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''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.〔"Candidatus Pelagibacter Ubique." European Bioinformatics Institute. European Bioinformatics Institute, 2011. Web. 08 Jan. 2012. http://www.ebi.ac.uk/2can/genomes/bacteria/Candidatus_Pelagibacter_ubique.html 〕 It is rod or crescent shaped and one 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. 30% of the cell's volume is taken up by its genome. It is gram negative.〔(), Gauthier, Nicholas; Zinman, Guy; D’Antonio, Matteo; Abraham, Michael. Comparative Microbial Genomics DTU course. 2005.〕 It recycles dissolved organic carbon. It undergoes regular seasonal cycles in abundance - in summer reaching ~50% of the cells in the temperate ocean surface waters. Thus it plays a major role in the Earth's carbon cycle. Its discovery was the subject of "Oceans of Microbes", Episode 5 of "Intimate Strangers: Unseen Life on Earth" by PBS.〔View "Oceans of Microbes" http://www.podcastdirectory.com/podshows/4339749〕 ==Cultivation== Several strains of ''Pelagibacter ubique'' have been cultured thanks to improved isolation techniques. The most studied strain is HTCC1062 (high-throughput cultivation collection).〔 The factors that regulate SAR11 populations are still largely unknown. They have sensors for nitrogen, phosphate, and iron limitation, and a very unusual requirement for reduced sulfur compounds. It is hypothesised that they have been molded by evolution in a low nutrient ecosystem, such as the Sargasso Sea where it was first discovered.〔Giovannoni Lab http://giovannonilab.science.oregonstate.edu/〕 A population of ''P. ubique'' cells can double every 29 hours, which is fairly slow, but they can replicate under low nutrient conditions. ''P. ubique'' can be grown on a defined, artificial medium with additions of reduced sulfur, glycine, pyruvate and vitamins. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Pelagibacter ubique」の詳細全文を読む スポンサード リンク
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