Valonia ventricosa, a species ofalga with a diameter that ranges typically from 1 to 4 centimetres (0.4 to 1.6 in), is among the largest unicellular species
Aunicellular organism, also known as asingle-celled organism, is anorganism that consists of a singlecell, unlike amulticellular organism that consists of multiple cells. Organisms fall into two general categories:prokaryotic organisms andeukaryotic organisms. Most prokaryotes are unicellular and are classified intobacteria andarchaea. Many eukaryotes are multicellular, but some are unicellular such asprotozoa, unicellularalgae, and unicellularfungi. Unicellular organisms are thought to be the oldest form of life, with early organisms emerging 3.5–3.8 billion years ago.[1][2]
Although some prokaryotes live incolonies, 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.Gametes, for example, are reproductive unicells for multicellular organisms.[3]
Theorigin of life is largely still a mystery. Primitiveprotocells are thought to be the precursors to today's unicellular organisms.
In one theory, known as theRNA world hypothesis, early RNA molecules would have been the basis for catalyzing organic chemical reactions and self-replication.[4]
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 replicatorribozyme may have replicated other replicator ribozymes of different RNA sequences if not kept separate.[5] Such hypothetic[clarification needed] cells with an RNA genome instead of the usual DNA genome are called 'ribocells' or 'ribocytes'.[4]
Whenamphiphiles likelipids are placed in water, the hydrophobic tails aggregate to formmicelles andvesicles, with the hydrophilic ends facing outwards.[6][5] Primitive cells likely used self-assembling fatty-acid vesicles to separate chemical reactions and the environment.[5] Because of their simplicity and ability to self-assemble in water, it is likely that these simplemembranes predated other forms of early biological molecules.[6]
Prokaryotes lack membrane-bound organelles, such asmitochondria or anucleus.[7] Instead, most prokaryotes have an irregular region that contains DNA, known as thenucleoid.[8] Most prokaryotes have a single, circularchromosome, which is in contrast to eukaryotes, which typically have linear chromosomes.[9] 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.[10] Prokaryotes are relatively ubiquitous in the environment and some (known as extremophiles) thrive in extreme environments.[citation needed]
Modernstromatolites in Shark Bay, Western Australia. It can take a century for a stromatolite to grow 5 cm.[11]Bacteria in a capule
Bacteria are one of the world's oldest forms of life, and are found virtually everywhere on Earth.[10] Many common bacteria haveplasmids, 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 calledbinary fission. However, about 80 different species can undergo a sexual process referred to as naturalgenetic transformation.[14] Transformation is a bacterial process for transferring DNA from one cell to another, and is apparently an adaptation forrepairing DNA damage in the recipient cell.[15] In addition, plasmids can be exchanged through the use of apilus in a process known asconjugation.[13]
The photosyntheticcyanobacteria are arguably the most successful bacteria, and changed the early atmosphere of the earth by oxygenating it.[16]Stromatolites, structures made up of layers ofcalcium 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 theArchaean (4 billion to 2.5 billion years ago),Proterozoic (2.5 billion to 540 million years ago), andPhanerozoic (540 million years ago to present day) eons.[17] Much of the fossilized stromatolites of the world can be found inWestern Australia.[17] There, some of the oldest stromatolites have been found, some dating back to about 3,430 million years ago.[17]
Clonalaging occurs naturally inbacteria, and is apparently due to the accumulation of damage that can happen even in the absence of external stressors.[18]
A bottom-dwelling community found deep in the European Arctic.[19]
Hydrothermal vents release heat andhydrogen sulfide, allowing extremophiles to survive usingchemolithotrophic growth.[20]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.[21][22] 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.[22] As their name suggests, Archaea comes from a Greek wordarchaios, meaning original, ancient, or primitive.[23]
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]. Examples of these Archaeanextremophiles are as follows:
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 ofadenosine triphosphate.[28] They are the only known organisms capable of producing methane.[29] Under stressful environmental conditions that causeDNA 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]
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 billion years ago.[32] This was an important step in evolution. In contrast to prokaryotes, eukaryotes reproduce by usingmitosis andmeiosis. Sex appears to be a ubiquitous and ancient, and inherent attribute ofeukaryotic life.[33] Meiosis, a true sexual process, allows for efficientrecombinational repair of DNA damage[15] and a greater range of genetic diversity by combining the DNA of the parents followed byrecombination.[32] Metabolic functions in eukaryotes are more specialized as well by sectioning specific processes into organelles.[citation needed]
Theendosymbiotic 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 toRickettsia, with the parasitic ability to enter a cell.[34] 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.[34] Chloroplasts probably became symbionts through a similar set of events, and are most likely descendants of cyanobacteria.[35] 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 thanfermentation alone.[citation needed]
Ciliophora, or ciliates, are a group of protists that utilize cilia for locomotion. Examples includeParamecium,Stentors, andVorticella.[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 havetrichocysts, 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: amacronucleus for normal metabolic control and a separatemicronucleus that undergoes meiosis.[41] Examples of such ciliates areParamecium andTetrahymena that likely employ meiotic recombination for repairing DNA damage acquired under stressful conditions.[citation needed]
Unicellular algae are plant-like autotrophs and containchlorophyll.[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 bemixotrophic (both autotrophic and heterotrophic).[45] An example isEuglena 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]
Unicellular fungi include theyeasts. Fungi are found in most habitats, although most are found on land.[51] Yeasts reproduce through mitosis, and many use a process calledbudding, where most of thecytoplasm 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 researchcancer andneurodegenerative diseases as well as to understand thecell cycle.[53][54] Furthermore, research usingS. cerevisiae has played a central role in understanding the mechanism of meioticrecombination and the adaptive function ofmeiosis.Candida spp. are responsible forcandidiasis, causing infections of the mouth and/or throat (known as thrush) and vagina (commonly called yeast infection).[55]
Most unicellular organisms are ofmicroscopic size and are thus classified asmicroorganisms. However, some unicellular protists and bacteria aremacroscopic and visible to the naked eye.[56] Examples include:
Brefeldia maxima, aslime 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 kg[57]
^Barton, Larry L.; Fardeau, Marie-Laure; Fauque, Guy D. (2014-01-01). "Hydrogen Sulfide: A Toxic Gas Produced by Dissimilatory Sulfate and Sulfur Reduction and Consumed by Microbial Oxidation".The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment. Metal Ions in Life Sciences. Vol. 14. pp. 237–277.doi:10.1007/978-94-017-9269-1_10.ISBN978-94-017-9268-4.ISSN1559-0836.PMID25416397.
^"Archaea".www.microbeworld.org. Archived fromthe original on 2015-11-23. Retrieved2015-11-22.
^Sugibayashi, Rika; Harumoto, Terue (2000-12-29). "Defensive function of trichocysts in Paramecium tetraurelia against heterotrich ciliate Climacostomum virens".European Journal of Protistology.36 (4):415–422.doi:10.1016/S0932-4739(00)80047-4.
