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.

[6] Due to their small genome size and limited metabolic function, Pelagibacterales have become a model organism for 'streamlining theory'.

[5] P. ubique and related species are oligotrophs (scavengers) and feed on dissolved organic carbon and nitrogen.

[3] 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,[7] as well as some cofactors.

Pelagibacter Subgroups Ib, II, IIIa, IIIb, IV and V Proto-mitochondria Neorickettsia Wolbachia Anaplasma Ehrlichia Midichloria Orientia Rickettsia