Pelagibacterales Explained

The Pelagibacterales are an order in the Alphaproteobacteria composed of free-living marine bacteria that make up roughly one in three cells at the ocean's surface.^{[1] [2]} Overall, members of the Pelagibacterales are estimated to make up between a quarter and a half of all prokaryotic cells in the ocean.

Initially, this taxon was known solely by metagenomic data and was known as the SAR11 clade. It was first placed in the Rickettsiales, but was later raised to the rank of order, and then placed as sister order to the Rickettsiales in the subclass Rickettsidae. It includes the highly abundant marine species Pelagibacter ubique.

Bacteria in this order are unusually small.^[3] Due to their small genome size and limited metabolic function, Pelagibacterales have become a model organism for 'streamlining theory'.^[4]

P. ubique and related species are oligotrophs (scavengers) and feed on dissolved organic carbon and nitrogen.^[2] They are unable to fix carbon or nitrogen, but can perform the TCA cycle with glyoxylate bypass and are able to synthesise all amino acids except glycine,^[5] as well as some cofactors.^[6] They also have an unusual and unexpected requirement for reduced sulfur.^[7]

P. ubique and members of the oceanic subgroup I possess gluconeogenesis, but not a typical glycolysis pathway, whereas other subgroups are capable of typical glycolysis.^[8]

Unlike Acaryochloris marina, P. ubique is not photosynthetic — specifically, it does not use light to increase the bond energy of an electron pair — but it does possess proteorhodopsin (including retinol biosynthesis) for ATP production from light.^[9]

SAR11 bacteria are responsible for much of the dissolved methane in the ocean surface. They extract phosphate from methylphosphonic acid.^[10]

Although the taxon derives its name from the type species P. ubique (status Candidatus species), this species has not yet been validly published, and therefore neither the order name nor the species name has official taxonomic standing.^[11]

Subgroups

Currently, the order is divided into five subgroups:^[12]

• Subgroup Ia, open ocean, crown group — includes P. ubique HTCC1062
• Subgroup Ib, open ocean, sister clade to Ia
• Subgroup II, coastal, basal to Ia + Ib
• Subgroup III, brackish, basal to I + II along with its sister clade IV
• Subgroup IV, also known as the LD12 clade, freshwater^[13]
• Subgroup V, which includes alphaproteobacterium HIMB59, basal to the remainder

The above results in a cladogram of the Pelagibacterales as follows:

Phylogenetic placement and endosymbiotic theory

A 2011 study by researchers of the University of Hawaiʻi at Mānoa and Oregon State University, indicated that SAR11 could be the ancestor of mitochondria in most eukaryotic cells.^[1] However, this result could represent a tree reconstruction artifact due to compositional bias.^[14]

Notes and References

1. J. Cameron Thrash . Alex Boyd . Megan J. Huggett . Jana Grote . Paul Carini . Ryan J. Yoder . Barbara Robbertse . Joseph W. Spatafora . Michael S. Rappé . Stephen J. Giovannoni . Phylogenomic evidence for a common ancestor of mitochondria and the SAR11 clade . Scientific Reports. June 2011 . 10.1038/srep00013 . 22355532 . 1 . 13 . 3216501. 2011NatSR...1E..13T .
2. Morris RM, Rappé MS, Connon SA, etal . SAR11 clade dominates ocean surface bacterioplankton communities . Nature . 420 . 6917 . 806–10 . 2002 . 12490947 . 10.1038/nature01240 . 2002Natur.420..806M . 4360530 .
3. Rappé MS, Connon SA, Vergin KL, Giovannoni SJ . Cultivation of the ubiquitous SAR11 marine bacterioplankton clade . Nature . 418 . 6898 . 630–3 . August 2002 . 12167859 . 10.1038/nature00917 . 2002Natur.418..630R . 4352877 .
4. Giovannoni. Stephen J.. 2017-01-03. SAR11 Bacteria: The Most Abundant Plankton in the Oceans. Annual Review of Marine Science. 9. 231–255. 10.1146/annurev-marine-010814-015934. 1941-0611. 27687974. 2017ARMS....9..231G. free.
5. H. James Tripp . Michael S. Schwalbach . Michelle M. Meyer . Joshua B. Kitner . Ronald R. Breaker . Stephen J. Giovannoni . amp . Unique glycine-activated riboswitch linked to glycine-serine auxotrophy in SAR11 . Environmental Microbiology. January 2009 . 10.1111/j.1462-2920.2008.01758.x . 19125817 . 2621071 . 11 . 1 . 230–8.
6. Giovannoni . S. J. . Tripp . H. J. . Givan . S. . Podar . M. . Vergin . K. L. . Baptista . D. . Bibbs . L. . Eads . J. . Richardson . T. H. . Noordewier . M. . Rappé . M. S. . Short . J. M. . Carrington . J. C. . Mathur . E. J. . Genome Streamlining in a Cosmopolitan Oceanic Bacterium . 10.1126/science.1114057 . Science . 309 . 5738 . 1242–1245 . 2005 . 16109880. 2005Sci...309.1242G . 16221415 .
7. H. James Tripp . Joshua B. Kitner . Michael S. Schwalbach . John W. H. Dacey . Larry J. Wilhelm . Stephen J. Giovannoni . amp . SAR11 marine bacteria require exogenous reduced sulfur for growth . Nature. April 2008 . 10.1038/nature06776 . 18337719 . 452 . 7188 . 741–4. 2008Natur.452..741T . 205212536 .
8. Schwalbach . M. S. . Tripp . H. J. . Steindler . L. . Smith . D. P. . Giovannoni . S. J. . 10.1111/j.1462-2920.2009.02092.x . The presence of the glycolysis operon in SAR11 genomes is positively correlated with ocean productivity . Environmental Microbiology . 12 . 2 . 490–500 . 2010 . 19889000.
9. Giovannoni . S. J. . Bibbs . L. . Cho . J. C. . Stapels . M. D. . Desiderio . R. . Vergin . K. L. . Rappé . M. S. . Laney . S. . Wilhelm . L. J. . Tripp . H. J. . Mathur . E. J. . Barofsky . D. F. . Proteorhodopsin in the ubiquitous marine bacterium SAR11 . Nature . 438 . 7064 . 82–85 . 2005 . 16267553 . 10.1038/nature04032. 2005Natur.438...82G . 4414677 .
10. 10.1038/ncomms5346. Methane production by phosphate-starved SAR11 chemoheterotrophic marine bacteria. Nature Communications. 5. 2014. Carini . P. . White . A. E. . Campbell . E. O. . Giovannoni . S. J. . 25000228 . 4346. 2014NatCo...5.4346C. free .
11. Book: Bergey's Manual of Systematic Bacteriology. 2C. The Proteobacteria. George M. Garrity. Don J. Brenner. Noel R. Krieg. James T. Staley. Springer. New York. 2nd. 978-0-387-24145-6. 1388. July 26, 2005. 1984(Williams & Wilkins). British Library no. GBA561951.
12. Robert M. Morris, K.L.V., Jang-Cheon Cho, Michael S. Rappé, Craig A. Carlson, Stephen J. Giovannoni, Temporal and Spatial Response of Bacterioplankton Lineages to Annual Convective Overturn at the Bermuda Atlantic Time-Series Study Site" Limnology and Oceanography 50(5) p. 1687-1696.
13. Salcher, M.M., J. Pernthaler, and T. Posch, Seasonal bloom dynamics and ecophysiology of the freshwater sister clade of SAR11 bacteria 'that rule the waves' (LD12). ISME J, 2011.
14. Rodríguez-Ezpeleta N, Embley TM . 2012. The SAR11 group of alpha-proteobacteria is not related to the origin of mitochondria. PLOS ONE. 22291975. 10.1371/journal.pone.0030520. 7. 1. e30520. 3264578. 2012PLoSO...730520R. free.