Hydrobacteria
Hydrobacteria is a taxon containing approximately one-third of prokaryote species, mostly gram-negative bacteria and their relatives. It was found to be the closest relative of an even larger group of Bacteria, Terrabacteria, which are mostly gram positive bacteria. The name Hydrobacteria (hydro = "water") refers to the moist environment inferred for the common ancestor of those species. In contrast, species of Terrabacteria possess adaptations for life on land.
| Hydrobacteria | |
|---|---|
| Escherichia coli cells magnified 25,000 times | |
| Scientific classification | |
| Domain: | Bacteria |
| Clade: | Hydrobacteria Battistuzzi and Hedges 2009 |
| Superphyla/Phyla | |
| |
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, and Spirochaetota.
Some unrooted molecular phylogenetic analyses have not supported this dichotomy of Terrabacteria and Hydrobacteria, but the most recent genomic analyses, including those that have focused on rooting the tree, have found these two groups to be monophyletic.
Hydrobacteria and Terrabacteria were inferred to have diverged approximately 3 billion years ago, suggesting that land (continents) had been colonized by prokaryotes at that time. Together, Hydrobacteria and Terrabacteria form a large group containing 97% of prokaryotes and 99% of all species of Bacteria known by 2009, and placed in the taxon Selabacteria, in allusion to their phototrophic abilities (selas = light). Currently, the bacterial phyla that are outside of Hydrobacteria + Terrabacteria, and thus justifying the taxon Selabacteria, are debated and may or may not include Fusobacteria.
"Gracilicutes," which was described in 1978 by Gibbons and Murray, is sometimes used in place of Hydrobacteria. However, "Gracilicutes" included cyanobacteria (a member of Terrabacteria) and was not constructed under the now generally accepted three-domain system. More recently, a redefinition of "Gracilicutes" was proposed but it did not include a molecular phylogeny or statistical analyses. Also, it did not follow the three-domain system, claiming instead that the lineage of eukaryotes + Archaea is nested within Bacteria as a close relative of Actinomycetota, a tree not supported in any molecular phylogeny.