Pelagibacter, with the single species P. ubique, are possibly the most numerous bacteria in the world (perhaps 1028 individual cells). They are an abundant member of the alphaproteobacteria - SAR11 clade and were originally known only from their rRNA genes, which were first identified in environmental samples from the Sargasso Sea in 1990. The bacteria responsible 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 length of 0.37-0.89 µm and a diameter of only 0.12-0.20 µm.
References
Michael S. Rappé, Stephanie A. Connon, Kevin L. Vergin, Stephen J. Giovannoni (2002). "Cultivation of the ubiquitous SAR11 marine bacterioplankton clade". Nature418: 630-633.
R. M. Morris et al. (2002). "SAR11 clade dominates ocean surface bacterioplankton communities". Nature420: 806 - 810.
Stephen J. Giovannoni, H. James Tripp et. al (2005). "Genome Streamlining in a Cosmopolitan Oceanic Bacterium". Science309: 1242-1245. doi:10.1126/science.1114057
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