Even the simplest microscope can open a window on a menagerie of diverse organisms in a drop of pond water. Over 300 years ago, the Dutch naturalist Antonie van Leeuwenhoek was one of the first to observe and describe living organisms invisible to the naked eye. Using his single-lens microscope, Leeuwenhoek saw microbes that behaved like tiny animals, swimming actively and capturing prey. Others were as green as grass, growing as single cells, tiny clusters, or chains of cells as fine as a hair.

Many of the organisms in Leeuwenhoek’s samples were single-celled eukaryotes. Today, we refer to these organisms collectively as protists. They’re members of the kingdom Protista.

Most protists are unicellular—that is, the individual organism lives its entire life as a single cell. But some protists exist as colonies of cells, and some are multicellular, though they don’t have the complex structure of plants, animals, and fungi. Protists can live almost anywhere there is water: moist soil, lakes and streams, the oceans, and even the body fluids of other organisms.

The kingdom Protista is a category of convenience, used for the eukaryotes that don’t fit well into the other three kingdoms. The organisms currently considered protists are amazingly diverse, and their taxonomic relationships are the subject of intense study. Many systematists believe that groups of related protists should be placed in their own kingdoms to properly reflect their differences from each other and from the multicellular eukaryotes.

This activity broadly surveys the diversity of fascinating and beautiful organisms in the kingdom Protista, many of which are hidden from the naked eye by their small size. Anyone with a microscope can rediscover the wonder Leeuwenhoek experienced at seeing protists hundreds of years ago. We’ll begin with a look at the evolutionary origins of the eukaryotic cell and the general features of members of the kingdom Protista. After that, we’ll consider the variety of protists grouped informally according to lifestyle and nutrition.

First, we’ll look at protozoans, those protists that behave like tiny animals. Then we’ll explore some of the protists that derive energy from sunlight like plants. Finally, we’ll consider some protists that have features reminiscent of fungi.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education