What are Protists?

Because it is easier to use pictures rather than words, the pictures at the top of the page show representatives of this complex group of organisms. What they all have in common is that they need water as a habitat. Most of them are very small and you need a microscope to see them. In the picture above we see from left to right: testate amoebae, a green alga, a red alga and a golden alga.

In former times a taxonomical kingdom named “Protists” or “Protoctists” existed, including all unicellular organized organsims with nuclei, see “Five Kingdoms” by Lynn Margulis and Karlene Schwartz, New York 1982. The term “protist” is used today only informally and includes unicellular organized organisms with (eukaryotes) and without a cell nucleus (prokaryotes), see “Biodiversity and Earth History” by Jens Boenigk, Sabina Wodniok and Edvard Glücksman, Berlin 2015.

Some important characteristics of protists:

• The organization of protists is mostly unicellular or in chains of cells without functional differenciation.
• In eukaryotic protists, the cells have organelles, for example for nutrition, all types of metabolism, osmoregulation and, in certain cases, also for defense.
• Their cell sizes range from a few micrometers (some flagellates) to a few millimeters (e.g. Spirostomum species, Ciliophora).
• Representatives of the multicellular protist groups of brown and red algae as well as some slime mold species can reach sizes of several square meters.

 
Unlike animals and higher plants, single-celled organisms do not have sexual reproduction with embryogenesis. However, the cells of eukaryotic single-celled organisms also have a cell nucleus that controls vegetative and generative life processes.

For a detailed taxonomic overview please look at EOL, a project of Smithsonian Institute called “Encyclopedia of Life”.