Dinoflagellates are microscopic, (usually) unicellular, flagellated, often photosynthetic protists, commonly regarded as "algae" (Division Dinoflagellata). They are characterized by a transverse flagellum that encircles the body (often in a groove known as the cingulum) and a longitudinal flagellum oriented perpendicular to the transverse flagellum. This imparts a distinctive spiral to their swimming motion. Both flagella are inserted at the same point in the cell wall, by convention defining the ventral surface. This point is usually slightly depressed, and is termed the sulcus. In heterotrophic dinoflagellates (ones that eat other organisms), this is the point where a conical feeding structure, the peduncle, is projected in order to consume food.
Dinoflagellates possess a unique nuclear structure at some stage of their life cycle - a dinokaryotic nucleus (as opposed to eukaryotic or prokaryotic), in which the chromosomes are perminently condensed. The cell wall of many dinoflagellates is divided into plates of cellulose ("armor") within amphiesmal vesicles, known as a theca. These plates form a distinctive geometry/topology known as tabulation, which is the main means for classification.
Both heterotrophic (eat other organisms) and autotrophic (photosynthetic) dinoflagellates are known. Some are 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, both sexual and asexual, motile and non-motile. Some species form cysts composed of sporopollenin (an organic polymer), and preserve as fossils. Often the tabulation of the cell wall is somehow expressed in the shape and/or ornamentation of the cyst.
Besides being important primary producers, and therefore an important part of the food chain, dinoflagellates are also known for producing nasty toxins, particularly when they occur in large numbers, called "red tides" because the cells are so abundant they make the water change colour. Besides being bad for a large range of marine life, red tides can also introduce non-fatal or fatal amounts of toxins into animals (particularly shellfish) that may be eaten by humans, who are also affected by the toxins. Many of these toxins are quite potent, and if not fatal, can still cause neurological and all sorts of other nasty effects. Add this to the rather ominous suspicion that red tides may be more common thanks to human inputs of phosphates and warmer global temperatures, and you can probably see why we have a vested interest in finding out more about them - both medical and economic.
These images are scanning electron microscope images from a plankton tow conducted by Graham Williams ( e-mail:williamsg@agc.bio.ns.ca) and Bill MacMillan ( e-mail:macmillanw@agc.bio.ns.ca) in the harbor of Halifax, Nova Scotia on July 23, 1994.
Ceratium sp.A dinoflagellate with large "horns" and a gonyaulacoid tabulation. This is the dorsal surface, opposite the sulcus. The distinctive groove going around the central part of the body is where the transverse flagellum occurs (not preserved). The fine holes in the cell wall are the apertures for trichocysts. Scale bar 50 microns.
An enlarged view
Dinophysis sp.The anterior part of this dinoflagellate is greatly reduced in size, so the cingulum forms a distinctive groove near the "top" of this image. The wall-like structures on the left occur at the edges of the sulcus, and are known as "lists". The tabulation in the posterior part of the body ("below" the cingulum) is dominated by two large plates that form almost the entire theca. The suture between these plates occurs along the right, lower, and left (sulcal) edges of the body in this image. Scale bar 10 microns.
Protoperidinium sp.
Evitt, W.R., 1985. Sporopollenin Dinoflagellate Cysts: Their Morphology and Interpretation. American Association of Stratigraphic Palynologists Foundation, p.1-333. ISBN 0-931871-00-X
Fensome, R.A.; Taylor, F.J.R.; Norris, G.; Sarjeant, W.A.S.; Wharton, D.I; and Williams, G.L., 1993. A Classification of Living and Fossil Dinoflagellates. American Museum of Natural History, Micropaleontology, Special Publication Number 7, p.1-351. ISBN 0160-2071
Edwards, L.E., 1993. Chapter 7: Dinoflagellates. IN: Lipps, J.H. (ed.), Fossil Prokaryotes and Protists. Blackwell Scientific Publications:Boston, p.105-129. ISBN 0-86542-073-4
Sarjeant, W.A.S., 1974. Fossil and living dinoflagellates. Academic Press: London, p.1-182. ISBN 0-12-6191506
Taylor, F.J.R. (ed.), 1987. The Biology of Dinoflagellates. Botanical Monographs, Volume 21. Blackwell Scientific Publications:Oxford, p.1-785. ISBN 0-632-00915-2
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