SAR11 clade
Encyclopedia
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 bacterial or archaea
n cells in the ocean.
SAR11 bacteria are classified as alphaproteobacteria
, and include the highly abundant marine species Pelagibacter ubique
. This species and other results indicate that SAR11 cells are unusually small.
"Pelagibacter ubique" and its relatives are a oligotrophs — scavengers — and feed off dissolved organic carbon and nitrogen. It is unable to fix carbon or nitrogen, but it possess TCA cycle with glyoxylate bypass and is able to synthesise all amino-acids, bar glycine, and some cofactors.
The ocean is nutrient poor, but the compounds present are small but varied:
"Pelagibacter ubique" and members of the oceanic subgroup I possess gluconeogenesis but not a canonical glycolysis pathway, whereas the other subgroups are capable of canonical glycolysis.
Unlike Acaryochloris marina
, it is non-photosynthetic, but possesses proteorhodopsin (incl. retinol biosynthesis) for ATP production.
The family "Pelagibacteraceae" is believed to be basal to the other three families in the order Rickettsiales. However, it is named after the type species "Pelagibacter ubique", which has not been fully described—hence the "candidatus
" status—consequently both taxa do not have an official standing in taxonomy and the Rickettsiales is currently defined with the trait that its members "Multiply only inside host cells". If the location of the family is correct, it offers a unique insight in the transition from free-living to parasitic/symbiotic life.
, seems to indicate that SAR11 could be the ancestor of mitochondrion
existing in most eukaryotic cells.
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
that is extremely common in the ocean
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
. 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 bacterial or archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
n cells in the ocean.
SAR11 bacteria are classified as alphaproteobacteria
Alphaproteobacteria
Alphaproteobacteria is a class of Proteobacteria. Like all Proteobacteria, they are Gram-negative.-Characteristics:The Alphaproteobacteria comprise most phototrophic genera, but also several genera metabolising C1-compounds , symbionts of plants and animals, and a group of pathogens, the...
, and include the highly abundant marine species Pelagibacter ubique
Pelagibacter ubique
Pelagibacter, with the single species P. ubique, was isolated in 2002 and given a specific name, although it has not yet been validly published according to the bacteriological code. It is an abundant member of the SAR11 clade in the phylum Alphaproteobacteria...
. This species and other results indicate that SAR11 cells are unusually small.
"Pelagibacter ubique" and its relatives are a oligotrophs — scavengers — and feed off dissolved organic carbon and nitrogen. It is unable to fix carbon or nitrogen, but it possess TCA cycle with glyoxylate bypass and is able to synthesise all amino-acids, bar glycine, and some cofactors.
The ocean is nutrient poor, but the compounds present are small but varied:
- Humic acid substances are a principal component
- the marine invertebrate osmolyte glycine betaine that is converted into glycine, serine and other amino acids.
- the algal osmolytes 3-dimethylsulphoniopropionate (DMSP), a major source of reduced sulphur
- iron is a limiting step
- other compounds such as methanol, formaldehyde, methylamine, DMSO, trimethylamine and trimethylamine N-oxide.
"Pelagibacter ubique" and members of the oceanic subgroup I possess gluconeogenesis but not a canonical glycolysis pathway, whereas the other subgroups are capable of canonical glycolysis.
Unlike Acaryochloris marina
Acaryochloris marina
Acaryochloris marina is a symbiotic species of the phylum Cyanobacteria that produces Chlorophyll d, allowing it to utilise far-red light, at 710 nm wavelength.-Description:...
, it is non-photosynthetic, but possesses proteorhodopsin (incl. retinol biosynthesis) for ATP production.
The family "Pelagibacteraceae" is believed to be basal to the other three families in the order Rickettsiales. However, it is named after the type species "Pelagibacter ubique", which has not been fully described—hence the "candidatus
Candidatus
Candidatus is in scientific classification a component of the taxonomic name for a bacterium that cannot be maintained in a Bacteriology Culture Collection. It is an interim taxonomic status for noncultivable organisms. An example would be "Candidatus Phytoplasma allocasuarinae"...
" status—consequently both taxa do not have an official standing in taxonomy and the Rickettsiales is currently defined with the trait that its members "Multiply only inside host cells". If the location of the family is correct, it offers a unique insight in the transition from free-living to parasitic/symbiotic life.
Subgroup
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, costal, 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 remeinder
Phylogenetic placement and Endosymbiotic theory
A recent study by researchers of the University of Hawaiʻi at Mānoa and the Oregon State UniversityOregon State University
Oregon State University is a coeducational, public research university located in Corvallis, Oregon, United States. The university offers undergraduate, graduate and doctoral degrees and a multitude of research opportunities. There are more than 200 academic degree programs offered through the...
, seems to indicate that SAR11 could be the ancestor of mitochondrion
Mitochondrion
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...
existing in most eukaryotic cells.