Bioluminescence is the emission of light from living organisms, without giving out appreciable or no heat. It is basically a 100% efficient system. Virtually all of the energy generated is converted into light with almost none lost in heat or sound production. It is literally a ‘cold fire’. The light results from a chemical reaction mediated by enzymes and involving specialized molecules in the organisms. Bioluminescence occurs in species too numerous to list but the most recognizable ones include dinoflagellates, some jellyfish and fireflies. Dinoflagellates and fireflies are by far the most common sources of bioluminescence in the ocean and on land respectively. Some deep sea fish are equipped with organs that produce luminescence to which prey is attracted. The flashes emitted by male and female fireflies are used as species specific signals for mating. The use of bioluminescence in an organism can be to evade predators, attacking its enemies, camouflage, food, attracting their mates or sometimes due to organisms inside an organism.
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Dinoflagellates are unicellular aquatic organisms which come under the order Dinoflagellida and the class Phytomastigophorea with two uneven flagella for locomotion. Several thousand species of dinoflagellates are known to mankind. Most contain chlorophyll and are photosynthetic. Among these there are the diatoms, which are the primary producers of energy in the ocean food chain. Like many complex one celled organisms, dinoflagellates show traits of both animals and plants and are claimed by zoologists as protozoans and by botanists as algae. They are mostly marine creatures and in warm shallow waters they sometimes reproduce in enormous numbers resulting in a bloom. Many species of dinoflagellates are bioluminescent. Both heterotrophic and autotrophic dinoflagellates are known. Some can be both. They form a significant part of primary planktonic production in both oceans and lakes. Most dinoflagellates go through moderately complex life cycles involving several steps, sexual and asexual, motile and non-motile. Some species form cysts composed of sporopollenin, and preserve as fossils.
Dinoflagellates display considerable morphological variations and many share a common anatomical pattern during at least one stage of their life cycle. Most of them have two flagella inserted into their cell wall via the flagellar pores at approximately the same location. In many one of the flagella wraps around the cell and is known as the transverse flagellum, while the other longitudinal flagellum extends tangentially to the cell, perpendicular to the plane of the transverse flagellum. The beating of the longitudinal flagellum and the transverse flagellum imparts a forward and spiraling swimming motion, and defines the anterior and the posterior. The flagellar pore and point of flagellar insertion defines the ventral with the opposite side dorsal. Left and right sides of the cell are then defined as in most organisms.
Basic anatomy of a thecate, dinokont dinoflagellate
A depression occurs on the ventral surface at the point of flagellar insertion, and is known as the sulcus. The transverse flagellum occurs in a furrow known as the cingulum which encircles the cell except where it is interrupted by the sulcus on the ventral surface. The cell wall of dinoflagellates is subdivided into multiple polygonal amphiesmal vesicles of varying numbers from half a dozen to hundreds. In some dinoflagellates, these vesicles are filled with relatively thick cellulose plates with bounding sutures. When this occurs, the cell wall is referred to as a theca. Dinoflagellates possessing a theca are often referred to as armored dinoflagellates, while the ones which lack are referred to as naked dinoflagellates.
Redrawn from Fensome et al. 1996
Schematic life cycle history of dinoflagellates
Coming to the life cycle of dinoflagellates which is multi-staged and about 6 stages can be clearly identified in peridiniales dinoflagellates. The six stages are:
When rapid growth and a population expansion is observed vegetative propagation dominates and takes over.
Now the schizonts act as gametes and pair up to form zygotes. Due to this process one or more theca may be lost.
A new theca is formed from the new diploid zygote.
The activity level of the cell decreases, and with time the flagella is lost. This zygote is termed as a hypnozygote. When the theca is separated and broken and decayed the cyst is formed and completed.
The cyst now settles down in the bottom on the sea.
After the period of dormancy the theca is grown again and it becomes motile.
For an organism to give off light, at least two chemicals are required in the presence of oxygen and the energy molecule ATP (Adenosine Tri Phosphate). The one which produces the light is generically called a “luciferin” and the one that drives or catalyzes the reaction is called a “luciferase.” Luciferase is the enzyme that catalyses the oxidation of luciferin which is the basic substrate in bioluminescent reactions.
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 are bound together in a single unit called a “photo protein.” This molecule can be triggered to produce light when a particular type of ion is added to the system (say calcium as it happens in the jellyfish, Aequorea victoria).
Dinoflagellate luciferin is thought to be derived from chlorophyll, and has a very similar structure. In the genus Gonyaulax, at pH 8 the molecule is “protected” from the luciferase by a “luciferin-binding protein”, but when the pH lowers to around 6, the free luciferin reacts and light is produced.
The structure of the luciferin in a dinoflagellate
The ability to produce luminescence is strictly dependent upon the day or night cycle. In a twelve hour light or twelve hour dark cycle, dinoflagellates will only flash brightly during the dark phase. Light emitted is brightest after several hours of darkness. Early in the morning, glowing activity is reduced and they no longer give off light upon shaking or disturbing them. During the day, the dinoflagellates appear as ellipse shaped cells, pigmented red, indicating the presence of chlorophyll which enables photosynthesis to occur so they may harvest light from the sun. The luminescence is transient and the cells soon return to their resting state. Most cells flash for less than a second, however others appear toglow for 1-6 seconds. Upon repeated stimulation, light emission is much reduced. Within about half an hour of rest, the luminescence becomes brighter again.
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12 hour light cycle
Bioluminescence is used to evade predators which act as a type of burglar alarm for defense mechanism in dinoflagellates. They produce light when the deformation of the cell by minute forces triggers its luminescence. When the cell is disturbed by a predator, it will give a light flash lasting 0.1 to 0.5 seconds. The flash is meant to attract a secondary predator that will be more likely to attack the predator that is trying to consume the dinoflagellate. The light flash also makes the predator jump and worry about other predators attacking it, making the predator less likely to prey on the dinoflagellate.2
In most dinoflagellates, bioluminescence is controlled by an internal biological rhythm. They are on a circadian rhythm. Towards the end of daylight, luminous chemicals are packaged in vesicles called scintillons. The scintillons then migrate to the cytoplasm from the area around the nucleus. It is not currently known how the scintillons are moved to the cytoplasm. During the night light is triggered by mechanical stimulation. When action potential generates in the vacuole, the action potential propagates throughout the rest of the cell. This allows protons to pass from the vacuole to the cytoplasm. The cytoplasm becomes acidified, normally by hydrogen ions and the process is activated in the scintillons.
Dinoflagellates have distinct chromosomes through the whole cell cycle although their condensation patterns vary during interphase, with a maximum unwinding corresponding with the peak of replication in S phase. They are attached to the nuclear envelope and have a unique organization. Free-living dinoflagellates have high chromosome numbers per haploid genome while parasitic dinoflagellates have only a few chromosomes. Chromosomal ultra structure varies during interphase, and lacks the typical banding pattern of mitotic eukaryotic chromosomes, reflecting the genome compartmentalization. Dinochromosomes show a banded and arched organization by transmission electron microscopy (TEM) and freeze-etching that corresponds to a cholesteric organization of their DNA with a constant left-handed twist. Whole-mount chromosomes have a left-handed screw-like configuration with differentiated roughly spherical ends. Dinomitosis occurs without nuclear envelope breakdown and nucleolar disassembly and with an extra nuclear mitotic spindle without direct contact with the chromosomes.
Dinoflagellates are true eukaryotes that experienced a secondary loss of histones during evolution, constituting the only living eukaryotic knockouts of histones. The ancestral group of the alveolates, that includes the dinoflagellates, had eukaryotic histones as observed in ciliates and apicomplexans suggesting that dinoflagellates may have experienced a secondary loss of histones, and a set of primitive bacterial HLP may have been reintroduced from a prokaryotic source by gene transfer. Dinoflagellates have significant genomic differences compared with higher eukaryotes at all levels, from base composition and methylation, to the structural organization of their DNA and chromosomal domains, that nevertheless led to a similar organization and functioning of nuclear domains. The exact way they use to regulate gene silencing and activation without histones is still unknown, although the high proportion of base methylation could be involved.
The very mention of red tides brings to mind the fear of dead fish and toxic seafood. Red tide is a naturally occurring, higher than normal concentration of the microscopic algae. The massive multiplication of these tiny, single-celled algae is usually found in warm saltwater and is commonly referred to as a bloom. Even though they are important producers and a key component to the food chain, dinoflagellates are also known for producing deadly toxins, especially when they are present in large numbers. They can not only kill a large range of marine species, but can also impart fatal toxins into several species, especially shellfish. Usually deadly to finfish, shellfish are relatively unaffected. These shellfish may then be eaten by humans, who are then affected by the stored toxins.
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