Candidatus Pelagibacter ubique is an oceanic carbon recycling bacteria that has one of the smallest and simplest genomes sequenced to date

Candidatus pelagibacter ubique belongs to a group of bacteria known as SAR 11, named because of their discovery in the Sargasso Sea. The Sargasso Sea is a particularly interesting environment to scientists, it is very low in nutrients and other resources and relatively devoid of life. SAR 11 is a dominant form of life in the Sargasso Sea where most other life forms cannot survive. C. Pelagibacter ubique is arguably the most abundant organism in seawater, where it accounts for approximately 25% of all microbial plankton cells. During summer periods it may exceed 50% of the cells in the surface waters of temperate oceans. It is estimated that the combined weight of these microbes exceeds that of all the fish in the world's oceans.

C. Pelagibacter ubique plays a key role in the recycling of organic carbon, which is used by photosynthetic algae to manufacture much of the Earth's supply of oxygen.

C. Pelagibacter ubique is the first cultured member of the SAR11 clade (which is one of the most successful clades of organisms on the planet) to be completely sequenced. It has the smallest genome (1,308,759 base pairs) of any cell known to replicate independently in nature.
It encodes the smallest number of predicted open reading frames known for a free-living microorganism. In contrast to parasitic bacteria and archaea with small genomes, P. ubique has complete biosynthetic pathways for all 20 amino acids and all but a few cofactors.

P. ubique's evolution is distinctly different from that of all other heterotrophic marine bacteria for which genome sequences are available. Evolution has divested it of all but the most fundamental cellular systems so that it replicates as efficiently as possible even when there are limited nutrient resources. The pattern of genome reduction observed is consistent with the hypothesis of genome streamlining driven by selection acting on a very large population which resides in a very low nutrient habitat. P. ubique appears to employ an adaptive strategy different from other heterotrophic marine bacteria and instead resembles the highly successful marine unicellular cyanobacteria in its simple metabolism and small genome size.

One objective of current research is to predict the organic carbon sources used by Pelagibacter by metabolic reconstruction and also to explore the proteome state of Pelagibacter cells in the oceans, so that they can be used as proxies to report the biological state of the system. Metabolic modelling of Pelagibacter is an attractive long range goal because it is one of the smallest and simplest cells known. Its remarkable success may be attributable to the integration and optimization of metabolic processes for efficiency at low nutrient fluxes.

Genetic analysis indicates that C. Pelagibacter ubique became the dominant life form in the oceans largely by being the simplest.

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