Volume 15, Number 4 • July-August 1997

Dinoflagellates, Protists and Pfiesteria

To botanists, dinoflagellates are microscopic plants, algae that are also referred to as phytoplankton; to zoologists, they are protozoans, one of the simplest, most primitive animals. The difference in classification arises because half of the known species of dinoflagellates are photosynthetic - like plants they manufacture their own food. The other half are heterotrophic, meaning they must feed on other organisms for nourishment as animals do. Dinoflagellates are now classified in the kingdom Protista, which includes simple single-celled organisms that cannot be classified as either plants or animals.

Many of the photosynthetic species, says JoAnn Burkholder, can switch between plant-like and animal-like modes of nutrition. "Pfiesteria," she says, "is actually a little animal . . . thousands of these little creatures would fit on the head of a pin." So far, of the 24 stages have been identified, at least four stages have been shown to be toxic. However, for much of the year, says Burkholder, "its various forms are usually harmless members of the food chain that survive by eating bacteria, algae and small organisms."

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