The Aquificales (Phylum Aquificae) are chemolithoautotrophic thermophilic eubacteria (mostly rods, tem, aeolicus) found in terrestrial geothermal and marine hydrothermal systems. These recently discovered chemolithoautotrophs are primary producers of bacterial biomass within high temperature ecosystems, thriving in temperatures up to 950C. The Aquificales display reduced metabolic flexibility, Aquifex aeolicus is one of the most thermophilic, bacteria known – it is able to grow on hydrogen, oxygen, carbon dioxide, and mineral salts.

Most Aquificales can grow with hydrogen as sole electron donor and oxygen as electron acceptor performing the reduction of O2 with H2 ("Knallgas" reaction): 2 H2 + O2 -> 2 H2O
Aquifex means ‘water-maker’, referring to this ability of Aquificales to oxidize hydrogen and oxygen gases to produce water. Most Aquificales can use thiosulfate or sulfur as an energy source (like chlorobium and other green sulfur bacteria) and produce sulfuric acid and H2S instead of water.

Within on the 16S rRNA tree, Aquificales represent one of the deepest and earliest branching groups within the phylogenetic tree. Together with the separate phylogenetic branch of the Thermotogales order, the Aquificales form a very deep phylogenetic branch within the domain Bacteria (more).

Aquificae > Aquificales > Aquifex > Aquifex aeolicus, Aquifex pyrophilus, Aquifex sp.

The genome of Aquifex aeolicus comprises 1,551,335 base pairs – only one-third the size of the E. coli genome. The genome is densely packed and contains genes that overlap others, and it contains no introns or protein splicing elements. Comparison of the Aquifex genome to other prokaryotic organisms demonstrated that 16% of its genes are equivalent to those of Archaea. It has been suggested that this indicates horizontal transfer, however, it has also been suggested that this indicates a conserved ancestral component shared with the archaea. Article Genome Aquifex aeolicus (abstract). Free Full Text Article. More information on the genome and chromosomal structure of A. aeolicus VF5 is available at TIGR.

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