Marine protists

Marine protists

Alga (phytoplankton)

Protozoan (zooplankton)

Protists are usually one-celled microorganisms. They include algae (autotrophs which make their own food) and protozoans (heterotrophs which eat the algae for food). In recent years, researchers have discovered many protists are mixotrophs, which can function in both modes.

Marine protists are defined by their habitat as protists that live in marine environments, that is, in the saltwater of seas or oceans or the brackish water of coastal estuaries. Life originated as marine single-celled prokaryotes (bacteria and archaea) and later evolved into more complex eukaryotes. Eukaryotes are the more developed life forms known as plants, animals, fungi and protists. Protists are the eukaryotes that cannot be classified as plants, fungi or animals. They are mostly single-celled and microscopic. The term protist came into use historically as a term of convenience for eukaryotes that cannot be strictly classified as plants, animals or fungi. They are not a part of modern cladistics because they are paraphyletic (lacking a common ancestor for all descendants).

Most protists are too small to be seen with the naked eye. They are highly diverse organisms currently organised into 18 phyla, but not easy to classify.^[1][2] Studies have shown high protist diversity exists in oceans, deep sea-vents and river sediments, suggesting large numbers of eukaryotic microbial communities have yet to be discovered.^[3][4] There has been little research on mixotrophic protists, but recent studies in marine environments found mixotrophic protists contribute a significant part of the protist biomass.^[5] Since protists are eukaryotes (and not prokaryotes) they possess within their cell at least one nucleus, as well as organelles such as mitochondria and Golgi bodies. Many protist species can switch between asexual reproduction and sexual reproduction involving meiosis and fertilization.^[6]

In contrast to the cells of prokaryotes, the cells of eukaryotes are highly organised. Plants, animals and fungi are usually multi-celled and are typically macroscopic. Most protists are single-celled and microscopic. But there are exceptions. Some single-celled marine protists are macroscopic. Some marine slime molds have unique life cycles that involve switching between unicellular, colonial, and multicellular forms.^[7] Other marine protist are neither single-celled nor microscopic, such as seaweed.

Protists have been described as a taxonomic grab bag of misfits where anything that does not fit into one of the main biological kingdoms can be placed.^[8] Some modern authors prefer to exclude multicellular organisms from the traditional definition of a protist, restricting protists to unicellular organisms.^[9][10] This more constrained definition excludes all brown, the multicellular red and green algae, and, sometimes, slime molds (slime molds excluded when multicellularity is defined as "complex").^[11]

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Marine life
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