The Pelagibacterales are an order in the Alphaproteobacteria composed of free-living 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, 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 clade 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 oligotrophs (scavengers) and feed on dissolved organic carbon and nitrogen. They are unable to fix carbon or nitrogen, but can perform the TCA cycle with glyoxylate bypass and are able to synthesise all amino acids except glycine, and 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.
The taxon derives its name from the type species Pelagibacter ubique(status Candidatus species). However, this species has not yet been validly published, and therefore neither the order name nor the species name has official taxonomic standing.
Currently, the (unofficial) order is divided into five subgroups:
Subgroup Ia, open ocean, crown group — includes Pelagibacter 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 LD12 clade, freshwater
Subgroup V, which includes alphaproteobacterium HIMB59, basal to the remainder
The above results in a cladogram of the Pelagibacterales as follows:
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^Salcher, M.M., J. Pernthaler, and T. Posch, Seasonal bloom dynamics and ecophysiology of the freshwater sister clade of SAR11 bacteria 'that rule the waves' (LD12). ISME J, 2011.