Protist

Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy. Protists live in almost any environment that contains liquid water. Many protists, such as algae, are photosynthetic and are vital primary producers in ecosystems, particularly in the ocean as part of the plankton. Others are saprophytes, or organisms that decompose dead organisms, and parasites, which harm their host.

Classification

Protists were traditionally subdivided into several groups based on similarities to the "higher" kingdoms such as:

• Protozoa, which includes amoebae, flagellates, ciliates, sporozoans, and many other forms of "animal-like" microbes.
• Algae, which are plant-like protists that can be unicellular or multicellular.
• Fungus-like protists, which include slime molds and water molds.

However, modern classifications attempt to assign protists into monophyletic groups, which are sets of organisms that includes all the descendants of a common ancestor.

Characteristics

Protists are a diverse group of organisms, with a wide range of characteristics. Some protists, like the amoeba, move by pseudopodia, while others, like paramecium, move by cilia. Some protists, like the euglena, have chloroplasts and can perform photosynthesis, while others, like the plasmodium, are parasitic and cause disease.

Reproduction

Protists reproduce by a variety of mechanisms. Most protists reproduce asexually under favorable conditions, but will sometimes reproduce sexually under stress. Some protists, like the amoeba, reproduce by binary fission, while others, like the paramecium, reproduce by conjugation.

Ecological role

Protists play a vital role in their ecosystems. Many protists, such as algae, are photosynthetic and are vital primary producers in ecosystems, particularly in the ocean as part of the plankton. Others are saprophytes, decomposing dead organisms and recycling their nutrients back into the ecosystem.

See also

• Eukaryote
• Protozoa
• Algae
• Fungus-like protists

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Protist

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  Euglena gracilis

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  Giant kelp (Macrocystis pyrifera) holdfast

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