What is a Dinoflagellate?

Dinoflagellates are the most common sources of bioluminescence at the surface of the ocean. This section describes the life history and ecology of dinoflagellates, and explains how and why they produce bioluminescence.

What is a Dinoflagellate?

Dinoflagellates are unicellular protists; most exhibit the following characteristics:

They are planktonic.
Ninety percent of all dinoflagellates are marine plankton.
They are small.
Although many of them are microscopic, the largest, Noctiluca, may be as large as 2 mm in diameter!
They are motile.
Dinoflagellates swim by means of two flagella, movable protein strands which propel the cell through the water. The longitudinal flagellum extends out from the sulcal groove of the hypotheca (posterior part of cell); when it whips back and forth it propels the cell forward. The flattened ___ flagellum lies in the cingulum, the groove that extends around the equator of the cell. Its motion provides maneuvering and forward movement. As a result of the action of the two flagella the cell spirals as it moves.
Many are covered by cellulose plates.
The cell is surrounding by a series of membranes called the amphiesma. In "armored" species cellulose deposited between the membranes forms rigid plates called thecae. "Naked" cells lack thecae.
Their chromosomes are always condensed.
In addition, the DNA is not associated with histones as in other eukaryotic cells. Dinoflagellates contain a lot of DNA, which explains the large size of the nucleus. The metabolic requirements of supporting the large amount of DNA may explain the low growth rates of dinoflagellates compared to other unicellular protists.
Not all dinoflagellates are photosynthetic.
Many dinoflagellates are photosynthetic, manufacturing their own food using the energy from sunlight, and providing a food source for other organisms. The photosynthetic dinoflagellates are important primary producers in coastal waters.
Some photosynthetic dinoflagellates are symbiotic, living in the cells of their hosts, such as corals. Called zooxanthellae, they are found in many marine invertebrates, including sponges, corals, jellyfish, and flatworms, as well as within protists, such as ciliates, foraminiferans, and colonial radiolarians.
Approximately half of all species are heterotrophic, eating other plankton, and sometimes each other, by snaring or stinging their prey. Non-photosynthetic species of dinoflagellates feed on diatoms or other protists (including other dinoflagellates); Noctiluca is large enough to eat zooplankton and fish eggs. Some species are parasites on algae, zooplankton, fish or other organisms.
Reproduction
The most form of reproduction is asexual, where daughter cells form by simple mitosis and division of the cell. The daughter cells will be genetically identical to that of the original cell. The thecal plates may either be divided, or completely shed and then reformed.
Under some conditions sexual reproduction may occur. Motile gametes are formed as a result of mitosis, because dinoflagellates are usually haploid. When two gametes fuse a motile planozygote may be formed.
Red Tides
Red tides are conditions when a dinoflagellate population increases to huge numbers. This "bloom" may be caused by nutrient and hydrographic conditions, although the environmental conditions which result in red tides are not completely understood. The water is discolored red or brown due to the presence of dinoflagellate cells numbering up to 20 million cells per liter. Red tides are composed primarily of one species of dinoflagellate which has been rapidly growing.
Some red tides are luminescent; check out the bioluminescence stimulated in breaking waves during the May 1997 Gonyaulax polyedra red tide in San Diego. A synopsis of the putative mechanisms responsible for this red tides is kindly provided by Prof. Wolfgang Burger, Interim Director of SIO:
"My understanding is this: if, after an upwelling or mixing event (storm?) there is plenty of sunshine, which warms the water and makes for stable stratification, conditions are right for a bloom. If, in addition, Gonyaulax cysts waiting around on the bottom have been stirred up into the water, and have by some means (change in temperature? light?) detected that the time is good for popping open, sufficient seeds are released to start the process. Rapid reproduction ensues (by cell division; these are unicellular organisms) and crowds out everything else, by taking away the light (the water was brown!) and perhaps also by chemical means."
Some both not all red tides are toxic. In toxic red tides, the dinoflagellates produce a chemical which acts as a neurotoxin in other animals. When the dinoflagellates are ingested by shellfish, for example, the chemicals accumulate in the shellfish tissue in high enough levels to cause serious neurological affects in birds, animals, or people which ingest the shellfish.
The are several types of neurotoxins produced by dinoflagellates. These chemicals may affect nerve action by interfering with the movement of ions across cell membranes, thus affecting muscle activity. The toxin saxitoxin, produced by Protogonyaulax catanella off the west coast of North America, and Gessnerium monilatum off the east coast, accumulates in shellfish. Eating contaminated shellfish causes paralytic shellfish poisoning (PSP), while ciguatera is caused by eating contaminated fish. The worst cases of PSP result in respiratory failure and death within 12 hours. Another toxin which accumulates in shellfish is brevitoxin, produced by the dinoflagellate Ptychodiscus brevis. A toxin produced by the dinoflagellate Dinophysis causes diarrhetic shellfish poisoning (DSP), which results in digestive upset but which is not fatal.

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What is a Dinoflagellate? / Updated 5/25/97 / biolum@ucsd.edu