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A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms and eukaryotic organisms. Most prokaryotes are unicellular and are classified into bacteria and archaea
At the base of the ocean food web are single-celled algae and other plant-like organisms known as phytoplankton. The second trophic level (primary consumers) is occupied by zooplankton which feed off the phytoplankton. Higher order consumers complete the web. There has been increasing recognition in recent years that marine microorganisms.
It is one of the largest single-celled organisms known, at up to 20 centimetres (8 in) across. [4] It was first described in 1882 by the oceanographer John Murray , after being discovered on an expedition in the ship Triton which dredged the deep ocean bed off the west coast of Scotland in an effort to find organisms new to science.
At first, Haeckel included a few multicellular organisms in this kingdom, but in later work, he restricted the Protista to single-celled organisms, or simple colonies whose individual cells are not differentiated into different kinds of tissues. [21] Frederick Chapman's The foraminifera: an introduction to the study of the protozoa (1902)
Valonia ventricosa is among the largest known single-celled organisms. Its thallus consists of a thin-walled, tough, multinucleate cell with a diameter that ranges typically from 1 to 4 centimetres (0.4 to 1.6 in), although it may achieve a diameter of up to 5.1 centimetres (2.0 in) in rarer cases.
The vegetative phase in the life cycle of a zooxanthellae is the predominant form of the organism. [14] In this form, the single-celled organism has a thin cell wall. As opposed to the zoospore, the zooxanthella contains numerous chloroplasts. Once the cell continues growing, however, chloroplasts decrease in abundance. [14]
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 archaeal cell was a member of the Asgard group. The bacterium was one of the Alphaproteobacteria, which had the ability to use oxygen in its respiration. This enabled it – and the archaeal cells that included it – to survive in the presence of oxygen, which was poisonous to other organisms adapted to reducing conditions.
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