| Pseudomonadati | |
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| Escherichia coli cells magnified 25,000 times | |
Scientific classification | |
| Domain: | Bacteria |
| Kingdom: | Pseudomonadati (Gibbons & Murray 1978) Oren & Göker 2024 |
| Phyla[1] | |
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| Synonyms | |
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Pseudomonadati[3] is akingdom containing approximately one-third ofprokaryote species, mostlygram-negative bacteria and their relatives.[2] It is the closest relative of an even larger kingdom ofBacteria, theTerrabacteria (also known as Bacillati), which are mostlygram-positive bacteria.[4][2]
Thesynonym's name "Hydrobacteria" (hydro = "water") refers to the moist environment inferred for the common ancestor of those species. In contrast, species ofTerrabacteria possess adaptations for life on land.[4][2] Since 2024, the onlyvalidly published name for this group is kingdom Pseudomonadati (there used to be none, because no levels abovephylum could exist in earlier versions of theProkaryotic Code).[5]
"Gracilicutes," which was described in 1978 by Gibbons and Murray,[6] is sometimes used in place of Pseudomonadati. However, "Gracilicutes" includedCyanobacteria (a member ofTerrabacteria) and was not constructed under the now generally acceptedthree-domain system.[6] More recently, a redefinition of "Gracilicutes" was proposed[7] but it did not include amolecular phylogeny or statistical analyses. Also, it did not follow thethree-domain system, claiming instead that the lineage ofeukaryotes +Archaea is nested within Bacteria as a close relative ofActinomycetota, a tree not supported in any molecular phylogeny.
Pseudomonadati andTerrabacteria were inferred to have diverged approximately 3 billion years ago, suggesting that land (continents) had been colonized by prokaryotes at that time.[2]
They include these superphyla and phyla:Acidobacteriota,Aquificota,Bdellovibrionota,Campylobacterota,Deferribacterota, Dependentiae,Desulfobacterota, Desulfuromonadota,Elusimicrobiota,FCB superphylum,Myxococcota,Nitrospirota,Proteobacteria,PVC superphylum, andSpirochaetota.[8][9]
Some unrooted molecular phylogenetic analyses[10][11] have not supported this dichotomy ofTerrabacteria and Pseudomonadati, but the most recent genomic analyses,[8][9] including those that have focused on rooting the tree,[8] have found these two groups to be monophyletic.
Together, Pseudomonadati and Terrabacteria form a large group containing 97% ofprokaryotes and 99% of all species ofBacteria known by 2009, and placed by Battistuzzi and Hedges in the proposedtaxonSelabacteria, in allusion to their phototrophic abilities (selas = light).[12] Currently, the bacterial phyla that are outside of Pseudomonadati +Terrabacteria, and thus justifying the taxon Selabacteria, are debated and may or may not includeFusobacteria.[8][9]
The definition of two major divisions within the domainBacteria, Pseudomonadati, andTerrabacteria, has come largely from rooted phylogenetic analyses of genomes.[4][2][8][9] Unrooted analyses have not fully supported this division,[11][10] drawing attention to the importance of rooted trees of life.
The two recent analyses of bacterial phylogeny both supported the division of Pseudomonadati and Terrabacteria.[8][9] However, they interpreted the evolution of thecell wall differently, with one concluding that the last common ancestor of Bacteria was a monoderm (gram-positive bacteria[8]) and the other concluding that it was a diderm (gram-negative bacteria[9]). The following tree is redrawn from one of those two recent studies,[8] showing the phylogeny of bacterial phyla and superphyla, with the position of Fusobacteria being unresolved and DST being the closest relative ofTerrabacteria:
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