TheAquificotaphylum is a diverse collection of bacteria that live in harsh environmental settings.[2][3] The nameAquificota was given to this phylum based on an early genus identified within this group,Aquifex (“water maker”), which is able to produce water by oxidizing hydrogen.[4] They have been found in springs, pools, and oceans. They areautotrophs, and are the primary carbon fixers in their environments. These bacteria areGram-negative, non-spore-formingrods.[5] They are true bacteria (domainBacteria) as opposed to the other inhabitants of extreme environments, theArchaea.
TheAquificota currently contain 15 genera and 42 validly published species.[6] The phylum comprises three class with each of them having their respective order.[7][8] Aquificales consists of the familiesAquificaceae andHydrogenothermaceae, while theDesulfurobacteriaceae are the only family within the Desulfurobacteriales.Thermosulfidibacter takaii is not assigned to a family within the phylum based on its phylogenetic distinctness from both orders.[9] It is currently classified as a member of Aquificales, but it has shown more physiological similarity to the Desulfobacteriaceae.
Comparative genomic studies have identified severalconserved signature indels (CSIs) that are specific for all species belonging to the phylumAquificota and provide potential molecular markers.[8] The order Aquificales can be distinguished from Desulfobacteriales by several CSIs across different proteins that are specific for each group. Additional CSIs have been found at the family level, and can be used to demarcateAquificota andHydrogenothermaceae from all other bacteria.[8] In parallel with the observed CSI distribution, the orders within theAquificota are also physiologically distinct from one another. Members of the Desulfurobacteriales are strictanaerobes that exclusively oxidize hydrogen for energy, whereas those belonging to the Aquificales aremicroaerophilic, and capable of oxidizing other compounds (such as sulfur or thiosulfate) in addition to hydrogen.[10][11][12]
Several CSIs have also been identified that are specific for the species from theAquificota and provide potential molecular markers for this phylum.[2] Additionally, a 51-amino-acid insertion has been identified inSecA preprotein translocase which is shared by all members of theAquificota, as well as all members of the orderThermotogales.[13] Phylogenetic studies demonstrated that the presence of the same CSI within these two unrelated groups of bacteria is not due tolateral gene transfer, rather the CSI likely developed independently in these two groups of thermophiles due toselective pressure.[13] The 51 amino acid insertion is located on the surface of SecA near the binding site of ADP/ATP. Molecular dynamic simulations revealed a network water molecules forming an intermediate interaction between residues of the 51 aa CSI and ADP molecules, which serves to stabilize the hydrogen bonds formed between ADP/ATP and the protein. It is suggested that the network of hydrogen bonds formed between the water molecules, CSI residues and ADP/ATP helps to maintain ATP/ADP binding to the SecA protein at high temperatures, which contributes to the bacteria’s overall thermostability.[13]
In the 16S rRNA gene trees, theAquificota species branch in the proximity of the phylumThermotogota (another phylum comprisinghyperthermophilic organisms) close to the archaeal-bacterial branch point.[14][11] However, a close relationship of theAquificota to the Thermotogota and the deep branching of theAquificota is not supported by some phylogenetic studies based upon other gene/protein sequences[15][16][17][18] and also by CSIs in several highly conserved universal proteins 16S-23S-5S operons.[19] 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,[20] the inference that theAquificota do not constitute a deep-branch lineage is also independently strongly supported by CSIs 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 theCampylobacterota.[19] A specific relationship of theAquificota to the Proteobacteria is supported by a two-amino-acid CSI in the proteininorganic pyrophosphatase, which is uniquely found in species from these two phyla.[19]Cavalier-Smith has also suggested that theAquificota are closely related to the Proteobacteria.[21] In contrast to the above cited analyses that are based on a few indels or on single genes, analyses on informational genes, which appeared to be less often transferred to theAquifex lineage than noninformational genes, most often placed the Aquificales close to the Thermotogales.[22] These authors explain the frequently observed grouping ofAquificota with Campylobacterota as result of frequenthorizontal gene transfer due to shared ecological niches.
Along with the Thermotogota, theAquificota arethermophilic eubacteria.[3]
^abcGupta RS, Lali R (September 2013). "Molecular signatures for the phylum Aquificae and its different clades: Proposal for division of the phylum Aquificae into the emended order Aquificales, containing the families Aquificaceae and Hydrogenothermaceae, and a new order Desulfurobacterialesord. nov., containing the family Desulfurobacteriaceae".Antonie van Leeuwenhoek.104 (3):349–368.doi:10.1007/s10482-013-9957-6.PMID23812969.S2CID559778.
^Guiral M, Prunetti L, Aussignargues C, Ciaccafava A, Infossi P, Ilbert M, Lojou E, Giudici-Orticoni MT (2012). "The Hyperthermophilic Bacterium Aquifex aeolicus".The hyperthermophilic bacterium Aquifex aeolicus: from respiratory pathways to extremely resistant enzymes and biotechnological applications. Advances in Microbial Physiology. Vol. 61. pp. 125–194.doi:10.1016/B978-0-12-394423-8.00004-4.ISBN9780123944238.PMID23046953.{{cite book}}:|journal= ignored (help)
^abReysenbach, A.-L. (2001) Phylum BII. Thermotogae phy. nov. In: Bergey's Manual of Systematic Bacteriology, pp. 369-387. Eds D. R. Boone, R. W. Castenholz. Springer-Verlag: Berlin.
^Gupta, RS (2014) The Phylum Aquificae. The Prokaryotes 417-445. Springer Berlin Heidelberg.
^Huber, R. and Hannig, M. (2006) Thermotogales. Prokaryotes 7: 899-922.
^Klenk, H. P., Meier, T. D., Durovic, P. and others (1999) RNA polymerase ofAquifex pyrophilus: Implications for the evolution of the bacterial rpoBC operon and extremely thermophilic bacteria. J Mol Evol 48: 528-541.
^Gupta, R. S. (2000) The phylogeny of Proteobacteria: relationships to other eubacterial phyla and eukaryotes. FEMS Microbiol Rev 24: 367-402.
^Ciccarelli, F. D., Doerks, T., von Mering, C., Creevey, C. J., Snel, B., and Bork, P. (2006) Toward automatic reconstruction of a highly resolved tree of life. Science 311: 1283-1287.
^Di Giulio, M. (2003) The universal ancestor was a thermophile or a hyperthermophile: Tests and further evidence. J Theor Biol 221: 425-436.
^abcGriffiths, E. and Gupta, R. S. (2004) Signature sequences in diverse proteins provide evidence for the late divergence of the order Aquificales. International Microbiol 7: 41-52.
^Meyer, T. E. and Bansal, A. K. (2005) Stabilization against hyperthermal denaturation through increased CG content can explain the discrepancy between whole genome and 16S rRNA analyses. Biochemistry 44: 11458-11465.
^Boussau B, Guéguen L, Gouy M. Accounting for horizontal gene transfers explains conflicting hypotheses regarding the position of Aquificales in the phylogeny of Bacteria. BMC Evol Biol. 2008 Oct 3;8:272.doi:10.1186/1471-2148-8-272.
