SAR11 clade

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Scientific classification (Candidatus)
Domain: Bacteria
Phylum: Proteobacteria
Class: Alphaproteobacteria
Order: Rickettsiales

The SAR11 clade, or Pelagibacteraceae,[1] defines a lineage of bacteria that is extremely common in the ocean.[2] Bacteria in the SAR11 clade make up roughly one in three cells at the ocean's surface. Overall, SAR11 bacteria are estimated to make up between a quarter and a half of all prokaryotic cells in the ocean.

SAR11 bacteria are classified as alphaproteobacteria, and include the highly abundant marine species Pelagibacter ubique. Bacteria in this clade are unusually small.[3]

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,[4] and some cofactors.[5] They also have an unusual and unexpected requirement for reduced sulfur.[6]

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.[7]

Unlike Acaryochloris marina, it is non-photosynthetic, but possesses proteorhodopsin (incl. retinol biosynthesis) for ATP production.[8]

SAR11 bacteria are responsible for much of the dissolved methane in the ocean surface. They extract phosphate from methylphosphonic acid.[9]

"Pelagibacteraceae" appear to be basal to the other three families in the order Rickettsiales.[10] The family 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. Rickettsiales are currently defined by at least one shared feature: multiplying only inside host cells.[11] If the position of "Pelagibacteraceae" is correct, it might be possible to derive unique insight into the evolution from free-living to parasitic and symbiotic.[citation needed]


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]

Phylogeny of Rickettsiales
Other alphaproteobacteria

Rhodospirillales, Sphingomonadales, Rhodobacteraceae, Rhizobiales, etc.

SAR11 clade

Pelagibacter ubique









Robust phylogeny of Rickettsiales from Williams et al. (2007)[14]


  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". Scientific Reports. doi:10.1038/srep00013. 
  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. ^ 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. 
  4. ^ 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. 
  5. ^ 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.  edit
  6. ^ 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. 
  7. ^ 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.  edit
  8. ^ 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.  edit
  9. ^ 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.  edit
  10. ^ Thrash, J. C.; Boyd, A.; Huggett, M. J.; Grote, J.; Carini, P.; Yoder, R. J.; Robbertse, B.; Spatafora, J. W.; Rappé, M. S.; Giovannoni, S. J. (2011). "Phylogenomic evidence for a common ancestor of mitochondria and the SAR11 clade". Scientific Reports 1. doi:10.1038/srep00013.  edit
  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. ^ Williams, K. P.; Sobral, B. W.; Dickerman, A. W. (2007). "A Robust Species Tree for the Alphaproteobacteria". Journal of Bacteriology 189 (13): 4578–4586. doi:10.1128/JB.00269-07. PMC 1913456. PMID 17483224.  edit