Hydrobacteria
Escherichia coli cells magnified 25,000 times
Scientific classification
Domain:	Bacteria
Clade:	Hydrobacteria Battistuzzi and Hedges 2009 [1]
Superphyla/Phyla
Acidobacteriota Aquificota Bdellovibrionota Campylobacterota Deferribacterota Dependentiae Desulfobacterota Desulfuromonadota Elusimicrobiota FCB superphylum Myxococcota Nitrospirota Proteobacteria PVC superphylum Spirochaetota

Hydrobacteria is a taxon containing approximately one-third of prokaryote species, mostly gram-negative bacteria and their relatives. ^[1] It was found to be the closest relative of an even larger group of Bacteria, Terrabacteria, which are mostly gram-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 of Terrabacteria possess adaptations for life on land. ^{[2] [1]}

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. ^{[3] [4]}

Some unrooted molecular phylogenetic analyses ^{[5] [6]} have not supported this dichotomy of Terrabacteria and Hydrobacteria, but the most recent genomic analyses, ^{[3] [4]} including those that have focused on rooting the tree, ^[3] 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. ^[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 in the taxon Selabacteria, in allusion to their phototrophic abilities (selas = light). ^[7] 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. ^{[3] [4]}

"Gracilicutes," which was described in 1978 by Gibbons and Murray, ^[8] 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. ^[8] More recently, a redefinition of "Gracilicutes" was proposed ^[9] 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.

Phylogeny

The definition of two major divisions within the domain Bacteria, Hydrobacteria and Terrabacteria, has come largely from rooted phylogenetic analyses of genomes. ^{[2] [1] [3] [4]} Unrooted analyses have not fully supported this division, ^{[6] [5]} 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. ^{[3] [4]} 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 ^[3]) and the other concluding that it was a diderm ( gram-negative bacteria ^[4]). The following tree is redrawn from one of those two recent studies, ^[3] showing the phylogeny of bacterial phyla and superphyla, with the position of Fusobacteria being unresolved and DST being the closest relative of Terrabacteria:

References