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