Pelagibacterales (SAR11): Difference between revisions
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SAR11 clade possesses many unusual features for a free-living organism, including an extremely small, streamlined genome with few paralogs, no pseudogenes, and many missing genes and pathways that are otherwise common in bacteria(8). However, the SAR11 clade is phylogenetically diverse, spanning 18% 16S rRNA gene divergence. They have greater genomic rearrangement at operon boundaries than within operons, as well as hyper variable regions, possibly allowing these organisms to acquire new genetic material with adaptive significance (8). | SAR11 clade possesses many unusual features for a free-living organism, including an extremely small, streamlined genome with few paralogs, no pseudogenes, and many missing genes and pathways that are otherwise common in bacteria(8). However, the SAR11 clade is phylogenetically diverse, spanning 18% 16S rRNA gene divergence. They have greater genomic rearrangement at operon boundaries than within operons, as well as hyper variable regions, possibly allowing these organisms to acquire new genetic material with adaptive significance (8). | ||
== | ==Characteristics of SAR11 genomes== | ||
==Ecology== | ==Ecology== | ||
Revision as of 14:37, 15 May 2015
Classification
Domain: Bacteria
Phylum: Proteobacteria
Class: Alphaproteobacteria
Subclass: Rickettsidae
Order: Pelagibacterales
Description and Significance
Pelagibacterales (SAR11) is an order in the Alphaproteobacteria composed of free-living planktonic oligotrophic facultative photochemotroph bacteria. (4)(5) They are most abundant group of planktonic cells in marine systems and possibly the most numerous bacterium in the world. Typically accounting for ~25% of prokaryotic cells in seawater worldwide.(3)
The order was originally named SAR11 following its discovery in the Sargasso Sea in 1990 by Professor Stephen Giovannoni and colleagues, from Oregon State University.(2)(5) It was first placed in the order of Rickettsiales, but after rRNA gene-based phyogenetic analysis, in 2013 it was raised to the rank of order, and then placed as sister order to the Rickettsiales in the subclass Rickettsidae.(5)
These gram negative, rod shaped bacteria are one of the smallest free cell living organisms, with a cell volume less than 1/500th the volume of E.coli. They have a high surface to volume ratio to better absorb nutrients from its oligotrophic environment. Unsurprisingly, it is also known to have one of the smallest genomes of free living cells.(7) Pelagibacterales have the complete biosynthetic pathway for all 20 amino acids and all but a few cofactors. There are also no pseudogenes, introns, transposons, or extrachromosomal elements yet observed for any cell. (7)
Subgroups
Currently the SAR11 clade is divided into five subgroups (8):
Subgroup Ia: HTCC1062, HTCC1002, HTCC9565, HTCC7211, HIMB5
Subgroup Ib: SAR193, SAR11
Subgroup II: Artic95B-1, SAR11
Subgroup IIIa: HIMB114, OM155
Subgroup IIIb: S9D-28, LD12
Subgroup IV: DQ009255
Subgroup V: HIMB59, DQ009262
SAR11 clade possesses many unusual features for a free-living organism, including an extremely small, streamlined genome with few paralogs, no pseudogenes, and many missing genes and pathways that are otherwise common in bacteria(8). However, the SAR11 clade is phylogenetically diverse, spanning 18% 16S rRNA gene divergence. They have greater genomic rearrangement at operon boundaries than within operons, as well as hyper variable regions, possibly allowing these organisms to acquire new genetic material with adaptive significance (8).
Characteristics of SAR11 genomes
Ecology
Oenococcus oeni stabilizes wine communities by consuming available nutrients and lowering potential growth of other microbes, but its malolactic fermentation can be beneficial or detrimental to the production of wine depending on grapes, climate, and style of wine. Variations between strains and fermentation conditions have the potential to impact general quality and production of wine. Industrial winemakers use a standardized strain of O. oeni, but the many external and environmental variables will dictate the success of the wine. O. oeni is not the only lactic acid bacteria that can perform secondary fermentation. There are a variety of lactic acid bacteria that can dominate bacterial community in response to temperature, nutrients, sulfur dioxide content, pH, ethanol levels, and inoculation densities. O. oeni commonly dominates secondary fermentation from its extreme tolerance to pH and ethanol levels. The molecule diacetyl is produced as a byproduct of lactic acid bacteria in secondary fermentation. In wine, the levels of diacetyl create buttery and caramel flavor notes. It is generated when there is little or no malic acid to be consumed so citric acid is used. This byproduct is sought out by some winemakers while it is avoided by others. Rarely do the diacetyl levels reach a point of spoiling the wine (10).
Because of its heterofermentive properties, Oenoccocus may be viewed as an ecosystem engineer. O. oeni plays a major role in establishing the environment for which other microbes will interact. Its end products and life strategies positively feedback into creating a more harsh environment for other microbes like yeasts and fungi while making the conditions more ideal for other lactic acid bacteria (5).
References
(1) "Rebounding bacteria". 2013
(2) “SAR11 bacteria thrive — despite viruses”.“Nature”
(6) A war without end - with Earth’s carbon cycle held in the balance
Author
Page authored by Digvinder Singh Mavi student of Prof. Katherine Mcmahon at University of Wisconsin-Madison.
