Lingulodinium polyedrum bioluminescing in surf

Scintillons are small structures in

cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. Th ...

that produce light. Among

bioluminescent Bioluminescence is the production and emission of light by living organisms. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some fungi, microorganisms including some bi ...

organisms, only

dinoflagellates The dinoflagellates ( Greek δῖνος ''dinos'' "whirling" and Latin ''flagellum'' "whip, scourge") are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered algae. Dinoflagellates ...

have scintillons.

Description

Dinoflagellate light production

Marine dinoflagellates at night can emit blue light by

bioluminescence Bioluminescence is the production and emission of light by living organisms. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some fungi, microorganisms including some ...

, a process also called "the phosphorescence of the seas". Light production in these single celled organisms is produced by small structures in the cytoplasm called ''scintillons''. Among bioluminescent organisms, only dinoflagellates have scintillons. In the dinoflagellates, the biochemical reaction that produces light involves a

luciferase Luciferase is a generic term for the class of oxidative enzymes that produce bioluminescence, and is usually distinguished from a photoprotein. The name was first used by Raphaël Dubois who invented the words ''luciferin'' and ''luciferase'' ...

-catalysed oxidation of a linear tetrapyrrole called

luciferin Luciferin (from the Latin ''lucifer'', "light-bearer") is a generic term for the light-emitting compound found in organisms that generate bioluminescence. Luciferins typically undergo an enzyme-catalyzed reaction with molecular oxygen. The resul ...

. The dinoflagellate ''

Lingulodinium polyedra ''Lingulodinium polyedra'' is a species of motile photosynthetic dinoflagellates. ''L. polyedra'' are often the cause of red tides in southern California, leading to bioluminescent displays on beaches at night. Life cycle As part of its life ...

'' (previously called ''Gonyaulax polyedra'') also contains a second protein called luciferin binding protein (LBP) that has been proposed to protect luciferin from non-luminescent oxidation. Luciferin is released from LBP by a decrease in pH, and the same decreased pH also activates the luciferase. Light production in the dinoflagellates occurs in bioluminescent organelles called scintillons and can be stimulated by agitation of the surrounding seawater.

Use of term scintillon

The name scintillon was first used to describe

ic particles isolated from a bioluminescent species of dinoflagellate that were able to produce a flash of light in response to a decrease in pH. Scintillons were first observed in ''L. polyedra'' by fluorescence microscopy, where they appear as small blue dots close to the cell surface. This blue fluorescence is due to the presence of the bioluminescence reaction substrate, a naturally fluorescent molecule called luciferin. When light production is stimulated by addition of dilute acid to the cells under the microscope, the site of light production corresponds to the location of the scintillons. Furthermore, the natural luciferin fluorescence is reduced after the light producing reaction.

Observation after freezing

Cells observed under the

electron microscope An electron microscope is a microscope that uses a beam of accelerated electrons as a source of illumination. As the wavelength of an electron can be up to 100,000 times shorter than that of visible light photons, electron microscopes have a h ...

after a technique involving rapid freezing of the cells followed by substitution of water with a polymer (Fast-freeze Fixation/Freeze Substitution) contain a large number of electron dense bodies around the cell periphery. These structures correspond in size and location to the fluorescent bodies confirmed to be scintillons by their light emission, and they show colocalization of anti-luciferase and anti-LBP labeling meaning both bioluminescence proteins are found in the structures. Scintillons appear as cytoplasmic drops hanging in the vacuolar space, as they are almost completely surrounded by the vacuolar membrane. This structure led to the proposal that a voltage gated proton channel in the vacuolar membrane could allow an action potential to be propagated along the vacuolar membrane. This would in turn let protons enter into the cytoplasm around all the scintillons in the cells virtually simultaneously producing an intense but brief flash of light. Voltage gated proton channels were subsequently identified in a dinoflagellate confirming their predicted existence.

Variance in emittance of light over time

Scintillons have been extensively purified from ''L. polyedra'' by

centrifugation Centrifugation is a mechanical process which involves the use of the centrifugal force to separate particles from a solution according to their size, shape, density, medium viscosity and rotor speed. The denser components of the mixture migrate ...

, and these purified scintillon preparations contain luciferase and luciferin binding protein as the only detectable protein components. The amount of luciferase, LBP and luciferin all vary over the course of a daily (circadian) period, as do the number of scintillons in the cell. These observations suggest that the circadian control of bioluminescence involves a daily synthesis and degradation of luciferase and LBP. When synthesized, these two proteins aggregate together and migrate to the vacuole membrane where LBP binds luciferin and the scintillons acquires an ability to produce light upon stimulation.

Variety in differing species

Scintillons are not identical in different species. Scintillons isolated from dinoflagellates belonging to the genus '' Pyrocystis'' such as ''P. lunula'' (previously ''Dissodinium lunula'') or ''P. noctiluca'' are less dense than those of ''L. polyedra'' and do not contain LBP. Little is known about the structure or composition of scintillons in species other than ''L. polyedra''.

References

{{Reflist Bioluminescence Cytoplasm