The Aquificae phylum is a diverse collection of bacteria that live in harsh environmental settings. They have been found in springs, pools, and oceans. Members of the genus Aquifex, for example, are productive in water between 105 and 138 °C. They are autotrophs, and are the primary carbon fixers in these environments. They are true bacteria (domain bacteria) as opposed to the other inhabitants of extreme environments, the Archaea.
Molecular signatures and phylogenetic position
Comparative genomic studies have identified 6 conserved signature indels (CSIs) that are specific for the species from the phylum Aquificae and provide potential molecular markers for this phylum. Of these 6 CSIs, 3 CSIs consisting of a 2 amino acid insertion, a 5-6 amino acid insertion and a 6 amino acid deletion are found in DNA polymerase I. The other 3 CSIs include a 6 to 7 amino acid insert in the GidA protein, a 52 amino acid insertion in RpoC beta’ subunit, and a 4 amino acid insertion in the EF-Tu proteins. Of these CSIs, the insert in EF-Tu was only found in members of family Aquificaceae. Additionally, a 51 amino acid insertion has been identified in SecA preprotein translocase which is shared by various members of the phylum Aquificae as well as 2 Thermotoga species. The presence of the insertion in the Thermotoga species may be due to a horizontal gene transfer. In the 16S rRNA gene trees, the Aquificae species branch in the proximity of the phylum Thermotogae (another phylum comprising hyperthermophilic organisms) close to the archaeal-bacterial branch point. However, a close relationship of the Aquificae to Thermotogae and the deep branching of Aquificae is not supported by some phylogenetic studies based upon other gene/protein sequences and also by conserved signature indels in several highly conserved universal proteins. The deep branching of Aquificae species in the rRNA gene tree was ascribed to be an artefact resulting from the very high G+C content of their 16S-23S-5S operons. In contrast to the very high G+C content of their rRNAs (i.e. more than 62%), which is required for stability of their secondary structures at high growth temperatures The inference that the Aquificae species do not constitute a deep branch lineage is also independently strongly supported by conserved signature indels in a number of important proteins (viz. Hsp70, Hsp60, RpoB, RpoB and AlaRS), which support its placement in the proximity of the phylum Proteobacteria, particularly the Epsilonproteobacteria. A specific relationship of the Aquificae to Proteobacteria is supported by a 2 aa conserved signature indel in the protein inorganic pyrophosphatase, which is uniquely found in species from these two phyla. Cavalier-Smith has also suggested that Aquificae is closely related to Proteobacteria. In contrast to the above cited analyses that are based on a few indels or on single genes, analyses informational genes, which appeared to be less often transferred to the Aquifex lineage than non-informational genes, most often placed Aquificales close to Thermotogales. These authors explain the frequently observed grouping of Aquificae with Epsilonproteobacteria as result of frequent horizontal gene transfer due to shared ecological niche.
The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN)  and National Center for Biotechnology Information (NCBI) and the phylogeny is based on 16S rRNA-based LTP release 111 by The All-Species Living Tree Project  
♠ Strain found at the National Center for Biotechnology Information (NCBI) but not listed in the List of Prokaryotic names with Standing in Nomenclature (LPSN)
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