Pelagibacter ubique
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{{Taxobox | color = lightgrey | name = Pelagibacter ubique | regnum = Bacteria | phylum = Proteobacteria | classis = Alpha Proteobacteria | ordo = Rickettsiales | genus = Pelagibacter | species = P. ubique | binomial = Candidatus Pelagibacter ubique | binomial_authority = Rappé et al. 2002 }} Pelagibacter, with the single species P. ubique, are possibly the most numerous bacteria in the world (perhaps 1028 individual cells). 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 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 diameter of only 0.12-0.20 µm.
Pelagibacter has just 1,354 genes, compared to humans, which have around 30,000 genes. It has none of the genetic clutter that most genomes have accumulated over time: there are no duplicate gene copies, no viral genes, and no junk DNA. The shorter length effectively means less 'work' to copy each generation as the bacteria reproduces. Also, base pairs are used that contain less nitrogen, since nitrogen is a relatively difficult nutrient for living things to obtain. The result, supported by their abundance, is a very efficient life form.