A photocyte is a type of cell that catalyses enzymatic reactions to produce light through bioluminescence. Photocytes are typically located within certain layers of epithelial tissue, functioning either individually, in groups, or as part of a larger light-producing organ known as a photophore. They contain distinctive structures called photocyte granules. These specialized cells occur in various multicellular animals, including coelenterates (such as cnidarians and ctenophores), annelids, arthropods (including insects), and fishes. Although some fungi exhibit bioluminescence, they do not possess specialized cells analogous to photocytes.
Mechanism of light production Nerve impulses may initiate light production by stimulating the photocyte to release the enzyme luciferase into a reaction chamber containing the substrate luciferin. In some species, this release occurs continuously without neural stimulation, through osmotic diffusion. Molecular oxygen is actively gated through surrounding tracheal cells, which normally limit oxygen diffusion from blood vessels. The subsequent reaction between oxygen, luciferase, and luciferin generates light energy and a byproduct, typically carbon dioxide. This biochemical reaction takes place within the peroxisome of the cell.
Early researchers proposed that adenosine triphosphate (ATP) served as the energy source for photocyte reactions. However, ATP produces only a small fraction of the energy generated by the luciferase reaction, making any resulting light emission too weak to be visible to the human eye. The wavelengths of light produced by most photocytes are centred around 490 nm, although emissions as energetic as 250 nm have been reported.
Variations in colour among different photocytes are typically caused by colour filters in other parts of the photophore, which modify the wavelength of the emitted light before it exits the endoderm. The visible colour range varies between bioluminescent species.
The specific combinations of luciferase and luciferin found in photocytes are unique to each species and are thought to result from evolutionary divergence.
Anatomy and physiology
Firefly larvae Light production in Photuris pensylvanica larvae occurs in approximately 2,000 photocytes located within the insect's highly innervated light organ, which is structurally simpler than that of the adult organism. The transparent photocytes are distinguishable from the opaque dorsal layer cells that cover them. Both nervous and intracellular mechanisms contribute to light production in these cells.
Fireflies can regulate the amount of oxygen travelling through their tracheal system to the light organ, thereby influencing oxygen availability for light emission. This regulation is achieved by modifying the quantity of fluid within the tracheal system. Because oxygen diffuses more slowly through water than through air, this mechanism allows fireflies to control the oxygen supply reaching the photocytes. Spiracles can also open and close to control the flow of air through the tracheal system, although this response typically occurs only under stress conditions.
Neural mechanism of light production Experiments have shown that applying an electric current of 5 to 15 volts for 50 milliseconds to the segmental nerve innervating the light organ produces a glow approximately 1.5 seconds later, lasting for five to ten seconds. Stimulation of this nerve generates multiple impulses, with the frequency of impulses proportional to the intensity of stimulation. A higher frequency of impulses results in a consistent latency period, and continuous nerve activity corresponds with sustained light emission. Light intensity increases with impulse frequency up to around 30 impulses per second, beyond which no further increase in brightness occurs. This suggests physiological limitations within the synaptic or photocyte light-producing mechanisms.
A series of action potentials produces intermittent light flashes, with higher frequencies increasing the likelihood of light emission. Nerve impulses are associated with depolarization of the photocyte membrane, and stronger depolarization events correlate with brighter light output. The nerve innervating the light organ contains only two axons, but these branch repeatedly to reach numerous photocytes. Each cell is connected to several nerve terminals, and each terminal may be associated with multiple synapses.
The junction between the neuron and the photocyte differs from typical synaptic junctions between neurons or between neurons and muscle fibres in the neuromuscular junction. Depolarization in the photocyte following neural stimulation occurs about one hundred times more slowly than in these other junction types, a delay not attributable to diffusion limitations, as the synapse between the neuron and photocyte is relatively small. It has also been observed that the neurons controlling the light response terminate at tracheal cells rather than directly at the photocytes.
Intracellular mechanism The resting potential of photocytes ranges between 50 and 65 millivolts. It is generally accepted that light emission follows depolarization of the photocyte membrane, although some studies suggest the reverse. Depolarization increases the diffusion rate of ions across the membrane and occurs approximately 0.5 seconds after a nerve impulse, reaching a peak at around one second. Higher frequencies of neural stimulation are associated with smaller depolarization events.
Exposure to certain neurotransmitters, including epinephrine, norepinephrine, and synephrine, can induce light emission without corresponding depolarization of the photocyte membrane.
Mnemiopsis leidyi In Mnemiopsis leidyi, photocytes are unevenly distributed near the plate cilia cells. Gastric cells form a barrier that separates the photocytes from the openings of the radial canals along which they are located.
Porichthys In Porichthys notatus, light production is triggered through an adrenergic mechanism. The sympathetic nervous system of the fish initiates bioluminescence in the photocytes. When stimulated by norepinephrine, epinephrine, or phenylephrine, the photocytes produce a brief flash of light followed by a gradual decrease in intensity. Stimulation with isoproterenol results only in a slow, fading illumination.
The amplitude of the initial flash, referred to as the "fast response", increases with higher concentrations of the stimulating neurotransmitter. Considerable variation in luminescence intensity has been observed among photocytes from different fish, as well as across seasons depending on when the samples were collected. Stimulation with phenylephrine produces a less intense response than stimulation with epinephrine or norepinephrine. Phentolamine has been shown to completely inhibit the effects of phenylephrine and partially inhibit those of epinephrine and norepinephrine. Clonidine suppresses the fast response but has no effect on the slow response.
The photocytes of Porichthys are known to be extensively innervated.
Amphiura filiformis Mechanical stimulation of the arm spines can cause Amphiura filiformis to emit bioluminescent light in the blue range. The species has been found to possess a luciferase enzyme, which has been localized to clusters of photocytes situated at the tips of the arms and around the spines. Structures believed to be photocytes, based on available evidence, have also been identified near the spine nerve plexus, mucous cells, and probable pigment cells.
Bioluminescence in A. filiformis is controlled by the animal's nervous system. Acetylcholine can stimulate the photocytes through nicotinic receptors.
Amphipholis squamata In Amphipholis squamata, bioluminescence originates from the arm spines, produced by photocytes located within the spinal ganglia. Acetylcholine has been shown to stimulate these photocytes to emit light.
Mollusks Bioluminescent snails have been observed to exhibit a high degree of control over light emission, although the mechanism underlying this control remains unknown. Species within the genus Phuphania have been shown to retain their ability to produce light even after extended periods of hibernation. The means by which these snails maintain bioluminescent capability over long durations is not yet understood, though some hypotheses suggest it may be related to mechanisms observed in certain bioluminescent fungi.
Other species of fish In many fish species, adrenaline has been found to stimulate photocytes to emit light. It is generally believed that impulses from the sympathetic nervous system provide the stimulus responsible for activating light production in these cells.
Embryological development
Mnemiopsis leidyi In Mnemiopsis leidyi, the ability to produce light first appears during the development of the plate cilia cells. The bioluminescent cells in the embryo share many characteristics with the photocytes observed in the adult organism. Cells of the M macromere lineage differentiate into photocytes and separate from other cell lineages during differential division.
The maturation of these photocytes and the intensification of light production occur rapidly, typically within ten hours of the initial observation of bioluminescence. The egg of M. leidyi contains two cytoplasmic regions: cortical and yolky. The cytoplasmic composition inherited by daughter cells during division influences their differentiation. Cortical cells produce light only if they inherit yolk in their cytoplasm; cells lacking yolk do not exhibit bioluminescence.
Evolution of photocytes Luciferin molecules are largely conserved across different species, whereas luciferase enzymes exhibit a greater degree of diversity. Approximately 80% of bioluminescent species are found in aquatic habitats.
Etmopterus spinax The evolution of light-producing cells (photocytes) in sharks is believed to have occurred independently at least twice through convergent evolution. Evidence indicates that the bioluminescent properties of the shark Etmopterus spinax likely evolved as a form of camouflage, particularly for counter-illumination. However, luminescence on the lateral sides of the shark suggests additional functions beyond camouflage.
Bioluminescence is believed to have evolved only in sharks among the cartilaginous fishes. The specific functions of bioluminescence in sharks have not been fully determined.
Evolution in fireflies All five families of luminescent beetles—Phengodidae, Rhagophthalidae, Elateridae, Sinopyrophoridae, and Lampyridae (fireflies)—are grouped within the Lampyroid clade. The luciferase enzymes and luciferin proteins expressed in the photocytes of fireflies are homologous to those found in beetle species within the families Phengodidae, Rhagophthalidae, and Elateridae (click beetles). Studies indicate that all bioluminescent beetle species investigated utilize highly similar mechanisms for light production in photocytes. The beetle genus Sinopyrophoridae is also bioluminescent, though the precise mechanism has not yet been fully characterized; it is believed to be homologous with other bioluminescent beetle genera.
The first complete genome of a bioluminescent beetle, Pyrocoelia pectoralis (a firefly), was sequenced in 2017. By 2018, the genomes of three additional bioluminescent beetle species had been sequenced. Bioluminescence in beetles serves multiple functions, including predator deterrence and mate attraction.