|Scientific classification (Candidatus)|
The SAR11 clade, or Pelagibacteraceae, defines a lineage of bacteria that is extremely common in the ocean. 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.
Pelagibacter ubique and related species are oligotrophs — scavengers — and feed on dissolved organic carbon and nitrogen. 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, and some cofactors. They also have an unusual and unexpected requirement for reduced sulfur.
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.
"Pelagibacteraceae" appear to be basal to the other three families in the order Rickettsiales. 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. 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.
Currently the (unofficial) family is divided into five subgroups:
- 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
- Subgroup V, which includes alphaproteobacterium HIMB59, basal to the remainder
Phylogenetic placement and Endosymbiotic theory
A recent study by researchers of the University of Hawaiʻi at Mānoa and the Oregon State University, seems to indicate that SAR11 could be the ancestor of mitochondria existing in most eukaryotic cells.