The "''

Candidatus In prokaryote nomenclature, ''Candidatus'' (abbreviated ''Ca.''; Latin for "candidate of Roman office") is used to name prokaryotic taxa that are well characterized but yet- uncultured. Contemporary sequencing approaches, such as 16S ribosomal R ...

'' Pelagibacterales" are an

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

in the

Alphaproteobacteria ''Alphaproteobacteria'' or ''α-proteobacteria'', also called ''α-Purple bacteria'' in earlier literature, is a class of bacteria in the phylum '' Pseudomonadota'' (formerly "Proteobacteria"). The '' Magnetococcales'' and '' Mariprofundales'' ar ...

composed of free-living

marine bacteria Marine prokaryotes are marine bacteria and marine archaea. They are defined by their habitat as prokaryotes that live in marine environments, that is, in the saltwater of seas or oceans or the brackish water of coastal estuaries. All cellu ...

that make up roughly one in three cells at the ocean's surface. Overall, members of the ''Pelagibacterales'' are estimated to make up between a quarter and a half of all

prokaryotic A prokaryote (; less commonly spelled procaryote) is a single-celled organism whose cell lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Greek (), meaning 'before', and (), meaning 'nut' ...

cells in the ocean. Initially, this taxon was known solely by metagenomic data and was known as the SAR11 clade. It was first placed in the Rickettsiales, but was later raised to the rank of order, and then placed as sister order to the Rickettsiales in the subclass Rickettsidae. It includes the highly abundant marine species ''

Pelagibacter ubique "''Candidatus'' Pelagibacter", with the single species "''Ca.'' P. communis", was isolated in 2002 and given a specific name, although it has not yet been described as required by the bacteriological code. It is an abundant member of the SAR11 ...

''. Bacteria in this order are unusually small. Due to their small genome size and limited metabolic function, ''Pelagibacterales'' have become a model organism for ' streamlining theory'. ''P. ubique'' and related species are

oligotroph An oligotroph is an organism that can live in an environment that offers very low levels of nutrients. They may be contrasted with copiotrophs, which prefer nutritionally rich environments. Oligotrophs are characterized by slow growth, low rates o ...

s (scavengers) and feed on dissolved organic carbon and nitrogen. They are unable to fix carbon or nitrogen, but can perform the

TCA cycle The citric acid cycle—also known as the Krebs cycle, Szent–Györgyi–Krebs cycle, or TCA cycle (tricarboxylic acid cycle)—is a series of chemical reaction, biochemical reactions that release the energy stored in nutrients through acetyl-Co ...

with glyoxylate bypass and are able to synthesise all amino acids except glycine, as well as some cofactors. They also have an unusual and unexpected requirement for reduced sulfur. ''P. ubique'' and members of the oceanic subgroup I possess

gluconeogenesis Gluconeogenesis (GNG) is a metabolic pathway that results in the biosynthesis of glucose from certain non-carbohydrate carbon substrates. It is a ubiquitous process, present in plants, animals, fungi, bacteria, and other microorganisms. In verte ...

, but not a typical

glycolysis Glycolysis is the metabolic pathway that converts glucose () into pyruvic acid, pyruvate and, in most organisms, occurs in the liquid part of cells (the cytosol). The Thermodynamic free energy, free energy released in this process is used to form ...

pathway, whereas other subgroups are capable of typical glycolysis. Unlike '' Acaryochloris marina'', ''P. ubique'' is not

photosynthetic Photosynthesis ( ) is a Biological system, system of biological processes by which Photoautotrophism, photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical ener ...

— specifically, it does not use light to increase the bond energy of an electron pair — but it does possess

proteorhodopsin Proteorhodopsin (PR or pRhodopsin) belongs to the Protein family, family of Bacteria, bacterial Transmembrane protein, transmembrane Rhodopsin, rhodopsins (Retinylidene protein, retinylidene proteins). In 1971, the first Microorganism, microbial ...

(including

retinol Retinol, also called vitamin A1, is a fat-soluble vitamin in the vitamin A family that is found in food and used as a dietary supplement. Retinol or other forms of vitamin A are needed for vision, cellular development, maintenance of skin and ...

biosynthesis) for ATP production from light. SAR11 bacteria are responsible for much of the dissolved

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...

in the ocean surface. They extract phosphate from

methylphosphonic acid Methylphosphonic acid is an organophosphorus compound with the chemical formula CH3P(O)(OH)2. The phosphorus center is tetrahedral and is bonded to a methyl group, two OH groups and an oxygen. Methylphosphonic acid is a white, non-volatile solid t ...

. Although the taxon derives its name from the type species ''P. ubique'' (status'' Candidatus'' species), this species has not yet been validly published, and therefore neither the order name nor the species name has official taxonomic standing.

Subgroups

Currently, the order is divided into five subgroups: * Subgroup Ia, open ocean, crown group — includes ''P. ubique'' HTCC1062 * Subgroup Ib, open ocean, sister clade to Ia * Subgroup II, coastal, basal to Ia + Ib * Subgroup III, brackish, basal to I + II along with its sister clade IV * Subgroup IV, also known as the LD12 clade, freshwater * Subgroup V, which includes alphaproteobacterium HIMB59, basal to the remainder The above results in a cladogram of the Pelagibacterales as follows:

Phylogenetic placement and endosymbiotic theory

A 2011 study by researchers of the

University of Hawaiʻi at Mānoa The University of Hawaii at Mānoa is a Public university, public Land-grant university, land-grant research university in Honolulu, Hawaii, United States. It is the flagship campus of the University of Hawaiʻi system and houses the main offic ...

and

Oregon State University Oregon State University (OSU) is a Public university, public Land-grant university, land-grant research university in Corvallis, Oregon, United States. OSU offers more than 200 undergraduate degree programs and a variety of graduate and doctor ...

, indicated that SAR11 could be the ancestor of

mitochondria A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...

in most eukaryotic cells. However, this result could represent a tree reconstruction artifact due to compositional bias.

References

{{Taxonbar, from=Q7388740 Alphaproteobacteria Candidatus taxa

Subgroups

Phylogenetic placement and endosymbiotic theory

See also

References