Unicellular organism

Bacteria are one of the world's oldest forms of life, and are found virtually everywhere on Earth.^[10] Many common bacteria have plasmids, which are short, circular, self-replicating DNA molecules that are separate from the bacterial chromosome.^[12] Plasmids can carry genes responsible for novel abilities, of current critical importance being antibiotic resistance.^[13] Bacteria predominantly reproduce asexually through a process called binary fission. However, about 80 different species can undergo a sexual process referred to as natural genetic transformation.^[14] Transformation is a bacterial process for transferring DNA from one cell to another, and is apparently an adaptation for repairing DNA damage in the recipient cell.^[15] In addition, plasmids can be exchanged through the use of a pilus in a process known as conjugation.^[13]

The photosynthetic cyanobacteria are arguably the most successful bacteria, and changed the early atmosphere of the Earth by oxygenating it.^[16]Stromatolites, structures made up of layers of calcium carbonate and trapped sediment left over from cyanobacteria and associated community bacteria, left behind extensive fossil records.^[16][17] The existence of stromatolites gives an excellent record as to the development of cyanobacteria, which are represented across the Archaean (4 billion to 2.5 billion years ago), Proterozoic (2.5 billion to 540 million years ago), and Phanerozoic (540 million years ago to present day) eons.^[17] Much of the fossilized stromatolites of the world can be found in Western Australia.^[17] There, some of the oldest stromatolites have been found, some dating back to about 3,430 million years ago.^[17]

Clonal aging occurs naturally in bacteria, and is apparently due to the accumulation of damage that can happen even in the absence of external stressors.^[18]

Las chimeneas hidrotermales liberan calor y sulfuro de hidrógeno , lo que permite a los extremófilos sobrevivir mediante el crecimiento quimiolitotrófico . ^{[ 20 ]} Las arqueas son generalmente similares en apariencia a las bacterias, de ahí su clasificación original como bacterias, pero tienen diferencias moleculares significativas, sobre todo en su estructura de membrana y ARN ribosómico. ^{[ 21 ] [ 22 ]} Mediante la secuenciación del ARN ribosómico, se descubrió que las arqueas probablemente se separaron de las bacterias y fueron las precursoras de los eucariotas modernos, y en realidad están más relacionadas filogenéticamente con los eucariotas. ^{[ 22 ]} Como su nombre lo indica, Archaea proviene de la palabra griega archaios, que significa original, antiguo o primitivo. ^{[ 23 ]}

Algunas arqueas habitan los entornos biológicamente más inhóspitos de la Tierra, y se cree que esto imita, en cierto modo, las duras condiciones primigenias a las que probablemente estuvo expuesta la vida . Algunos ejemplos de estas arqueas extremófilas son los siguientes:

• Termófilos , temperatura óptima de crecimiento de 50  °C-110  °C, incluyendo los géneros Pyrobaculum , Pyrodictium , Pyrococcus , Thermus aquaticus y Melanopyrus . ^{[ 24 ]}
• Psicrófilos , temperatura óptima de crecimiento inferior a 15  °C, incluidos los géneros Methanogenium y Halorubrum . ^{[ 24 ]}
• Alcalifílicos , pH óptimo de crecimiento superior a 8, incluyendo el género Natronomonas . ^{[ 24 ] [ 25 ]}
• Acidófilos , pH óptimo de crecimiento inferior a 3, incluidos los géneros Sulfolobus y Picrophilus . ^{[ 24 ] [ 26 ]}
• Los piezófilos (también conocidos como barófilos ) prefieren altas presiones de hasta 130 MPa, como los ambientes de las profundidades oceánicas, incluidos los géneros Methanococcus y Pyrococcus . ^{[ 24 ]}
• Los halófilos crecen de forma óptima en altas concentraciones de sal entre 0,2 M y 5,2 M de NaCl , incluidos los géneros Haloarcula , Haloferax y Halococcus . ^{[ 24 ] [ 27 ]}

Methanogens 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.^[28] This process utilizes hydrogen to reduce carbon dioxide into methane, releasing energy into the usable form of adenosine triphosphate.^[28] They are the only known organisms capable of producing methane.^[29] Under stressful environmental conditions that cause DNA damage, some species of archaea aggregate and transfer DNA between cells.^[30] 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.^[31]

Los protozoos se definen principalmente por su método de locomoción, que incluye flagelos , cilios y pseudópodos . ^{[ 36 ]} Si bien ha habido un debate considerable sobre la clasificación de los protozoos debido a su gran diversidad, en un sistema actualmente se reconocen siete filos dentro del reino Protozoa: Euglenozoa , Amoebozoa , Choanozoa sensu Cavalier-Smith , Loukozoa , Percolozoa , Microsporidia y Sulcozoa . ^{[ 37 ] [ 38 ]} Los protozoos, al igual que las plantas y los animales, pueden considerarse heterótrofos o autótrofos . ^{[ 34 ]} Los autótrofos como Euglena son capaces de producir su energía mediante la fotosíntesis, mientras que los protozoos heterótrofos consumen alimento canalizándolo a través de un esófago similar a una boca o engulléndolo con pseudópodos, una forma de fagocitosis . ^{[ 34 ]} Si bien los protozoos se reproducen principalmente asexualmente, algunos protozoos son capaces de reproducirse sexualmente. ^{[ 34 ]} Los protozoos con capacidad sexual incluyen las especies patógenas Plasmodium falciparum , Toxoplasma gondii , Trypanosoma brucei , Giardia duodenalis y especies de Leishmania . ^{[ 15 ]}

Ciliophora, or ciliates, are a group of protists that utilize cilia for locomotion. Examples include Paramecium, Stentors, and Vorticella.^[39] Ciliates are widely abundant in almost all environments where water can be found, and the cilia beat rhythmically in order to propel the organism.^[40] Many ciliates have trichocysts, which are spear-like organelles that can be discharged to catch prey, anchor themselves, or for defense.^[41][42] 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.^[41] Examples of such ciliates are Paramecium and Tetrahymena that likely employ meiotic recombination for repairing DNA damage acquired under stressful conditions.

The Amebozoa utilize pseudopodia and cytoplasmic flow to move in their environment. Entamoeba histolytica is the cause of amebic dysentery.^[43]Entamoeba histolytica appears to be capable of meiosis.^[44]

Unicellular algae are plant-like autotrophs and contain chlorophyll.^[45] They include groups that have both multicellular and unicellular species:

• Euglenophyta, flagellated, mostly unicellular algae that occur often in fresh water.^[45] In contrast to most other algae, they lack cell walls and can be mixotrophic (both autotrophic and heterotrophic).^[45] An example is Euglena gracilis.
• Chlorophyta (green algae), mostly unicellular algae found in fresh water.^[45] The chlorophyta are of particular importance because they are believed to be most closely related to the evolution of land plants.^[46]
• Diatoms, unicellular algae that have siliceous cell walls.^[47] They are the most abundant form of algae in the ocean, although they can be found in fresh water as well.^[47] They account for about 40% of the world's primary marine production, and produce about 25% of the world's oxygen.^[48] Diatoms are very diverse, and comprise about 100,000 species.^[48]
• Dinoflagellates, unicellular flagellated algae, with some that are armored with cellulose.^[49] Dinoflagellates can be mixotrophic, and are the algae responsible for red tide.^[46] Some dinoflagellates, like Pyrocystis fusiformis, are capable of bioluminescence.^[50]

Unicellular fungi include the yeasts. Fungi are found in most habitats, although most are found on land.^[51] Yeasts reproduce through mitosis, and many use a process called budding, where most of the cytoplasm is held by the mother cell.^[51]Saccharomyces cerevisiae ferments carbohydrates into carbon dioxide and alcohol, and is used in the making of beer and bread.^[52]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 diseases as well as to understand the cell cycle.^[53][54] Furthermore, research using S. cerevisiae has played a central role in understanding the mechanism of meiotic recombination and the adaptive function of meiosis. Candida spp. are responsible for candidiasis, causing infections of the mouth and/or throat (known as thrush) and vagina (commonly called yeast infection).^[55]

Some parasitic cnidarians in the clade myxozoa experience unicellular life phases. The feeding stage of these organisms is a superficially worm-like unicellular plasmodium; within these cells, multicellular spores develop. The group experiences alternation of generations and is not truly unicellular.^[56]