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23-597: The Pelagibacterales are an order in the Alphaproteobacteria composed of free-living marine bacteria that make up roughly one in three cells at the ocean's surface. 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

46-466: A convenient "artificial key" according to his Systema Sexuale , largely based on the arrangement of flowers. In botany, classes are now rarely discussed. Since the first publication of the APG system in 1998, which proposed a taxonomy of the flowering plants up to the level of orders, many sources have preferred to treat ranks higher than orders as informal clades . Where formal ranks have been assigned,

69-458: A case of convergent evolution that would result in an artefactual clustering. However, several studies disagree. Furthermore, it has been found that the GC-content of ribosomal RNA (the traditional phylogenetic marker for prokaryotes) little reflects the GC-content of the genome. One example of this atypical decorrelation of ribosomal GC-content with phylogeny is that members of

92-420: A diverse taxon and comprise several phototrophic genera, several genera metabolising C1-compounds ( e.g. , Methylobacterium spp.), symbionts of plants ( e.g. , Rhizobium spp.), endosymbionts of arthropods ( Wolbachia ) and intracellular pathogens ( e.g. Rickettsia ). Moreover, the class is sister to the protomitochondrion , the bacterium that was engulfed by the eukaryotic ancestor and gave rise to

115-466: A few studies have been reported on natural genetic transformation in the Alphaproteobacteria , this process has been described in Agrobacterium tumefaciens , Methylobacterium organophilum , and Bradyrhizobium japonicum . Natural genetic transformation is a sexual process involving DNA transfer from one bacterial cell to another through the intervening medium, and the integration of

138-412: A general definition of a class is available, it has historically been conceived as embracing taxa that combine a distinct grade of organization—i.e. a 'level of complexity', measured in terms of how differentiated their organ systems are into distinct regions or sub-organs—with a distinct type of construction, which is to say a particular layout of organ systems. This said, the composition of each class

161-524: Is a class of bacteria in the phylum Pseudomonadota (formerly "Proteobacteria"). The Magnetococcales and Mariprofundales are considered basal or sister to the Alphaproteobacteria . The Alphaproteobacteria are highly diverse and possess few commonalities, but nevertheless share a common ancestor. Like all Proteobacteria , its members are gram-negative , although some of its intracellular parasitic members lack peptidoglycan and are consequently gram variable. The Alphaproteobacteria are

184-571: Is based on whole-genome analysis. Subclass names are based on Ferla et al . (2013).   Magnetococcales   Mariprofundales   Rickettsiales (including mitochondria )  " Pelagibacterales "   Sphingomonadales   Rhodospirillales   Rhodothalassiales   Iodidimonadales   Kordiimonadales   Emcibacterales   Sneathiellales   Hyphomicrobiales   Rhodobacterales   Micropepsales  " Parvularculales "   Caulobacterales Spirochaetota Although only

207-542: Is composed by a large diversity of magnetotactic bacteria , but only one is described, Magnetococcus marinus . The Rickettsidae is composed of the intracellular Rickettsiales and the free-living Pelagibacterales . The Caulobacteridae is composed of the Holosporales , Rhodospirillales , Sphingomonadales , Rhodobacterales , Caulobacterales , Kiloniellales , Kordiimonadales , Parvularculales and Sneathiellales . Comparative analyses of

230-404: Is not photosynthetic — specifically, it does not use light to increase the bond energy of an electron pair — but it does possess proteorhodopsin (including retinol biosynthesis) for ATP production from light. SAR11 bacteria are responsible for much of the dissolved methane in the ocean surface. They extract phosphate from methylphosphonic acid . Although the taxon derives its name from

253-431: Is ultimately determined by the subjective judgment of taxonomists . In the first edition of his Systema Naturae (1735), Carl Linnaeus divided all three of his kingdoms of nature ( minerals , plants , and animals ) into classes. Only in the animal kingdom are Linnaeus's classes similar to the classes used today; his classes and orders of plants were never intended to represent natural groups, but rather to provide

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276-490: The Holosporales have a much higher ribosomal GC-content than members of the Pelagibacterales and Rickettsiales , even though they are more closely related to species with high genomic GC-contents than to members of the latter two orders. The Class Alphaproteobacteria is divided into three subclasses Magnetococcidae , Rickettsidae and Caulobacteridae . The basal group is Magnetococcidae , which

299-477: The TCA cycle with glyoxylate bypass and are able to synthesise all amino acids except glycine, as well as some cofactors. They also have an unusual and unexpected requirement for reduced sulfur. P. ubique and members of the oceanic subgroup I possess gluconeogenesis , but not a typical glycolysis pathway, whereas other subgroups are capable of typical glycolysis. Unlike Acaryochloris marina , P. ubique

322-881: The University of Hawaiʻi at Mānoa and Oregon State University , indicated that SAR11 could be the ancestor of mitochondria in most eukaryotic cells. However, this result could represent a tree reconstruction artifact due to compositional bias.   Magnetococcus marinus   Holosporales   Hyphomicrobiales , Rhodobacteraceae ,   Rhodospirillales , Sphingomonadales , etc .   Pelagibacter   Subgroups Ib, II, IIIa, IIIb, IV and V   Proto-mitochondria   Neorickettsia   Wolbachia   Anaplasma   Ehrlichia   Midichloria   Orientia   Rickettsia Alphaproteobacteria Alphaproteobacteria or α-proteobacteria , also called α-Purple bacteria in earlier literature,

345-439: The mitochondria , which are organelles in eukaryotic cells (See endosymbiotic theory ). A species of technological interest is Rhizobium radiobacter (formerly Agrobacterium tumefaciens ): scientists often use this species to transfer foreign DNA into plant genomes. Aerobic anoxygenic phototrophic bacteria , such as Pelagibacter ubique , are alphaproteobacteria that are a widely distributed and may constitute over 10% of

368-613: The sequenced genomes have also led to discovery of many conserved insertion-deletions (indels) in widely distributed proteins and whole proteins (i.e. signature proteins ) that are distinctive characteristics of either all Alphaproteobacteria , or their different main orders (viz. Rhizobiales , Rhodobacterales , Rhodospirillales , Rickettsiales , Sphingomonadales and Caulobacterales ) and families (viz. Rickettsiaceae , Anaplasmataceae , Rhodospirillaceae , Acetobacteraceae , Bradyrhiozobiaceae , Brucellaceae and Bartonellaceae ). These molecular signatures provide novel means for

391-482: The circumscription of these taxonomic groups and for identification/assignment of new species into these groups. Phylogenetic analyses and conserved indels in large numbers of other proteins provide evidence that Alphaproteobacteria have branched off later than most other phyla and classes of Bacteria except Betaproteobacteria and Gammaproteobacteria . The phylogeny of Alphaproteobacteria has constantly been revisited and updated. There are some debates for

414-428: The donor sequence into the recipient genome by homologous recombination . Class (biology) The class as a distinct rank of biological classification having its own distinctive name – and not just called a top-level genus (genus summum) – was first introduced by French botanist Joseph Pitton de Tournefort in the classification of plants that appeared in his Eléments de botanique of 1694. Insofar as

437-413: The inclusion of Magnetococcidae in Alphaproteobacteria . For example, an independent proteobacterial class (" Candidatus Etaproteobacteria") for Magnetococcidae has been proposed. A recent phylogenomic study suggests the placement of the protomitochondrial clade between Magnetococcidae and all other alphaproteobacterial taxa, which suggests an early divergence of the protomitochondrial lineage from

460-650: The open ocean microbial community. There is some disagreement on the phylogeny of the orders , especially for the location of the Pelagibacterales , but overall there is some consensus. The discord stems from the large difference in gene content ( e.g. genome streamlining in Pelagibacter ubique ) and the large difference in GC-content between members of several orders. Specifically, Pelagibacterales , Rickettsiales and Holosporales contain species with AT-rich genomes. It has been argued that it could be

483-577: The rest of alphaproteobacteria, except for Magnetococcidae . This phylogeny also suggests that the protomitochondrial lineage does not necessarily have a close relationship to Rickettsidae . The following taxa have been assigned to the Alphaproteobacteria , but have not been assigned to one or more intervening taxonomic ranks: The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN). The phylogeny

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506-678: The type species P. ubique (status Candidatus species), this species has not yet been validly published, and therefore neither the order name nor the species name has official taxonomic standing. Currently, the order is divided into five subgroups: The above results in a cladogram of the Pelagibacterales as follows: Subgroup Ia (named  Pelagibacteraceae , includes  Pelagibacter ) Subgroup Ib Subgroup II Subgroup IIIa Subgroup IIIb Subgroup IV (named LD12 clade, includes SAR11 bacteria) Subgroup V (includes α-proteobacterium HIMB59) A 2011 study by researchers of

529-663: 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. It includes the highly abundant marine species Pelagibacter ubique . Bacteria in this order are unusually small. Due to their small genome size and limited metabolic function, Pelagibacterales have become a model organism for ' streamlining theory '. P. 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

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