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The Protista are predominantly unicellular forms, although colonial aggregations of various sorts are not uncommon. All are eukaryotes, and they may be either photoautotrophs or chemoheterotrophs. Perhaps the most fascinating example of the range of functional capabilities found in these cells are those displayed by Ochromonas malhamensis , a member of the golden algae or Chrysophycophyta. These algae are so named because their photosynthetic pigments are goldenyellow, as compared to the pigments of the green algae or Chlorophycophyta, for example.

When grown in sunlight, O. malhamensis carries out pho-tosynthesis and uses very few preformed molecules in its nutrition. It is very close to being a photoautotroph. When deprived of sunlight, but provided with such organic molecules as the proteins, nucleic acids, and certain lipids and carbohydrates found in a nutrient broth, it is a chemoheterotroph. It releases enzymes into its environment to break down macromolecules to amino acids, nucleotides, sugars, etc., which then are moved into the cell to be utilized. Finally, when O. malhamensis is denied light and a nutrient broth, but is provided with bacteria in its environment, it uses them as food. Here, too, it is a chemoheterotroph, since it takes the bacteria into food vacuoles where digestion occurs.

These three nutritional activities must be viewed ecologically to be properly appreciated. In effect, O. malhamensis can function as a producer (a photoautotroph), a decomposer (a chemoheterotroph living off organic macromolecules), and a consumer (a chemoheterotroph living off ingested food organisms). We see in these cells the capabilities that finally emerge in the three great kingdoms of multicellular organisms, i.e., the Metaphyta as producers, the Fungi as decomposers, and the Metazoa as consumers. The protistans represent the evolutionary playground where these three major trends were sorted out. That, combined with the emergence of multicellularity, is an important way to view the evolutionarydevelopments within the Protista. Here we can introduce two useful terms, the protophyta and the protozoa.

Protophyta is not a taxon used in any systematic schema. The term covers the unicellular eukaryotic algae and refers to those protists that are producers. Comparably, we have the protozoa, which are the protistan consumers. They have been treated as a taxon, but the diversity within the protozoans is better viewed in terms of several different protistan phyla. The term is nonetheless useful to indicate all those eukaryote cells that show animal tendencies, i.e., that are consumers or ingestors.