Pelagibacter, with the single species P. ubique, are possibly the most numerous bacteria in the world. They were originally named SAR11 and known only from their rRNA genes, which were first identified in environmental samples from the Sargasso Sea in 1990. The bacteria resonsible were isolated in 2002 and given a specific name, although it has not yet been validly published according to the bacteriological code.

Pelagibacter has a world-wide distribution and is found among the bacterioplankton. They are some of the smallest self-replicating cells known, with a diameter of only 0.12-0.20 µm.

References

• Michael S. Rappé, Stephanie A. Connon, Kevin L. Vergin, Stephen J. Giovannoni Cultivation of the ubiquitous SAR11 marine bacterioplankton clade Nature 418: 630-633 (2002).
• R. M. Morris et al. SAR11 clade dominates ocean surface bacterioplankton communities Nature 420, 806 - 810 (2002)