View source for Unicellular organism

{{short description|Organism that consists of only one cell}}
{{Infobox
| above = Unicellular organism
| image =[[File:Ventricaria ventricosa.JPG|220px|alt=]]
| caption = ''[[Valonia ventricosa]]'', a species of [[alga]] with a diameter that ranges typically from {{convert|1|to|4|cm|in|1}}, is among the largest unicellular species
}}
A '''unicellular organism''', also known as a '''single-celled organism''', is an [[organism]] that consists of a single [[cell (biology)|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]]. Many eukaryotes are multicellular, but some are unicellular such as [[protozoa]], unicellular [[algae]], and unicellular [[fungi]]. Unicellular organisms are thought to be the oldest form of life, with early organisms emerging 3.5–3.8&nbsp;billion years ago.<ref>{{cite web|title=How Did Multicellular Life Evolve?|first=Charles Q|last=Choi|url=https://astrobiology.nasa.gov/news/how-did-multicellular-life-evolve/|publisher=NASA Astrobiology|access-date=2025-05-16}}</ref><ref>{{cite book|title=The Cell: A Molecular Approach|edition=2nd|last=Cooper|first=GM|chapter=The Origin and Evolution of Cells |location=Sunderland, Massachusetts|publisher=Sinauer Associates|year=2000|url=https://www.ncbi.nlm.nih.gov/books/NBK9841/}}</ref>

Although some prokaryotes live in [[Colony (biology)|colonies]], they are not specialised cells with differing functions. These organisms live together, and each cell must carry out all life processes to survive. In contrast, even the simplest multicellular organisms have cells that depend on each other to survive.

Most multicellular organisms have a unicellular life-cycle stage. [[Gamete]]s, for example, are reproductive unicells for multicellular organisms.<ref>{{cite journal |title=Understanding "green" multicellularity: do seaweeds hold the key?|journal= Frontiers in Plant Science|date=2015-01-01 |pmc=4299406 |pmid=25653653 |page=737 |volume=5 |doi=10.3389/fpls.2014.00737 |first1=Juliet C. |last1=Coates |author2=Umm-E-Aiman |first3=Bénédicte |last3=Charrier|doi-access= free|bibcode= 2015FrPS....5..737C}}</ref> 

Some organisms are partially unicellular, like ''[[Dictyostelium discoideum]]''. Additionally, unicellular organisms can be [[multinucleate]], like ''[[Caulerpa]]'', ''[[Plasmodium]]'', and [[Myxogastria]].

== Evolutionary hypothesis ==
{{Life timeline}}
The [[origin of life]] is largely still a mystery.  Primitive [[protocell]]s are thought to be the precursors to today's unicellular organisms. 

In one theory, known as the [[RNA world|RNA world hypothesis]], early RNA molecules would have been the basis for catalyzing organic chemical reactions and self-replication.<ref name="NLane_Funke">{{cite book | vauthors = Lane N | author-link1 = Nick Lane | url = https://archive.org/details/vitalquestionene0000lane | url-access = registration | page = [https://archive.org/details/vitalquestionene0000lane/page/77 77] | title = The Vital Question – Energy, Evolution, and the Origins of Complex Life | publisher = WW Norton | date = 2015 | isbn = 978-0-393-08881-6 }}</ref>

Compartmentalization was necessary for chemical reactions to be more likely as well as to differentiate reactions with the external environment. For example, an early RNA replicator [[ribozyme]] may have replicated other replicator ribozymes of different RNA sequences if not kept separate.<ref name="Exploring">{{cite web |title=Exploring Life's Origins: Fatty Acids |url=http://exploringorigins.org/fattyacids.html |website=exploringorigins.org |access-date=2015-10-28}}</ref> Such hypothetic cells with an RNA genome instead of the usual DNA genome are called '[[ribocells]]' or 'ribocytes'.<ref name="NLane_Funke" />

When [[amphiphile]]s like [[lipid]]s are placed in water, the hydrophobic tails aggregate to form [[micelle]]s and [[Vesicle (biology and chemistry)|vesicles]], with the hydrophilic ends facing outwards.<ref name="Pohorille">{{cite journal |title=Self-assembly and function of primitive cell membranes |last1=Pohorille |first1=Andrew |author-link=Andrew Pohorille |date=2009-06-23 |journal=Research in Microbiology |doi= 10.1016/j.resmic.2009.06.004|pmid= 19580865|last2=Deamer |first2=David |volume=160 |issue=7 |pages=449–456|doi-access=free |bibcode=2009ResMb.160..449P }}</ref><ref name="Exploring" /> Primitive cells likely used self-assembling fatty-acid vesicles to separate chemical reactions and the environment.<ref name="Exploring" /> Because of their simplicity and ability to self-assemble in water, it is likely that these simple [[Lipid bilayer|membranes]] predated other forms of early biological molecules.<ref name="Pohorille" />

== Prokaryotes ==
Prokaryotes lack membrane-bound organelles, such as [[Mitochondrion|mitochondria]] or a [[Cell nucleus|nucleus]].<ref>{{cite web |title=Prokaryotes |url=http://webprojects.oit.ncsu.edu/project/bio183de/Black/prokaryote/prokaryote1.html |website=webprojects.oit.ncsu.edu |access-date=2015-11-22 |archive-date=2022-09-22 |archive-url=https://web.archive.org/web/20220922110555/https://webprojects.oit.ncsu.edu/project/bio183de/Black/prokaryote/prokaryote1.html }}</ref> Instead, most prokaryotes have an irregular region that contains DNA, known as the [[nucleoid]].<ref>{{cite journal |title=The bacterial nucleoid: nature, dynamics and sister segregation |journal=Current Opinion in Microbiology |date=2014-12-01 |pmc=4359759 |pmid=25460806 |pages=127–137 |volume=22 |series=Growth and development: eukaryotes/ prokaryotes |doi=10.1016/j.mib.2014.10.001 |first1=Nancy |last1=Kleckner |first2=Jay K. |last2=Fisher |first3=Mathieu |last3=Stouf |first4=Martin A. |last4=White |first5=David |last5=Bates |first6=Guillaume |last6=Witz}}</ref> Most prokaryotes have a single, circular [[chromosome]], which is in contrast to eukaryotes, which typically have linear chromosomes.<ref>{{cite web |title=Eukaryotic Chromosome Structure {{!}} Science Primer |url=http://scienceprimer.com/eukaryotic-chromosome-structure |website=scienceprimer.com |access-date=2015-11-22}}</ref> Nutritionally, prokaryotes have the ability to utilize a wide range of organic and inorganic material for use in metabolism, including sulfur, cellulose, ammonia, or nitrite.<ref name="Dwight G Smith">{{cite book |title=Bacteria |last=Smith |first=Dwight G |publisher=Salem Press Encyclopedia of Science |year=2015 |isbn=978-1-58765-084-0 |url-access=registration |url=https://archive.org/details/magillsencyclope0000unse }}</ref> Prokaryotes are relatively ubiquitous in the environment and some (known as extremophiles) thrive in extreme environments.{{cn|date=January 2025}}

=== Bacteria ===
[[File:Stromatolites in Sharkbay.jpg|thumb|Modern [[stromatolite]]s in Shark Bay, Western Australia. It can take a century for a stromatolite to grow 5 cm.<ref>{{cite web |title=Nature Fact Sheets – Stromatolites of Shark Bay » Shark Bay |url=http://www.sharkbay.org.au/nature-of-shark-bay/fact-sheets/stromatolites-of-shark-bay.aspx |website=www.sharkbay.org.au |access-date=2015-11-22}}</ref>]]
[[File:Bacteria with capsule.jpg|left|thumb|242x242px|'''Bacteria in a capule''']]
Bacteria are one of the world's oldest forms of life, and are found virtually everywhere in nature.<ref name="Dwight G Smith" /> Many common bacteria have [[plasmid]]s, which are short, circular, self-replicating DNA molecules that are separate from the bacterial chromosome.<ref>{{cite web |title=Conjugation (prokaryotes) |url=http://www.nature.com/scitable/definition/conjugation-prokaryotes-290 |website=www.nature.com |access-date=2015-11-22}}</ref> Plasmids can carry genes responsible for novel abilities, of current critical importance being antibiotic resistance.<ref name="Lactic">{{cite journal |title=Plasmids from Food Lactic Acid Bacteria: Diversity, Similarity, and New Developments |journal=International Journal of Molecular Sciences |date=2015-06-10 |pmc=4490491 |pmid=26068451 |pages=13172–13202 |volume=16 |issue=6 |doi=10.3390/ijms160613172 |first1=Yanhua |last1=Cui |first2=Tong |last2=Hu |first3=Xiaojun |last3=Qu |first4=Lanwei |last4=Zhang |first5=Zhongqing |last5=Ding |first6=Aijun |last6=Dong|doi-access=free }}</ref> Bacteria predominantly reproduce asexually through a process called [[Fission (biology)|binary fission]]. However, about 80 different species can undergo a sexual process referred to as natural [[transformation (genetics)|genetic transformation]].<ref name="pmid24509783">{{cite journal |vauthors=Johnston C, Martin B, Fichant G, Polard P, Claverys JP |title=Bacterial transformation: distribution, shared mechanisms and divergent control |journal=Nat. Rev. Microbiol. |volume=12 |issue=3 |pages=181–96 |year=2014 |pmid=24509783 |doi=10.1038/nrmicro3199 |s2cid=23559881 }}</ref> Transformation is a bacterial process for transferring DNA from one cell to another, and is apparently an adaptation for [[DNA repair|repairing DNA]] damage in the recipient cell.<ref name=Bernsteinpathogens>{{cite journal |last1=Bernstein |first1=Harris |last2=Bernstein |first2=Carol |last3=Michod |first3=Richard E. |title=Sex in microbial pathogens |journal=Infection, Genetics and Evolution |date=January 2018 |volume=57 |pages=8–25 |doi=10.1016/j.meegid.2017.10.024 |pmid=29111273 |doi-access=free |bibcode=2018InfGE..57....8B }}</ref> In addition, plasmids can be exchanged through the use of a [[pilus]] in a process known as [[Bacterial conjugation|conjugation]].<ref name="Lactic" />

The photosynthetic [[cyanobacteria]] are arguably the most successful bacteria, and changed the early atmosphere of the earth by oxygenating it.<ref name="Fossil record">{{cite web |title=Fossil Record of the Cyanobacteria |url=http://www.ucmp.berkeley.edu/bacteria/cyanofr.html |website=www.ucmp.berkeley.edu |access-date=2015-11-22}}</ref> [[Stromatolite]]s, structures made up of layers of [[calcium carbonate]] and trapped sediment left over from cyanobacteria and associated community bacteria, left behind extensive fossil records.<ref name="Fossil record" /><ref name="McNamara">{{cite book |title=Stromatolites |url=https://books.google.com/books?id=EbjBAgAAQBAJ |publisher=Western Australian Museum |date=2009-09-01 |isbn=978-1-920843-88-5 |first=Kenneth |last=McNamara}}</ref> The existence of stromatolites gives an excellent record as to the development of cyanobacteria, which are represented across the [[Archean|Archaean]] (4&nbsp;billion to 2.5&nbsp;billion years ago), [[Proterozoic]] (2.5&nbsp;billion to 540&nbsp;million years ago), and [[Phanerozoic]] (540&nbsp;million years ago to present day) eons.<ref name="McNamara" /> Much of the fossilized stromatolites of the world can be found in [[Western Australia]].<ref name="McNamara" /> There, some of the oldest stromatolites have been found, some dating back to about 3,430&nbsp;million years ago.<ref name="McNamara" />

Clonal [[ageing|aging]] occurs naturally in [[bacteria]], and is apparently due to the accumulation of damage that can happen even in the absence of external stressors.<ref>{{cite journal | last1 = Łapińska | first1 = U | last2 = Glover | first2 = G | last3 = Capilla-Lasheras | first3 = P | last4 = Young | first4 = AJ | last5 = Pagliara | first5 = S | year = 2019| title = Bacterial ageing in the absence of external stressors | journal = Philos Trans R Soc Lond B Biol Sci | volume =  374| issue = 1786| article-number =  20180442| doi = 10.1098/rstb.2018.0442 | pmid = 31587633 | pmc = 6792439 }}</ref>

=== Archaea ===
[[File:Echinoderms 600.jpg|thumb|215x215px|A bottom-dwelling community found deep in the European Arctic.<ref>{{cite web |title=NOAA Ocean Explorer: Arctic Exploration 2002: Background |url=http://oceanexplorer.noaa.gov/explorations/02arctic/background/benthos/benthos.html |website=oceanexplorer.noaa.gov |access-date=2015-11-22}}</ref>]]

Hydrothermal vents release heat and [[hydrogen sulfide]], allowing extremophiles to survive using [[chemolithotroph]]ic growth.<ref>{{cite book |date=2014-01-01 |issn=1559-0836 |pmid=25416397 |pages=237–277 |volume=14 |doi=10.1007/978-94-017-9269-1_10 |first1=Larry L. |last1=Barton |first2=Marie-Laure |last2=Fardeau |first3=Guy D. |last3=Fauque|title=The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment |chapter=Hydrogen Sulfide: A Toxic Gas Produced by Dissimilatory Sulfate and Sulfur Reduction and Consumed by Microbial Oxidation |series=Metal Ions in Life Sciences |isbn=978-94-017-9268-4 }}</ref>
Archaea are generally similar in appearance to bacteria, hence their original classification as bacteria, but have significant molecular differences most notably in their membrane structure and ribosomal RNA.<ref>{{cite web |title=Archaea |url=http://www.microbeworld.org/types-of-microbes/archaea |website=www.microbeworld.org |access-date=2015-11-22 |archive-date=2015-11-23 |archive-url=https://web.archive.org/web/20151123080925/http://www.microbeworld.org/types-of-microbes/archaea }}</ref><ref name="Archaeal">{{Cite book | last1= Londei | first1= Paola |chapter = Archaeal Ribosomes | title = eLS | title-link=Encyclopedia of Life Sciences | date = 2020 | doi =10.1002/9780470015902.a0000293.pub3| isbn= 978-0-470-01617-6 }}</ref> By sequencing the ribosomal RNA, it was found that the Archaea most likely split from bacteria and were the precursors to modern eukaryotes, and are actually more phylogenetically related to eukaryotes.<ref name="Archaeal" /> As their name suggests, Archaea comes from a Greek word ''archaios,'' meaning original, ancient, or primitive.<ref>{{cite web |title=archaea {{!}} prokaryote |url=http://www.britannica.com/science/archaea |website=Encyclopedia Britannica |access-date=2015-11-22}}</ref>

Some archaea inhabit the most biologically inhospitable environments on earth, and this is believed to in some ways mimic the early, harsh conditions that life was likely exposed to{{citation needed|date=June 2021}}. Examples of these Archaean [[extremophile]]s are as follows:
* [[Thermophile]]s, optimum growth temperature of 50&nbsp;°C-110&nbsp;°C, including the genera ''[[Pyrobaculum]]'', ''[[Pyrodictium]]'', ''[[Pyrococcus]]'', ''[[Thermus aquaticus]]'' and ''[[Melanopyrus]].''<ref name="Extremophiles overview">{{cite journal |title=Extremophiles: An Overview of Microorganism from Extreme Environment |url=https://www.researchgate.net/publication/261547855 |journal=International Journal of Agriculture, Environment and Biotechnology |volume=7 |issue=2 |page=371 |access-date=2015-11-22 |doi=10.5958/2230-732X.2014.00258.7|year=2014 |last1=Gupta |first1=G.N. |last2=Srivastava |first2=S. |last3=Khare |first3=S.K. |last4=Prakash |first4=V. |doi-access=free }}</ref>
* [[Psychrophile]]s, optimum growth temperature of less than 15&nbsp;°C, including the genera ''[[Methanogenium]]'' and ''[[Halorubrum]].''<ref name="Extremophiles overview" />
* [[Alkaliphile]]s, optimum growth pH of greater than 8, including the genus ''[[Natronomonas]]''.<ref name="Extremophiles overview" /><ref>{{cite journal |title=Living with two extremes: Conclusions from the genome sequence of Natronomonas pharaonis |journal=Genome Research |date=2005-10-01 |issn=1088-9051 |pmc=1240075 |pmid=16169924 |pages=1336–1343 |volume=15 |issue=10 |doi=10.1101/gr.3952905 |first1=Michaela |last1=Falb |first2=Friedhelm |last2=Pfeiffer |first3=Peter |last3=Palm |first4=Karin |last4=Rodewald |first5=Volker |last5=Hickmann |first6=Jörg |last6=Tittor |first7=Dieter |last7=Oesterhelt}}</ref>
* [[Acidophile]]s, optimum growth pH of less than 3, including the genera ''[[Sulfolobus]] and [[Picrophilus]]''.<ref name="Extremophiles overview" /><ref>{{cite book |chapter=Acidophiles  |title=eLS |title-link=Encyclopedia of Life Sciences |date=2018 |publisher=Wiley |doi=10.1002/9780470015902.a0000336.pub3 |isbn=978-0-470-01617-6 |first=Aharon |last=Oren}}</ref>
* [[Barophile|Piezophiles]], (also known as [[Piezophile|barophiles]]), prefer high pressure up to 130 MPa, such as deep ocean environments, including the genera ''[[Methanococcus]]'' and ''[[Pyrococcus]]''.<ref name="Extremophiles overview" />
* [[Halophile]]s, grow optimally in high salt concentrations between 0.2 M and 5.2 M [[Sodium chloride|NaCl]], including the genera ''[[Haloarcula]]'', ''[[Haloferax]]'', ''[[Halococcus]]''.<ref name="Extremophiles overview" /><ref>{{cite web |title="Extremophiles: Archaea and Bacteria": Map of Life |url=http://www.mapoflife.org/topics/topic_354_Extremophiles-Archaea-and-Bacteria/ |website=www.mapoflife.org |access-date=2015-11-22}}</ref>
[[Methanogen]]s are a significant subset of archaea and include many extremophiles, but are also ubiquitous in wetland environments as well as the ruminant and hindgut of animals.<ref name="rumencal">{{cite web |title=Methanogens |url=http://www.vet.ed.ac.uk/clive/cal/rumencal/Info/infMeth.html |website=www.vet.ed.ac.uk |access-date=2015-11-22 |archive-date=2022-09-21 |archive-url=https://web.archive.org/web/20220921150528/http://www.vet.ed.ac.uk/clive/cal/rumencal/Info/infMeth.html }}</ref> This process utilizes hydrogen to reduce carbon dioxide into methane, releasing energy into the usable form of [[adenosine triphosphate]].<ref name="rumencal" /> They are the only known organisms capable of producing methane.<ref>{{cite journal |title=Methanogens: methane producers of the rumen and mitigation strategies |journal=Archaea |date=2010-01-01 |issn=1472-3654 |pmc=3021854 |pmid=21253540 |article-number=945785 |volume=2010 |doi=10.1155/2010/945785 |first1=Sarah E. |last1=Hook |first2=André-Denis G. |last2=Wright |first3=Brian W. |last3=McBride|doi-access=free }}</ref> Under stressful environmental conditions that cause [[DNA damage (naturally occurring)|DNA damage]], some species of archaea aggregate and transfer DNA between cells.<ref name="pmid26884154">{{cite journal |vauthors=van Wolferen M, Wagner A, van der Does C, Albers SV |title=The archaeal Ced system imports DNA |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=113 |issue=9 |pages=2496–501 |year=2016 |pmid=26884154 |pmc=4780597 |doi=10.1073/pnas.1513740113 |bibcode=2016PNAS..113.2496V |doi-access=free }}</ref> The function of this transfer appears to be to replace damaged DNA sequence information in the recipient cell by undamaged sequence information from the donor cell.<ref>{{cite book |doi=10.1007/978-3-319-65536-9 |title=Biocommunication of Archaea |year=2017 |isbn=978-3-319-65535-2 |s2cid=26593032 |editor1-last=Witzany |editor1-first=Guenther }}</ref>

== Eukaryotes ==

Eukaryotic cells contain membrane bound organelles. Some examples include mitochondria, a nucleus, or the Golgi apparatus. Prokaryotic cells probably transitioned into eukaryotic cells between 2.0 and 1.4&nbsp;billion years ago.<ref name="Jay R Yett">{{cite book |title=Eukaryotes |last=Yett |first=Jay R. |publisher=Salem Press Encyclopedia of Science |year=2015 }}</ref> This was an important step in evolution. In contrast to prokaryotes, eukaryotes reproduce by using [[mitosis]] and [[meiosis]]. Sex appears to be a ubiquitous and ancient, and inherent attribute of [[eukaryote|eukaryotic]] life.<ref name="pmid26195746">{{cite journal |author1=Speijer, D. |author2=Lukeš, J. |author3=Eliáš, M. |title=Sex is a ubiquitous, ancient, and inherent attribute of eukaryotic life |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=112 |issue=29 |pages=8827–34 |year=2015 |pmid=26195746 |pmc=4517231 |doi=10.1073/pnas.1501725112 |bibcode=2015PNAS..112.8827S |doi-access=free }}</ref> Meiosis, a true sexual process, allows for efficient [[Genetic recombination|recombination]]al repair of DNA damage <ref name=Bernsteinpathogens /> and a greater range of genetic diversity by combining the DNA of the parents followed by [[Genetic recombination|recombination]].<ref name="Jay R Yett" /> Metabolic functions in eukaryotes are more specialized as well by sectioning specific processes into organelles.{{citation needed|date=June 2021}}

The [[Symbiogenesis|endosymbiotic theory]] holds that mitochondria and chloroplasts have bacterial origins. Both organelles contain their own sets of DNA and have bacteria-like ribosomes. It is likely that modern mitochondria were once a species similar to ''[[Rickettsia]]'', with the parasitic ability to enter a cell.<ref name="Origins">{{cite web |title=Origin of Mitochondria |url=http://www.nature.com/scitable/topicpage/the-origin-of-mitochondria-14232356 |publisher=Nature |access-date=2015-11-23}}</ref> However, if the bacteria were capable of respiration, it would have been beneficial for the larger cell to allow the parasite to live in return for energy and detoxification of oxygen.<ref name="Origins" /> Chloroplasts probably became symbionts through a similar set of events, and are most likely descendants of cyanobacteria.<ref>{{cite web |title=Endosymbiosis and The Origin of Eukaryotes |url=http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/Endosymbiosis.html |website=users.rcn.com |access-date=2015-11-23 |archive-date=2017-06-22 |archive-url=https://web.archive.org/web/20170622083131/http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/Endosymbiosis.html }}</ref> While not all eukaryotes have mitochondria or chloroplasts, mitochondria are found in most eukaryotes, and chloroplasts are found in all plants and algae. Photosynthesis and respiration are essentially the reverse of one another, and the advent of respiration coupled with photosynthesis enabled much greater access to energy than [[fermentation]] alone.{{citation needed|date=June 2021}}

=== Protozoa ===
[[File:Paramecia tetraurelia.jpeg|thumb|''Paramecium tetraurelia'', a ciliate, with oral groove visible]]

[[Protozoa]] are largely defined by their method of locomotion, including [[Flagellum|flagella]], [[Cilium|cilia]], and [[pseudopodia]].<ref name="Klose">{{cite book |title=Protozoa |last=Klose |first=Robert T |publisher=Salem Press Encyclopedia of Science |year=2015 }}</ref> While there has been considerable debate on the classification of protozoa caused by their sheer diversity, in one system there are currently seven phyla recognized under the kingdom Protozoa: [[Euglenozoa]], [[Amoebozoa]], [[Choanozoa|Choanozoa ''sensu'' Cavalier-Smith]], [[Loukozoa]], [[Percolozoa]], [[Microsporidia]] and [[Sulcozoa]].<ref>{{cite journal |title=A Higher Level Classification of All Living Organisms |journal=PLOS ONE |date=2015-04-29 |pmc=4418965 |pmid=25923521 |article-number=e0119248 |volume=10 |issue=4 |doi=10.1371/journal.pone.0119248 |first1=Michael A. |last1=Ruggiero |first2=Dennis P. |last2=Gordon |first3=Thomas M. |last3=Orrell |first4=Nicolas |last4=Bailly |first5=Thierry |last5=Bourgoin |first6=Richard C. |last6=Brusca |first7=Thomas |last7=Cavalier-Smith |first8=Michael D. |last8=Guiry |first9=Paul M. |last9=Kirk|bibcode=2015PLoSO..1019248R |doi-access=free }}</ref><ref>{{cite web |title=Protozoa |url=http://www.microbeworld.org/types-of-microbes/protista/protozoa |website=www.microbeworld.org |access-date=2015-11-23 |archive-date=2017-07-15 |archive-url=https://web.archive.org/web/20170715195337/http://www.microbeworld.org/types-of-microbes/protista/protozoa }}</ref> Protozoa, like plants and animals, can be considered heterotrophs or autotrophs.<ref name="Origins" /> Autotrophs like ''[[Euglena]]'' are capable of producing their energy using photosynthesis, while heterotrophic protozoa consume food by either funneling it through a mouth-like gullet or engulfing it with pseudopods, a form of [[phagocytosis]].<ref name="Origins" /> While protozoa reproduce mainly asexually, some protozoa are capable of sexual reproduction.<ref name="Origins" /> [[Protozoa]] with sexual capability include the pathogenic species ''[[Plasmodium falciparum]]'', ''[[Toxoplasma gondii]]'', ''[[Trypanosoma brucei]]'', ''[[Giardia duodenalis]]'' and ''[[Leishmania]]'' species.<ref name=Bernsteinpathogens />

[[Ciliate|Ciliophora]], or ciliates, are a group of protists that utilize cilia for locomotion. Examples include ''[[Paramecium]]'', [[Stentor (protozoa)|''Stentors'']], and ''[[Vorticella]]''.<ref>{{cite web |title=Ciliophora: ciliates, move with cilia |url=http://www.microscope-microscope.org/applications/pond-critters/protozoans/ciliphora/ciliophora.htm |website=www.microscope-microscope.org |access-date=2015-11-23 |archive-date=2018-03-12 |archive-url=https://web.archive.org/web/20180312073748/http://www.microscope-microscope.org/applications/pond-critters/protozoans/ciliphora/ciliophora.htm }}</ref> Ciliates are widely abundant in almost all environments where water can be found, and the cilia beat rhythmically in order to propel the organism.<ref>{{cite web |title=Introduction to the Ciliata |url=http://www.ucmp.berkeley.edu/protista/ciliata.html |website=www.ucmp.berkeley.edu |access-date=2015-11-23}}</ref> Many ciliates have [[trichocyst]]s, which are spear-like organelles that can be discharged to catch prey, anchor themselves, or for defense.<ref name="Britannica">{{cite web |title=ciliate {{!}} protozoan |url=http://www.britannica.com/science/ciliate |website=Encyclopedia Britannica |access-date=2015-11-23}}</ref><ref>{{cite journal |title=Defensive function of trichocysts in Paramecium tetraurelia against heterotrich ciliate Climacostomum virens |journal=European Journal of Protistology |date=2000-12-29 |pages=415–422 |volume=36 |issue=4 |doi=10.1016/S0932-4739(00)80047-4 |first1=Rika |last1=Sugibayashi |first2=Terue |last2=Harumoto}}</ref> Ciliates are also capable of sexual reproduction, and utilize two nuclei unique to ciliates: a [[macronucleus]] for normal metabolic control and a separate [[micronucleus]] that undergoes meiosis.<ref name="Britannica" /> Examples of such ciliates are ''[[Paramecium]]'' and ''[[Tetrahymena]]'' that likely employ meiotic recombination for repairing DNA damage acquired under stressful conditions.{{citation needed|date=June 2021}}

The Amebozoa utilize pseudopodia and cytoplasmic flow to move in their environment. ''[[Entamoeba histolytica]]'' is the cause of amebic dysentery.<ref>{{cite web |title=amoeba {{!}} protozoan order |url=http://www.britannica.com/science/amoeba-order |website=Encyclopedia Britannica |access-date=2015-11-23}}</ref> ''[[Entamoeba histolytica]]'' appears to be capable of [[meiosis]].<ref name="pmid26422142">{{cite journal |vauthors=Kelso AA, Say AF, Sharma D, Ledford LL, Turchick A, Saski CA, King AV, Attaway CC, Temesvari LA, Sehorn MG |title=Entamoeba histolytica Dmc1 Catalyzes Homologous DNA Pairing and Strand Exchange That Is Stimulated by Calcium and Hop2-Mnd1 |journal=PLOS ONE |volume=10 |issue=9 |article-number=e0139399 |year=2015 |pmid=26422142 |pmc=4589404 |doi=10.1371/journal.pone.0139399 |bibcode=2015PLoSO..1039399K |doi-access=free }}</ref>

=== Unicellular algae ===
[[File:CSIRO ScienceImage 7632 SEM diatom.jpg|thumb|A scanning electron microscope image of a [[diatom]]]]

Unicellular algae are plant-like autotrophs and contain [[chlorophyll]].<ref name="FIP">{{cite web |title=algae Facts, information, pictures {{!}} Encyclopedia.com articles about algae |url=http://www.encyclopedia.com/topic/algae.aspx |website=www.encyclopedia.com |access-date=2015-11-23}}</ref> They include groups that have both multicellular and unicellular species:
* [[Euglenid|Euglenophyta]], flagellated, mostly unicellular algae that occur often in fresh water.<ref name="FIP" /> In contrast to most other algae, they lack cell walls and can be [[mixotroph]]ic (both autotrophic and heterotrophic).<ref name="FIP" /> An example is ''[[Euglena gracilis]]''.
* [[Chlorophyta]] (green algae), mostly unicellular algae found in fresh water.<ref name="FIP" /> The chlorophyta are of particular importance because they are believed to be most closely related to the evolution of land plants.<ref name="biologyreference.com">{{cite web |title=Algae – Biology Encyclopedia – cells, plant, body, human, organisms, cycle, life, used, specific |url=http://www.biologyreference.com/A-Ar/Algae.html |website=www.biologyreference.com |access-date=2015-11-23}}</ref>
* [[Diatom]]s, unicellular algae that have siliceous cell walls.<ref name="MBari">{{cite web |title=siliceous cell walls |url=http://www.mbari.org/staff/conn/botany/diatoms/john/basics/silica/silica.htm |website=www.mbari.org |access-date=2015-11-23 |archive-date=2015-11-21 |archive-url=https://web.archive.org/web/20151121172127/http://www.mbari.org/staff/conn/botany/diatoms/john/basics/silica/silica.htm }}</ref> They are the most abundant form of algae in the ocean, although they can be found in fresh water as well.<ref name="MBari" /> They account for about 40% of the world's primary marine production, and produce about 25% of the world's oxygen.<ref name="Genoscope">{{cite web |title=Diatoms are the most important group of photosynthetic eukaryotes – Site du Genoscope |url=http://www.genoscope.cns.fr/spip/Phaeodactylum-tricornutum,463.html |website=www.genoscope.cns.fr |access-date=2015-11-23}}</ref> Diatoms are very diverse, and comprise about 100,000 species.<ref name="Genoscope" />
* [[Dinoflagellate]]s, unicellular flagellated algae, with some that are armored with [[cellulose]].<ref>{{cite web |title=Algae Classification: DINOPHYTA |url=https://naturalhistory.si.edu/research/botany/research/algae/algae-classification |website=Smithsonian National Museum of Natural History }}</ref> Dinoflagellates can be mixotrophic, and are the algae responsible for [[red tide]].<ref name="biologyreference.com"/> Some dinoflagellates, like ''[[Pyrocystis fusiformis]]'', are capable of [[bioluminescence]].<ref>{{cite web |title=BL Web: Growing dinoflagellates at home |url= http://biolum.eemb.ucsb.edu/organism/dinohome.html |website=biolum.eemb.ucsb.edu |access-date=2015-11-23}}</ref>

=== Unicellular fungi ===
[[File:Kg3.jpg|thumb|Transmission electron microscope image of budding ''[[Ogataea polymorpha]]'']]

Unicellular fungi include the [[yeast]]s. Fungi are found in most habitats, although most are found on land.<ref name="Microbiology online">{{cite web |title=Microbiology Online {{!}} Microbiology Society {{!}} About Microbiology – Introducing microbes – Fungi |url=http://www.microbiologyonline.org.uk/about-microbiology/introducing-microbes/fungi |website=www.microbiologyonline.org.uk |access-date=2015-11-23 |archive-date=2016-11-19 |archive-url=https://web.archive.org/web/20161119102320/http://www.microbiologyonline.org.uk/about-microbiology/introducing-microbes/fungi }}</ref> Yeasts reproduce through mitosis, and many use a process called [[budding]], where most of the [[cytoplasm]] is held by the mother cell.<ref name="Microbiology online" /> ''[[Saccharomyces cerevisiae]]'' ferments carbohydrates into carbon dioxide and alcohol, and is used in the making of beer and bread.<ref>{{cite journal |last1=Alba-Lois |first1=Luisa |last2=Segal-Kischinevzky |first2=Claudia |date=2010 |title=Yeast Fermentation and the Making of Beer and Wine |journal=Nature Education |volume=3 |issue=9 |page=17 |url=http://www.nature.com/scitable/topicpage/yeast-fermentation-and-the-making-of-beer-14372813 |access-date=2015-11-23}}</ref> ''S. cerevisiae'' is also an important model organism, since it is a eukaryotic organism that is easy to grow. It has been used to research [[cancer]] and [[neurodegenerative disease]]s as well as to understand the [[cell cycle]].<ref>{{cite web |title=Saccharomyces cerevisiae – MicrobeWiki |url=https://microbewiki.kenyon.edu/index.php/Saccharomyces_cerevisiae |website=MicrobeWiki |access-date=2015-11-23}}</ref><ref>{{cite web |title=Model organisms: yeast |url=https://www.yourgenome.org/theme/model-organisms-yeast/ |website=www.yourgenome.org |access-date=2025-03-25}}</ref> Furthermore, research using ''[[Saccharomyces cerevisiae|S. cerevisiae]]'' has played a central role in understanding the mechanism of meiotic [[Genetic recombination|recombination]] and the adaptive function of [[meiosis]]. [[Candida (fungus)|Candida spp]]. are responsible for [[candidiasis]], causing infections of the mouth and/or throat (known as thrush) and vagina (commonly called yeast infection).<ref>{{cite web |title=Candidiasis {{!}} Types of Diseases {{!}} Fungal Diseases {{!}} CDC |url=https://www.cdc.gov/fungal/diseases/candidiasis/ |website=www.cdc.gov |access-date=2015-11-23}}</ref>

== Macroscopic unicellular organisms ==
Most unicellular organisms are of [[Microscopic scale|microscopic]] size and are thus classified as [[microorganism]]s. However, some unicellular protists and bacteria are [[Macroscopic scale|macroscopic]] and visible to the naked eye.<ref>[http://www.mpg.de/english/illustrationsDocumentation/documentation/pressReleases/1999/news17_99.htm Max Planck Society Research News Release] Accessed 21 May 2009</ref> Examples include:
* ''[[Brefeldia maxima]]'', a [[slime mold]], examples have been reported up to a centimetre thick with a surface area of over a square metre and weighed up to around 20&nbsp;kg<ref>{{cite book | last = Ing | first = Bruce | title = The myxomycetes of Britain and Ireland: an identification handbook | publisher = Richmond Pub. Co | location = Slough, England | year = 1999 | isbn = 0-85546-251-5 | page = 4}}</ref>
* [[Xenophyophore]]s, protozoans of the phylum [[Foraminifera]], are the largest examples known, with ''[[Syringammina fragilissima]]'' achieving a diameter of up to {{convert|20|cm|in|abbr=on}}<ref>[http://scrippsnews.ucsd.edu/Releases/?releaseID=1206 Researchers Identify Mysterious Life Forms in the Desert.] Accessed 2011-10-24.</ref>
* ''[[Nummulite]]'', foraminiferans
* ''[[Valonia ventricosa]]'', an [[alga]] of the class [[Chlorophyceae]], can reach a diameter of {{convert|1|to|4|cm|in|abbr=on|sigfig=1}}<ref name="becky">{{Cite news|last=Bauer|first=Becky|title=Gazing Balls in the Sea|url=http://www.allatsea.net/article/October_2008/Gazing_Balls_in_the_Sea|access-date=27 August 2010|newspaper=All at Sea|date=October 2008|archive-url=https://web.archive.org/web/20100917073206/http://allatsea.net/article/October_2008/Gazing_Balls_in_the_Sea|archive-date=17 September 2010}}</ref><ref>{{Cite book|url=https://books.google.com/books?id=tu0sqBp8eAAC&pg=PA91|page=91|title=Coral reefs of the southern Gulf of Mexico|author1=John Wesley Tunnell |author2=Ernesto A. Chávez |author3=Kim Withers |publisher=Texas A&M University Press|year=2007|isbn=978-1-58544-617-9}}</ref>
* ''[[Acetabularia]]'', algae
* ''[[Caulerpa]]'', algae,<ref>{{cite web|url=http://www.wisegeek.com/what-is-the-largest-biological-cell.htm |title=What is the Largest Biological Cell? (with pictures) |publisher=Wisegeek.com |date=2014-02-23 |access-date=2014-03-01}}</ref>{{Unreliable source?|date=March 2014}} may grow to 3 metres long<ref name="Helmenstine">{{cite web|url=https://sciencenotes.org/what-is-the-largest-unicellular-organism/|title=What Is the Largest Unicellular Organism?|publisher=sciencenotes.org| date=2018-11-29 | access-date=2020-01-07|author= Anne Helmenstine}}</ref>
* ''[[Gromia sphaerica]]'', amoeba, {{convert|5|to|38|mm|in|abbr=on|sigfig=1}}<ref name="Helmenstine"/>
* ''[[Thiomargarita magnifica]]'' is the largest bacterium, reaching a length of up to 20&nbsp;mm
* ''[[Epulopiscium fishelsoni]]'', a bacterium
* ''[[Stentor (ciliate)|Stentor]]'', ciliates nicknamed trumpet animalcules
* ''[[Bursaria (ciliate)|Bursaria]]'', largest colpodean ciliates.

==See also==
{{Div col}}
* [[Abiogenesis]]
* [[Asexual reproduction]]
* [[Colonial organism]]
* [[Individuality#Biology|Individuality in biology]]
* [[Largest organisms]]
* [[Modularity#Modularity in biology|Modularity in biology]]
* [[Multicellular organism]]
* [[Sexual reproduction]]
* [[Superorganism]]
{{div col end}}

==References==
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{{Reflist}}

{{Evolution}}
{{Authority control}}
[[Category:Microorganisms]]