The Pelagibacterales are an order in the

Alphaproteobacteria Alphaproteobacteria is a class of bacteria in the phylum Pseudomonadota (formerly Proteobacteria). The Magnetococcales and Mariprofundales are considered basal or sister to the Alphaproteobacteria. The Alphaproteobacteria are highly diverse and ...

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 cellular ...

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 () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connec ...

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 c ...

''. 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 (CAC)—also known as the Krebs cycle or the TCA cycle (tricarboxylic acid cycle)—is a series of chemical reactions to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and protein ...

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, but not a typical glycolysis pathway, whereas other subgroups are capable of typical glycolysis. Unlike '' Acaryochloris marina'', ''P. ubique'' is not photosynthetic — specifically, it does not use light to increase the bond energy of an electron pair — but it does possess

proteorhodopsin Proteorhodopsin (also known as pRhodopsin) is a family of transmembrane proteins that use retinal as a chromophore for light-mediated functionality, in this case, a proton pump. pRhodopsin is found in marine planktonic bacteria, archaea and euka ...

(including

retinol Retinol, also called vitamin A1, is a fat-soluble vitamin in the vitamin A family found in food and used as a dietary supplement. As a supplement it is used to treat and prevent vitamin A deficiency, especially that which results in xeroph ...

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 relative abundance of methane on Ea ...

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 * Subg * 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 (University of Hawaii—Mānoa, UH Mānoa, Hawai'i, or simply UH) is a public land-grant research university in Mānoa, a neighborhood in Honolulu, Hawaii. It is the flagship campus of the University of Haw ...

and

Oregon State University Oregon State University (OSU) is a public land-grant, research university in Corvallis, Oregon. OSU offers more than 200 undergraduate-degree programs along with a variety of graduate and doctoral degrees. It has the 10th largest engineering c ...

, indicated that SAR11 could be the ancestor of mitochondria in most eukaryotic cells. However, this result could represent a tree reconstruction artifact due to compositional bias.

References

{{Taxonbar, from=Q7388740 Alphaproteobacteria