The ''Hydrogenophilaceae'' are a family of the class '' Hydrogenophilalia'' in the phylum ''Pseudomonadota'' ("Proteobacteria"), with two genera – '' Hydrogenophilus'' and '' Tepidiphilus''. Like all ''

Pseudomonadota Pseudomonadota (synonym "Proteobacteria") is a major phylum of gram-negative bacteria. Currently, they are considered the predominant phylum within the domain of bacteria. They are naturally found as pathogenic and free-living (non- parasitic) ...

'', they are

Gram-negative Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelope consists ...

. All known species are

thermophilic A thermophile is a type of extremophile that thrives at relatively high temperatures, between . Many thermophiles are archaea, though some of them are bacteria and fungi. Thermophilic eubacteria are suggested to have been among the earliest bact ...

, growing around 50 °C, and use molecular

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

or organic molecules as their source of electrons to support growth; some species are

autotroph An autotroph is an organism that can convert Abiotic component, abiotic sources of energy into energy stored in organic compounds, which can be used by Heterotroph, other organisms. Autotrophs produce complex organic compounds (such as carbohy ...

s. The genus ''

Thiobacillus ''Thiobacillus'' is a genus of Gram-negative Betaproteobacteria. ''Thiobacillus thioparus'' is the type species of the genus, and the type strain thereof is the StarkeyT strain, isolated by Robert Starkey in the 1930s from a field at Rutgers U ...

'' was previously considered to be a member in this family but was reclassified into the order ''

Nitrosomonadales The ''Nitrosomonadales'' are an order of the class ''Betaproteobacteria'' in the phylum ''Pseudomonadota''.Garrity, George M.; Brenner, Don J.; Krieg, Noel R.; Staley, James T. (eds.) (2005). Bergey's Manual of Systematic Bacteriology, Volume Two ...

'' at the same time that the '' Hydrogenophilales'' were removed from the ''

Betaproteobacteria ''Betaproteobacteria'' are a class of Gram-negative bacteria, and one of the six classes of the phylum '' Pseudomonadota'' (synonym Proteobacteria). Metabolism The ''Betaproteobacteria'' comprise over 75 genera and 400 species. Together, they ...

'' to form the class '' Hydrogenophilalia''. '' Hydrogenophilus thermoluteolus'' is a facultative chemolithoautotroph originally isolated from a

hot spring A hot spring, hydrothermal spring, or geothermal spring is a Spring (hydrology), spring produced by the emergence of Geothermal activity, geothermally heated groundwater onto the surface of the Earth. The groundwater is heated either by shallow ...

; however, it was detected 2004 in ice core samples retrieved from a depth around 3 km within the ice covering

Lake Vostok Lake Vostok () is the largest of Antarctica's 675 known subglacial lakes. Lake Vostok is located at the southern Pole of Cold, beneath Russia's Vostok Station under the surface of the central East Antarctic Ice Sheet, which is at above mean se ...

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean (also known as the Antarctic Ocean), it contains the geographic South Pole. ...

. The presence of DNA from (and potentially live cells of) thermophilic bacteria in the ice suggests that a geothermal system could exist beneath the cold water body of Lake Vostok, or simply that non-thermophilic strains of '' Hydrogenophilus'' exist and were present in the ice.

''Hydrogenophilalia''

The class ''Hydrogenophilalia'' in the ''

Bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

'' was circumscribed in 2017 when it was demonstrated that the order ''Hydrogenophilales'' was distinct from the ''

'' on the basis of physiology, biochemistry, fatty acid profiles, and phylogenetic analyses on the basis of the 16S rRNA gene and 53 ribosomal protein sequences concatenated using the rMLST platform for multilocus sequence typing. The class comprises one order, the '' Hydrogenophilales'' (type order), which contains thermophilic organisms - both

s and

heterotroph A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...

s, the former of which utilise molecular

as their electron donor, coupling its oxidation to the reduction of NAD⁺ with the enzyme

hydrogenase A hydrogenase is an enzyme that Catalysis, catalyses the reversible Redox, oxidation of molecular hydrogen (H2), as shown below: Hydrogen oxidation () is coupled to the reduction of electron acceptors such as oxygen, nitrate, Ferric, ferric i ...

. The very high proportion of ω-cyclohexyl fatty acids (specifically C19:0 ''cyclo'' and C17:0 ''cyclo'') versus straight counterparts was a major distinguishing feature versus the ''

'', and is probably involved in ensuring membrane stability at high growth temperatures. Members of the class can all use molecular

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

as a terminal electron acceptor (i.e. are aerobic) as well as

nitrate Nitrate is a polyatomic ion with the chemical formula . salt (chemistry), Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are solubility, soluble in wa ...

, which can be used by some members during

denitrification Denitrification is a microbially facilitated process where nitrate (NO3−) is reduced and ultimately produces molecular nitrogen (N2) through a series of intermediate gaseous nitrogen oxide products. Facultative anaerobic bacteria perform denitr ...

. The autotrophic members of the class do not use carboxysomes to concentrate

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

or improve

RuBisCO Ribulose-1,5-bisphosphate carboxylase/oxygenase, commonly known by the abbreviations RuBisCo, rubisco, RuBPCase, or RuBPco, is an enzyme () involved in the light-independent (or "dark") part of photosynthesis, including the carbon fixation by wh ...

efficiency as a carboxylase versus an oxygenase. The dominant respiratory quinone of the class is ubiquinone-8 and

menaquinone Vitamin K2 or menaquinone (MK) () is one of three types of vitamin K, the other two being vitamin K1 (phylloquinone) and K3 (menadione). K2 is both a tissue and bacterial product (derived from vitamin K1 in both cases) and is usually found in an ...

s or rhodoquinones are not observed, though they are in the neighbouring ''

''.

References

{{Taxonbar, from1=Q134834, from2=Q3143734, from3=Q123126187 Hydrogenophilales Hydrogenophilalia Taxa described in 2006