Dinoflagellates are the most common sources of bioluminescence at the surface of the ocean. This section explains how and why they produce bioluminescence.
Dinoflagellate Bioluminescence
The dinoflagellates are also known as Pyrrhophyta, meaning "fire plants". Some species produce bioluminescence, which is light produced as a result of a chemical reaction within specialized structures in the dinoflagellate cell. Agitation of seawater containing dinoflagellates will stimulate light flashes. The phenomenon was first noted in the genus Noctiluca in the 1800's, but the ability to produce bioluminescence is now known to occur in several marine species of dinoflagellates.
Bioluminescence acts as a burglar alarm. When the cell is disturbed by a grazing predator, such as a copepod (the burglar), it gives a light flash (the alarm) which lasts 0.1 to 0.5 sec. The flash attracts a secondary predator, such as a small fish (the police), which closes in looking for food. The fish is more liable to find and eat the larger copepod than the miniscule dinoflagellate cell. In this way the predators of the dinoflagellate are removed. When the copepod sees the luminescent flash it gives a jump, because staying put means it is vulnerable to predation. The result is that the feeding behavior of the copepod is disrupted and less dinoflagellate prey are consumed.
Light is produced as a result of a chemical reaction. Therfore, light production in dinoflagellates and all other luminescent organisms involves a chemiluminescent reaction in which a substrate, luciferin, is oxidized, releasing a large amount of energy in the form of light. Unlike light produced by light bulbs, in which a portion of the energy is wasted as heat, most of the energy released from the chemiluminescent oxidation of luciferin occurs in the form of light. Hence bioluminescence is commonly called "cold light". All bioluminescent reactions occur in the presence of oxygen.
In dinoflagellates, the luciferin is normally bound to a protein, called luciferin binding protein (LBP). At neutral pH, LBP stabilizes the luciferin from being spontaneously oxidized. When activated by a drop in pH, the luciferin dissociates from LBP, and associates with a protein, luciferase, which acts as a catalyst. In the process of being oxidized, luciferin briefly exists in an excited state, after which it decays to its ground state, releasing energy in the form of light. In dinoflagellates, the wavelength of maximum emission is approximately 472 nm, in the blue-green region of the visible spectrum. [Blue-green light transmits best through clear ocean water.]
In most dinoflagellates the expression of bioluminescence is controlled by an internal biological rhythm. Towards the end of the day the luminescent chemicals are packaged in vesicles called scintillons, which then migrate into the cytoplasm. During the night, light emission is triggered by mechanical stimulation of the cell according to the following hypothesized sequence of events: Following an unknown mechano-chemical transduction process, an action potential is generated in the internal vacuole membrane. The action potential propagates throughout the cell, allowing protons to pass from the vacuole, where they are sequestered, into the cytoplasm. As a result the cytoplasm is acidified, and the chemiluminescent process is activated in the scintillons according to the steps previously described.
Because of the circadian rhythm, most dinoflagellates produce much less bioluminescence during the day compared to the light produced at night. Even if you turn off the lights during the day, you won't get luminescence produced. This is because there are fewer scintillons present during the day, and possibly an uncoupling of the sensory and effector pathways. Bioluminescence reaches maximum levels approximately 2 hours into the dark. Bioluminescence can be mechanically stimulated by stirring or bubbling, chemically stimulated by an acid solution, and also stimulated by ultrasound.
Most dinoflagellates can be cultured in the laboratory. All it takes are some starter cells, the proper nutrients, and light. Laboratory cultures are usually grown in a temperature controlled chamber with a defined light cycle. A 12 hour light -- 12 hour dark cycle is the most common cycle to use. By adjusting the timing of when the dark phase starts, the cells can be conveniently used in laboratory experiments. In our laboratory, the dark phase starts at 11 am, allowing us to handle cells during their light phase when they are inexcitable, and then perfoming experiments (after lunch!) when bioluminescence has reached maximum levels.
Dinoflagellates are the most common sources of bioluminescence in the surface waters of the ocean. The light displays created by breaking waves, swimming fish, or boats are mainly due to dinoflagellates. The taxonomic composition of luminescent dinoflagellates will vary with location and time. In the San Diego area dinoflagellates are present year-round; in the northeast U.S., the combination of short day length and cold water temperatures forces cells to overwinter as cysts in the sediment. In the spring, the warmer temperatures and longer days allows cysts to excyst, forming motile cells once again.
During the May 1997 red tide of Gonyaulax polyedra in southern California, beautiful displays of bioluminescence occurred in breaking waves. In Puerto Rico, the dinoflagellate bioluminescence is particulary impressive. View the beautiful images by photographer Frank Borges LLosa.
Dinoflagellate Bioluminescence / Updated 8/29/96 / biolum@ucsd.edu