The Aquificaephylum is a diverse collection of bacteria that live in harsh environmental settings. The name 'Aquificae' 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. They have been found in springs, pools, and oceans. They are autotrophs, and are the primary carbon fixers in their environments. These bacteria are Gram-negative, non-spore-forming rods. They are true bacteria (domainBacteria) as opposed to the other inhabitants of extreme environments, the Archaea.
The Aquificae currently contain 15 genera and 42 validly published species. The phylum comprises a single class and two orders. Aquificales consists of the families Aquificaceae and Hydrogenothermaceae, while the Desulfurobacteriaceae are the only family within the Desulfobacteriales. Thermosulfidibacter takaii is not assigned to a family within the phylum based on its phylogenetic distinctness from both orders. It is currently classified as a member of Aquificales, but it has shown more physiological similarity to the Desulfobacteriaceae.
Molecular signatures and phylogenetic position
Comparative genomic studies have identified several conserved signature indels (CSIs) that are specific for all species belonging to the phylum Aquificae and provide potential molecular markers. 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 demarcate Aquificae and Hydrogenothermaceae from all other bacteria. In parallel with the observed CSI distribution, the orders within the Aquificae are also physiologically distinct from one another. Members of the Desulfurobacteriales are strict anaerobes that exclusively oxidize hydrogen for energy, whereas those belonging to the Aquificales are microaerophilic, and capable of oxidizing other compounds (such as sulfur or thiosulfate) in addition to hydrogen.
Several CSIs have also been identified that are specific for the species from the Aquificae and provide potential molecular markers for this phylum. Additionally, a 51-amino-acid insertion has been identified in SecA preprotein translocase which is shared by various members of the Aquificae, as well as two Thermotoga species. 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 the Thermotogae and the deep branching of the Aquificae is not supported by some phylogenetic studies based upon other gene/protein sequences and also by CSIs in several highly conserved universal proteins. Thus, the presence of the insertion in the Thermotoga species may be due to a horizontal gene transfer. 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 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 the Epsilonproteobacteria. A specific relationship of the Aquificae to the Proteobacteria is supported by a two-amino-acid CSI in the protein inorganic pyrophosphatase, which is uniquely found in species from these two phyla.Cavalier-Smith has also suggested that the Aquificae are closely related to the Proteobacteria. 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 the Aquifex lineage than noninformational genes, most often placed the Aquificales close to the Thermotogales. These authors explain the frequently observed grouping of Aquificae with Epsilonproteobacteria as result of frequent horizontal gene transfer due to shared ecological niches.
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