The Pelagibacterales are an order in the Alphaproteobacteria composed of free-living marine bacteria 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 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 Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...

, and then placed as sister order to the Rickettsiales in the subclass Rickettsidae. It includes the highly abundant marine species '' Pelagibacter ubique''. 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 Genomic streamlining is a theory in evolutionary biology and microbial ecology that suggests that there is a reproductive benefit to prokaryotes having a smaller genome size with less non-coding DNA and fewer non-essential genes. There is a lot of v ...

'. ''P. ubique'' and related species are oligotrophs (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 proteins ...

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 generation of glucose from certain non-carbohydrate carbon substrates. It is a ubiquitous process, present in plants, animals, fungi, bacteria, and other microorganisms. In vertebrat ...

, but not a typical

glycolysis Glycolysis is the metabolic pathway that converts glucose () into pyruvate (). The free energy released in this process is used to form the high-energy molecules adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NADH ...

pathway, whereas other subgroups are capable of typical glycolysis. Unlike ''

Acaryochloris marina ''Acaryochloris marina'' is a symbiotic species of the phylum Cyanobacteria that produces chlorophyll d, allowing it to use far-red light, at 770 nm wavelength. Description It was first discovered in 1993 from coastal isolates of coral ...

'', ''P. ubique'' is not

photosynthetic Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in c ...

— 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 e ...

(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 xerophtha ...

biosynthesis) for

ATP ATP may refer to: Companies and organizations * Association of Tennis Professionals, men's professional tennis governing body * American Technical Publishers, employee-owned publishing company * ', a Danish pension * Armenia Tree Project, non ...

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 Eart ...

in the ocean surface. They extract phosphate from methylphosphonic acid. 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 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 co ...

, indicated that SAR11 could be the ancestor of

mitochondria A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...

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

References

{{Taxonbar, from=Q7388740 Alphaproteobacteria