Pelagibacter ubique

Pelagibacter, with the single species P. ubique, was isolated in 2002 and given a specific name, although it has not yet been validly published according to the bacteriological code. It is an abundant member of the SAR11 clade in the phylum Alphaproteobacteria. SAR11 members are highly dominant organisms found in both salt and fresh water worldwide — possibly the most numerous bacterium in the world, and were originally known only from their rRNA genes, which were first identified in environmental samples from the Sargasso Sea in 1990 by Stephen Giovannoni's laboratory in the Department of Microbiology at Oregon State University and later found in oceans worldwide.P. ubique and its relatives may be the most abundant organisms in the ocean, and quite possibly the most abundant bacteria in the entire world. It can make up about 25% of all microbial plankton cells, and in the summer they may account for approximately half the cells present in temperate ocean surface water. The total abundance of P. ubique and relatives is estimated to be about 2 × 10²⁸ microbes.

Pelagibacterales

The Pelagibacterales is 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.

Pelagibacter 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.