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Concept
Candidatus Pelagibacter communis
Summary
"Candidatus Pelagibacter", with the single species "Ca. P. communis", was isolated in 2002 and given a specific name, although it has not yet been described as required by 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 and were originally known only from their rRNA genes, first identified in the Sargasso Sea in 1990 by Stephen Giovannoni's laboratory at Oregon State University and later found in oceans worldwide. "Ca. P. communis" 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 "Ca. P. communis" and relatives is estimated to be about 2 × 1028 microbes.
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. The Pelagibacter genome takes up about 30% of the cell's volume. It is gram negative. 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.
Several strains of "Candidatus Pelagibacter communis" 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.
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