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Scientific classification
Domain: Bacteria
Phylum: Proteobacteria
Class: Alphaproteobacteria
Subclass: Rickettsidae
Order: Pelagibacterales

The Pelagibacterales is an order in the Alphaproteobacteria composed of free-living bacteria that make up roughly one in three cells at the ocean's surface.[1][2][3] 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.[3]

It includes the highly abundant marine species Pelagibacter ubique. Bacteria in this clade are unusually small.[4]

Pelagibacter 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] and some cofactors.[6] They also have an unusual and unexpected requirement for reduced sulfur.[7]

Pelagibacter 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, it is non-photosynthetic —as in, it does use light to increase the bond energy of an electron pair—, but possesses proteorhodopsin (incl. 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]

The taxon derives its name from the type species Pelagibacter ubique. However, this species has not yet been validly published and, therefore, neither the familiar or the species has official taxonomic standing.[11]


Currently the (unofficial) family is divided into five subgroups:[12]

  • Subgroup Ia, open ocean, crown group — includes Pelagibacter 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 LD12 clade, freshwater[13]
  • Subgroup V, which includes alphaproteobacterium HIMB59, basal to the remainder

Phylogenetic placement and Endosymbiotic theory[edit]

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

Phylogeny of Rickettsiales

Magnetococcus marinus


Rhodospirillales, Sphingomonadales, Rhodobacteraceae, Rhizobiales, etc.




subgroups Ib, II, IIIa, IIIb, IV and V











Robust 16S + 23S phylogeny of Rickettsidae from Ferla et al. (2013)[15]


  1. ^ a b 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 (June 2011). "Phylogenomic evidence for a common ancestor of mitochondria and the SAR11 clade" (PDF). Scientific Reports 1: 13. doi:10.1038/srep00013. PMID 22355532. 
  2. ^ a b Morris RM, Rappé MS, Connon SA, et al. (2002). "SAR11 clade dominates ocean surface bacterioplankton communities". Nature 420 (6917): 806–10. doi:10.1038/nature01240. PMID 12490947. 
  3. ^ a b Ferla, M. P.; Thrash, J. C.; Giovannoni, S. J.; Patrick, W. M. (2013). "New rRNA gene-based phylogenies of the Alphaproteobacteria provide perspective on major groups, mitochondrial ancestry and phylogenetic instability". PLoS ONE 8 (12): e83383. doi:10.1371/journal.pone.0083383. PMC 3859672. PMID 24349502. 
  4. ^ Rappé MS, Connon SA, Vergin KL, Giovannoni SJ (August 2002). "Cultivation of the ubiquitous SAR11 marine bacterioplankton clade". Nature 418 (6898): 630–3. doi:10.1038/nature00917. PMID 12167859. 
  5. ^ H. James Tripp, Michael S. Schwalbach, Michelle M. Meyer, Joshua B. Kitner, Ronald R. Breaker, and Stephen J. Giovannoni (January 2009). "Unique glycine-activated riboswitch linked to glycine-serine auxotrophy in SAR11". Environmental Microbiology 11 (1): 230–8. doi:10.1111/j.1462-2920.2008.01758.x. PMC 2621071. PMID 19125817. 
  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. (2005). "Genome Streamlining in a Cosmopolitan Oceanic Bacterium". Science 309 (5738): 1242–1245. doi:10.1126/science.1114057. PMID 16109880. 
  7. ^ H. James Tripp, Joshua B. Kitner, Michael S. Schwalbach, John W. H. Dacey, Larry J. Wilhelm, and Stephen J. Giovannoni (April 2008). "SAR11 marine bacteria require exogenous reduced sulfur for growth". Nature 452 (7188): 741–4. doi:10.1038/nature06776. PMID 18337719. 
  8. ^ Schwalbach, M. S.; Tripp, H. J.; Steindler, L.; Smith, D. P.; Giovannoni, S. J. (2010). "The presence of the glycolysis operon in SAR11 genomes is positively correlated with ocean productivity". Environmental Microbiology 12 (2): 490–500. doi:10.1111/j.1462-2920.2009.02092.x. PMID 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. (2005). "Proteorhodopsin in the ubiquitous marine bacterium SAR11". Nature 438 (7064): 82–85. doi:10.1038/nature04032. PMID 16267553. 
  10. ^ Carini, P.; White, A. E.; Campbell, E. O.; Giovannoni, S. J. (2014). "Methane production by phosphate-starved SAR11 chemoheterotrophic marine bacteria". Nature Communications 5. doi:10.1038/ncomms5346. 
  11. ^ Don J. Brenner, Noel R. Krieg, James T. Staley (July 26, 2005) [1984(Williams & Wilkins)]. George M. Garrity, ed. The Proteobacteria. Bergey's Manual of Systematic Bacteriology 2C (2nd ed.). New York: Springer. p. 1388. ISBN 978-0-387-24145-6. 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. 7 (1): e30520. doi:10.1371/journal.pone.0030520. PMID 22291975. 
  15. ^ Ferla, M. P.; Thrash, J. C.; Giovannoni, S. J.; Patrick, W. M. (2013). "New rRNA gene-based phylogenies of the Alphaproteobacteria provide perspective on major groups, mitochondrial ancestry and phylogenetic instability". PLoS ONE 8 (12): e83383. doi:10.1371/journal.pone.0083383. PMC 3859672. PMID 24349502.  edit