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Bioluminescence

Bioluminescence is simply light produced by a chemical reaction which originates in an organism.

It can be expected anytime and in any region or depth in the sea. Its most common occurrence to the sailor is in the often brilliantly luminescent bow wave or wake of a surface ship. In these instances the causal organisms are almost always dinoflagellates, single-cell algae, often numbering many hundreds per liter.

They are mechanically excited to produce light by the ship's passage or even by the movement of porpoises and smaller fish. The deep-sea fish Aristostomias has more than one light organ. Read more about this and other amazing adaptations. (Illustration © Steven Haddock)

Bioluminescence is a primarily marine phenomenon. It is the predominant source of light in the largest fraction of the habitable volume of the earth, the deep ocean . In contrast, bioluminescence is essentially absent (with a few exceptions) in fresh water, even in Lake Baikal. On land it is most commonly seen as glowing fungus on wood, or in the few families of luminous insects. (For firefly information, try here.)

Bioluminescence has evolved many times in the sea as evidenced by the several distinct chemical mechanisms by which light is emitted and the large number of only distantly related taxonomic groups that have many bioluminescent members.

Bioluminescent bacteria occur nearly everywhere, and probably most spectacularly as the rare "milky sea" phenomenon, particularly in the Indian Ocean where mariners report steaming for hours through a sea glowing with a soft white light as far as the eye can see

Spectrum

In the sea, bioluminescent light is concentrated in the blue window of greatest optical transparency of seawater. Most organisms emit between 440 nm and 479 nm. Some cnidarians have green fluorescent proteins that absorb an initially blue emission and emit it shifted towards the green (~505 nm). One remarkable fish has a similar mechanism to shift the initial emission into the red for use in viewing prey in the near infrared with its red-sensitive eyes. (More detail in the organism section.) Measurements in situ at various depths confirm emission clustering in the blue to green region of the spectrum.

Intensity

The luminescence of a single dinoflagellate is readily visible to the dark adapted human eye, as the demonstration will show. Most dinoflagellates emit about 6e8 photons in a flash lasting only about 0.1 second. Much larger organisms such as jellyfish emit about 2e11 photons per second for sometimes tens of seconds. The intensity of luminescence by photosynthetic dinoflagellates is strongly influenced by the intensity of sunlight the previous day. The brighter the sunlight the brighter the flash.



Kinetics

Some organisms emit light continuously, but most emit flashes of durations ranging from about 0.1 s to 10 s. Some dinoflagellates can respond repetitively to excitation over a short period. In most multicellular species luminescence is neurally controlled. Thus in some fish the sympathetic nervous system controls luminescence by way of the neurotransmitter nor-adrenaline. In fireflies the transmitter is glutamate. In most marine invertebrates the transmitters are unknown. In such forms the "trigger" to luminescence is some detected behaviorally significant event.

In single cell organisms like dinoflagellates or radiolarians luminescence is triggered by deformation of the cell surface by minute forces(1 dyne per square cm). Mechanical deformation causes an action potential sweeping over the vacuole membrane and this is thought to induce light emission by admitting protons from the acidic vacuole into contact with the cellular elements that contain the light emission chemistry. (Details in the Research Forum).

In a some instances in marine invertebrates with eyes or other light receptors, light emission can be induced by photic excitation, even by another luminescing organism. Called "empathetic" luminescence, this phenomenon has as yet undemonstrated potential to enhance the luminescence generated by a moving source by photic transfer from the luminescent organisms mechanically triggered by the moving source

Bioluminescence is light produced by a chemical reaction within an organism.





At least two chemicals are required. The one which produces the light is generically called a "luciferin" and the one that drives or catalyzes the reaction is called a "luciferase."

The basic reaction follows the sequence illustrated above: The luciferase catalyzes the oxidation of luciferin Resulting in light and an inactive "oxyluciferin" In most cases, fresh luciferin must be brought into the system, either through the diet or by internal synthesis.

Sometimes the luciferin and luciferase (as well as a co-factor such as oxygen) are bound together in a single unit called a "photoprotein." This molecule can be triggered to produce light when a particular type of ion is added to the system (frequently calcium).



Bioluminescence is not the same as "fluorescence" or "phosphorescence". (See Myths for more explanation.) In fluorescence, energy from a source of light is absorbed and reëmitted as another photon. In bioluminescence or chemiluminescence the excitation energy is supplied by a chemical reaction rather than from a source of light. Here is a simplified view of fluorescence:

The mechanism of fluorescence (not bioluminescence)

An electron (yellow) "orbits" the nucleus (blue), minding its own business. A source of light of an appropriate wavelength (indicating its energy) strikes ... ...driving the electron into a higher-energy orbital. The electron is only stable there for a short time whereupon it... ...returns to the lower energy level... ...emitting the energy as a longer wavelength photon. The electron continues on its way... Note that due to energy loss while in the excited state, the photon emitted will always be longer wavelength (lower energy) than the exciting photon. See the Myths section for a little more explanation.