5.2: Introduction to Protists

Protists are a group of organisms that have traditionally been placed together because they are primarily single-celled eukaryotic organisms. They are often categorized into plant-like, animal-like, and fungal-like groups. However, they are not phylogenetically related as a major group and their taxonomy is in a state of flux. Scientists are studying the genetics of these organisms to more accurately develop a taxonomic scheme that is meaningful, instead of just dumping them all together. In today’s lab you will observe live specimens and prepared slides in order to observe the diversity of the group and get a sense of size of these unique and, despite their size, complex organisms. Many Protists are disease-causing organisms. Some terms you will need as you view the prepared slides (hint—always look at the label of the slide for clues about what you will observe on that slide):

• Trophozoite: the active feeding stage
• Sporozoite: the motile infective stage
• Merozoite: the result of schizogony (a trophozoite replicates its organelles and nuclei multiple times, then the cell divides into multiple daughter cells called merozoites)
• Cyst: the dormant stage, usually resistant to harsh or changing conditions, also the stage that is often part of transmission from host to host.

Taxonomic Supergroups:

• Amoebozoa: use pseudopodia for feeding and motility
  • Entamoeba, (and slime molds)
• Excavata: most have a flagellum which aids in feeding
  • Giardia, Trichomonas, Trypanosoma, Leishmania
• Chromalveolata: ciliates and apicomplexans (and Dinoflagellates, Diatoms, brown algae, golden algae)
  • Balantidium, Cryptosporidium, Toxoplasma, Plasmodium
  • Rhizaria: (radiolarians, forams)
• Archaeplastida: (Green algae)

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