The phylum aquificae

Abstract

The phylum Aquificae is comprised of thermophilic and hyperthermophilic bacteria, which flourish in marine and terrestrial hydrothermal environments. Most Aquificae species are chemolithoautotrophs; however, some also grow heterotrophically. The phylum presently contains a single class and a single order with three families. Of these families, Aquificaceae and Hydrogenothermaceae are aerobic or microaerophilic bacteria deriving energy by oxidation of hydrogen or reduced sulfur compounds by molecular oxygen. In contrast, the family Desulfurobacteriaceae is composed of strict anaerobes, and they obtain energy by reduction of sulfate, nitrate, elemental sulfur, or other compounds by molecular hydrogen. However, the above characteristics are not unique to members of the phylum Aquificae, and they cannot be used to distinguish this phylum from other bacteria. Analyses of the Aquificae genomes have identified numerous conserved signature indels (CSIs) in protein sequences that are specific for the phylum Aquificae or its different families/taxa, enabling demarcation of these clades in molecular terms. Many CSIs that are uniquely shared by the families Aquificaceae and Hydrogenothermaceae provide evidence that these two families shared a common ancestor exclusive of Desulfurobacteriaceae. In phylogenetic trees based upon different genes/proteins sequences, Aquificae branch either very deeply among bacteria, in the proximity of Thermotogae, or they are a late branching group in the neighborhood of Epsilonproteobacteria. Evidence reviewed here indicates that deep branching of these species in phylogenetic trees based upon rRNA and many translational related genes/proteins is heavily influenced by the unusually high G+C content of their rRNA genes, which is required for maintaining the structural integrity of rRNA at high temperature. Several CSIs in the elongation factor EF-Tu provide evidence that its gene, which is part of an operon containing the genes for EF-G and several ribosomal proteins, was laterally transferred from the phylum Thermotogae to Aquificae. The members of this phylum, due to their ability to grow at temperature >80 °C by oxidation of hydrogen, also provide an important resource for many biotechnological applications.