In some biological taxonomy schemes, protists (/ˈproʊtɨst/) are a large and diverse group of eukaryotic microorganisms, which belong to the kingdom Protista. There have been attempts to remove the kingdom from modern taxonomy but it is still very much in use.[1][2][3] The term Protoctista is also used for these organisms by various organisations and institutions.[4][5][6] Molecular information has been used to redefine this group in modern taxonomy as diverse and often distantly related phyla. The group of protists is now considered to mean diverse phyla that are not closely related through evolution and have different life cycles, trophic levels, modes of locomotion and cellular structures.[7][8] Besides their relatively simple levels of organization, the protists do not have much in common.[9] They are unicellular, or they are multicellular without specialized tissues; this simple cellular organization distinguishes the protists from other eukaryotes, such as fungi, animals and plants, although some fungi and animals are also unicellular[citation needed].
The term protista was first used by Ernst Haeckel in 1866. Protists were traditionally subdivided into several groups based on similarities to the "higher" kingdoms: the unicellular "animal-like" protozoa, the "plant-like" protophyta (mostly unicellular algae), and the "fungus-like" slime molds and water molds. These traditional subdivisions, largely based on superficial commonalities, have been replaced by classifications based on phylogenetics (evolutionary relatedness among organisms). However, the older terms are still used as informal names to describe the morphology and ecology of various protists.
Protists live in almost any environment that contains liquid water. Many protists, such as the algae, are photosynthetic and are vital primary producers in ecosystems, particularly in the ocean as part of the plankton. Other protists include pathogenic species such as the kinetoplastid Trypanosoma brucei, which causes sleeping sickness and species of the apicomplexan Plasmodium which cause malaria.
