LiTtLe aQuARiA: January 2010

What are dinoflagellates?

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.

Why bother?

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.

source: http://www.geo.ucalgary.ca/~macrae/palynology/dinoflagellates/dinoflagellates.html