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
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.
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.
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
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
Bioluminescence is light produced by a chemical reaction within
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
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 reėmitted 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
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