagnetic radiation released can be visible, ultraviolet, or infrared light.
In bioluminescence, however, less than 20% of the light generates thermal radiation or heat.
Naturally, we’re more familiar with the visible bioluminescence we can see, for example, in the light of a firefly, but many other species —even humans— emit light of one kind or another.
So, how does it happen?
What causes bioluminescence?
Here is when we must stop and remember the second law of thermodynamics. This law states that the entropy in closed systems is always increasing. This is because no thermodynamic process is 100% efficient —there is always a certain amount of energy that can’t be converted into work. This “useless” energy is released into the environment in the form of heat, which increases the molecular disorder (the entropy) of the system.
But the conversion of chemical energy into light that occurs in bioluminescence is so efficient that very little heat is released into the environment. This is why it is called “bioluminescence” — because luminescence is the emission of light by certain materials when they are relatively cool. So, this can be thought of as "cold light".
And how do living beings produce this light? It depends on the species.
The chemical reaction that generally results in bioluminescence requires two chemicals: luciferin and either luciferase or photoprotein. Luciferin is the compound that produces light, and the exact color produced is a result of the arrangement of luciferin molecules.
Some bioluminescent organisms can synthesize luciferin on their own, while others absorb it through other organisms, either by consuming them as food or by existing in a symbiotic relationship with a luciferin-producing organism. Squid, for example, have a symbiotic relationship with bioluminescent bacteria that live in the squid's light organs.
The enzyme luciferase interacts with oxidized luciferin to create a byproduct, called oxyluciferin, and it is this chemical reaction that creates light.
While most bioluminescent reactions involve luciferin and luciferase, some involve a chemical called a photoprotein. The photoprotein combines with luciferins and oxygen, along with another agent, such as calcium ion, to produce light.
The crystal jelly, a kind of hydromedusa living on the west coast of North America, uses a photoprotein called aequorin, which is activated by calcium ions. The calcium generates the catalysis and it’s so fast that it produces very brief flashes of light.
On the other hand, fireflies — perhaps the most famous bioluminescent living organisms — combine oxygen with calcium, adenosine triphosphate (ATP) and luciferin in the presence of luciferase in order to produce light. This reaction takes place in special organs located in their abdomen.
There are also bacteria that emit light. These are most commonly found in the marine environment. Single-celled creatures called dinoflagellates are planktonic surface dwellers that are responsible for producing a sparkly or milky effect on the ocean surface.
Bioluminescent dinoflagellates produce light using a luciferin-luciferase reaction. The luciferase found in dinoflagellates is related to the green chemical chlorophyll found in plants.
The practical use of bioluminescence
But, why do living beings emit light? Perhaps you’ve heard that fireflies use their light to attract mates, but they are not the only bioluminescent species that do so. Some worms and tiny crustaceans attract mates this way, too. Male ostracods, for example, actually vomit bioluminescent mucus to impress their females.
The sunlight illuminates the ocean for about 650 feet (200 m) of depth before it is unable to penetrate further into the water. This first layer covered by sunlight is called the photic zone and it’s where the majority -- about 90% of marine life inhabit.
As we go deeper and deeper, the ocean gets darker and darker. Soon, we see many of the remaining marine life that has shown incredible adaptations to the permanent, near-total darkness of their habitat. One of these adaptations is the ability to produce light to lure their prey.
Some species use bioluminescence to confuse attackers. Many species of squid, for example, startle predators by flashing them. The squid uses the momentary confusion of its attacker for a quick escape.
There are several species —like the firefly squid— that use bacterial bioluminescence for counterillumination. This is some sort of an active camouflage method in which the animals use light to mimic the brightness and wavelengths of the backgrounds, to create a sort of stealth effect where they blend into their environment.
This way, they remain unseen or appear larger or smaller than they actually are, confusing predators and/or setting up traps for their prey. This is most common in the mid-water zone, where there is still some dim light left.
Hatchetfish is another animal that uses counterillumination. Its light-producing organs point downward. By adjusting the amount of light coming from its undersides it can match the intensity of the light coming from above to become virtually invisible to any predators under it.
Bioluminescence in humans
According to a recent study, it turns out that most animals actually emit some light - including humans. It’s just that we can’t see it because it’s 1,000 times less intense than what we can detect with our naked eyes.
In a study conducted by the Tohoku Institute of Technology in 2009, scientists asked five volunteers to get into a light-sealed chamber and observed them with an ultra-sensitive camera capable of capturing individual photons. This way, they discovered that the volunteers glowed rhythmically during the day.
The light emissions had a peak at 4 PM and decreased towards the night, for which the scientists concluded that the phenomenon was linked to the human body’s internal clock. They verified it by disrupting the volunteers’ sleep patterns, which eventually disrupted the glow’s cycle, too.
Most of the photons were spread in the face, as shown in the images below from the original study:
Scientists don’t exactly know why this happens, but the link with the body’s circadian rhythms makes them believe that it’s related to the body’s metabolism.
It's likely that this bioluminescence is a side-effect of metabolic reactions, as highly reactive free radicals produced through cell respiration interact with free-floating lipids and proteins. The resulting "excited" molecules can react with chemicals called fluorophores to emit photons. These metabolic reactions are less frequent or intense at night, so less light is emitted then.
Researcher Hitoshi Okamura, a biologist at Kyoto University, says that if this is the case, spotting the human body’s light emissions could help spot medical conditions one day.
Will the ultra-sensitive cameras become a diagnostic tool in the future? Only time will tell.