Protist

Protist is a general term for eukaryotic organisms—those whose cells contain a nucleus—but which are not classified as animals, plants or fungi. Most are single-celled, though some, such as seaweeds, are multicellular.

Protists do not form a single, unified group derived from one common ancestor. They are a diverse collection of lineages united only by what they lack: membership in the other three kingdoms. For this reason, modern taxonomists use the term as a descriptive label rather than a formal taxonomic rank.

History of the term

The term was introduced in the nineteenth century to classify microscopic organisms that did not fit clearly into either the plant or animal category. Later, the five-kingdom system established Protista as an independent kingdom. With advances in DNA analysis, that classification has been abandoned.

General characteristics

Protists inhabit diverse environments: oceans, lakes, rivers and soil; some live inside other organisms. They obtain food in various ways: some are photosynthetic (making food from light), some consume other organisms, and some do both. Movement is achieved through cilia, flagella or pseudopodia (false feet).

Examples

Common protists include amoeba, paramecium, euglena, diatoms and various seaweeds (green, brown and red). Also included are disease-causing protists, notably Plasmodium, which causes malaria, and Trypanosoma.

Ecological importance

Photosynthetic protists, especially those in the ocean, play a major role in producing oxygen and forming the base of marine food chains. Others participate in the decomposition of organic matter.