Thermotogales

These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Introduction

Members of the Thermotogales represent a very deep phylogenetic branch within the 16S rRNA gene tree. Within the order, members of the genus Thermotoga have an upper temperature border of growth at 90°C and represent, together with members of the order Aquificales, the bacteria with the highest growth temperatures known so far. Representatives of the Thermotogales are widespread and cosmopolitan, and they thrive mainly in volcanically or geothermally heated environments. Owing to their strictly organotrophic way of life, they are consumers of microbial biomaterial within high temperature ecosystems.

For the Thermotogales, the following order criteria are characteristic: thermophilic, nonsporeforming, rod-shaped cells with an outer sheath-like envelope (‘toga’); Gram-negative, but meso-diaminopimelic acid not present in the peptidoglycan; strictly anaerobic, fermentative bacteria; acetate, carbon dioxide, and hydrogen metabolites from glucose fermentation; inhibition of growth by hydrogen; lysozyme-sensitive; unusual long-chain dicarboxylic fatty acids present in the lipids; and G+C content, 29–50 mol%.

Phylogeny

By 16S rRNA gene sequence comparison it was shown first in 1987 that Thermotoga maritima represents a slowly evolving lineage and a deep phylogenetic branch within the bacterial domain (Huber et al., 1986; Woese, 1987; Fig. 1). On the basis of this result, a thermophilic origin of the bacterial domain was proposed (Achenbach-Richter et al., 1987). The outstanding phylogenetic placement of T. maritima was strengthened by comparative analysis of other macromolecules such as 23S rRNA, elongation factor Tu and G, ß-subunit of the ATPase, fus gene and of ferredoxins (Bachleitner et al., 1989; Schleifer and Ludwig, 1989; Tiboni et al., 1991; Ludwig et al., 1993; Blamey et al., 1994; Darimont and Sterner, 1994) and by whole genome-based phylogenetic analysis (Fitz-Gibbon and House, 1999). In contrast, sequence comparisons of bacterial RNA-polymerase large subunits placed T. maritima next to the chloroplasts, and the analysis of the bacterial DNA polymerase III, class II, placed T. maritima next to Clostridium acetobutylicum (Palm et al., 1993). Furthermore, a 16S rRNA phylogenetic tree based on the most conserved positions proposed that the Planctomycetales is the first branching bacterial group and not a hyperthermophilic bacterium (Brochier and Philippe, 2002). On the basis of conserved inserts and deletions found in various proteins, Gupta and Griffith (2002) placed T. maritima phylogenetically within the Gram-positive Bacteria. However, owing to biochemical and ultrastructural features, T. maritima is considered to be a true Gram-negative bacterium (see also Identification).

Fig. 1.

Phylogenetic tree based upon 16S rRNA gene sequence comparison showing the phylogenetic position of the species Thermotoga maritima at the root of bacterial tree. (From Woese, 1987; courtesy of C. W. Woese.)

Taxonomy

Owing to the outstanding phylogenetic position of T. maritima in combination with unique morphological, physiological and biochemical criteria, the order Thermotogales consisting of the single family Thermotogaceae was described (Huber et al., 1992b). Within the Thermotogaceae, the genera Thermotoga (Huber et al., 1986; Huber and Stetter, 2001b), Thermosipho (Huber et al., 1989a; Ravot et al., 1996b; Huber and Stetter, 2001b), Fervidobacterium (Patel et al., 1985a; Huber et al., 1990; Huber and Stetter, 2001b), Geotoga, Petrotoga (Davey et al., 1993a; Davey et al., 2001a; Davey et al., 2001b) and Marinitoga (Wery et al., 2001) have been described. The species belonging to the different genera of the Thermotogales are listed in Table 1; their phylogenetic relationship is shown in Fig. 2. On the basis of 16S rRNA gene sequence analysis, T. maritima, Thermotoga neapolitana, Thermotoga petrophila and Thermotoga naphthophila show a very close relationship (Fig. 2). However, based on specific sequence signatures in the 16S rRNA gene (Escherichia coli numbering; Brosius et al., 1981; Woese, 1987), the T. maritima – T. neapolitana group (position 1031, C; 1290, G; 1364, U) can be distinguished from the T. naphthophila – T. petrophila group (position 1031, U; 1290, A; 1364, C). Recently, the phylum Thermotogae, consisting of the single class Thermotogae was proposed (Reysenbach, 2001).

Table 1. List of the described species within the Thermotogales.

Fig. 2.

16S rRNA-based phylogenetic tree of the Thermotogales. The tree topology is based on the ARB database of 11,800 sequence entries and was reconstructed using the ARB parsimony tool. A filter defining positions that share identical residues in at least 50% of all included sequences from the Thermotogales was used for reconstructing the tree. Reference sequences were chosen to represent the broadest diversity of bacteria. Accession numbers for the sequences are given in Table 1. The scale bar represents 0.10 fixed mutations per nucleotide position.

Habitat

Members of the Thermotogales are widespread and cosmopolitan. They thrive within continental solfatara springs of low salinity, shallow and deep-sea marine hydrothermal systems and high-temperature marine and continental oil fields.

From the genus Thermotoga, the type species T. maritima had been originally isolated from a geothermally heated, shallow marine sediment at Vulcano, Italy (Huber et al., 1986). The second species of this genus, T. neapolitana, was obtained from a submarine thermal vent at Lucrino near Naples, Italy (Belkin et al., 1986; Jannasch et al., 1988). Members of the marine T. maritima – T. neapolitana group are widespread within high temperature ecosystems and have also been isolated from shallow submarine hydrothermal systems on Sangeang Island (Indonesia), Ribeira Quente, Sao Miguel Island (the Azores), Kunashir Island (north of Japan) and the Fiji Island (Huber and Stetter, 1992b). Additional isolates were obtained from the Kolbeinsey Ridge north of Iceland in a depth of 106 m and from deep-sea hot sediments in a depth of 2000 m (Guaymas, Mexico; Huber and Stetter, 1992a; Huber and Stetter, 1992b). Thermotoga thermarum was originally isolated from continental solfataras with low ionic strength at Lac Abbé, Djibouti, Africa (Windberger et al., 1989). The closest cultivated relative of T. thermarum is the so far undescribed new Thermotoga species BB13-1-L6A, which thrives in the geothermally heated water (68°C; pH 6.8) of the Friedrichstollen in Baden-Baden (Germany; Fig. 3).

Fig. 3.

The new Thermotoga species BB13-1-L6A, isolated from geothermally heated water from the Friedrichstollen in Baden-Baden, Germany. The rod-shaped cell is covered by an elongated sheath-like outer membrane (“toga”), overballooning at both ends. Bar, 1 µm.

Evidence for the presence of Thermotogales relatives in deep-subsurface petroleum reservoirs was reported for the first time by Stetter et al. (1993). Thermotoga-related organisms (designated “Pb-isolates”) were obtained from seawater flooded oil fields, about 3000 m below the permafrost surface of the North Slope of Alaska (Prudhoe Bay, Endicott and Kuparuk oil fields; Stetter et al., 1993; Stetter and Huber, 2000). By 16S rRNA gene sequence comparison and sequence signature analysis, these Pb-isolates were found to group together with the recently described T. petrophila and T. naphthophila from the Kubiki oil reservoir in Niigata (Japan; Takahata et al., 2001). Further Thermotoga isolates from oil production wells were described as Thermotoga elfii (Africa; Ravot et al., 1995), Thermotoga subterranea (East Paris Basin; Jeanthon et al., 1995) and Thermotoga hypogea (Africa; Fardeau et al., 1997). Thermotoga-like organisms have been also reported from a continental oil reservoir (L’Haridon et al., 1995), and Thermotogales relatives were also found during a systematic survey in high temperature oil reservoirs with different salinities (Grassia et al., 1996).

Members of the genus Fervidobacterium seem to be restricted to biotopes with low salinity. Fervidobacterium nodosum was originally isolated from a hot spring in New Zealand (Patel et al., 1985a). Fervidobacterium islandicum was obtained from a continental solfatara field at Hveragerthi (Iceland; Huber et al., 1990) and further, so far undescribed Fervidobacterium relatives were isolated from continental hot springs in Tibet (Huber and Stetter, 1992a), from Nadi, Viti Levu (Fiji Island), and from Octopus Spring (Yellowstone National Park, United States). Fervidobacterium gondwanense was obtained from a sample taken from a runoff channel of the Great Artesian Basin of Australia (Andrews and Patel, 1996), Fervidobacterium pennivorans from a hot spring on San Miguel (the Azores, Portugal; Friedrich and Antranikian, 1996).

From marine hydrothermal springs and sandy sediments of the Gulf of Tadjoura southwest of Obock (Djibouti, Africa), three strains belonging to the genus Thermosipho have been isolated. They represented a single species, Thermosipho africanus (Huber et al., 1989a). Thermosipho melanesiensis, the second species within this genus, was obtained from the gills of Bathymodiolus brevior, a deep-sea hydrothermal vent mussel. This invertebrate was collected at the bottom of a black smoker in the Lau Basin (depth 1832 m), Southwest Pacific Ocean (Antoine et al., 1997). Recently, Thermosipho japonicus was isolated from a deep-sea hydrothermal vent in the Iheya Basin (Japan; Takai and Horikoshi, 2000a). Members of the genus Thermosipho thrive also in hot oil reservoirs. A so far undescribed, new Thermosipho species was obtained from reservoir fluids taken at the Thistle oil production platform (East Shetland Basin, North Sea; Stetter et al., 1993; Stetter and Huber, 2000), and recently, Thermosipho geolei from a continental petroleum reservoir in Western Siberia was published (Russia; L’Haridon et al., 2001).

From petroleum reservoirs located in Oklahoma and Texas, members of Geotoga and Petrotoga have been isolated for the first time (Davey et al., 1993a). The isolates have been described as Geotoga petraea, Geotoga subterranea and Petrotoga miotherma. Petrotoga mobilis was obtained from a North Sea oil production well (Lien et al., 1998) and two recent isolates were obtained from a continental petroleum reservoir in Western Siberia, namely Petrotoga olearia and Petrotoga sibirica (L’Haridon et al., 2002).

The type species of Marinitoga, Marinitoga camini, was isolated from a deep-sea hydrothermal chimney sample on the Mid-Atlantic Ridge (Wery et al., 2001) and Marinitoga piezophila was isolated from a chimney rock on the East-Pacific Rise (Alain et al., 2002).

Isolation

Members of the Thermotogales can be enriched in anaerobic culture media with a pH around 7 (Table 2) under a gas phase consisting of 300 kPa N₂ or 300 kPa N₂/CO₂ (80:20; vol/vol). Depending on the salt requirement of the organism, either half strength seawater media or media with low ionic strength are used (Table 2). The media, supplemented with a defined carbon source (e.g., maltose or starch; Table 3) and complex organic material (e.g., yeast extract or peptone; Table 3), are inoculated with original sampling material. Members of the T. maritima –T. neapolitana group can be selectively enriched at an incubation temperature of 85°C with starch as the only carbon and energy source. Keratinophilic (feather-degrading) Fervidobacterium species might be obtained in yeast extract/tryptone medium with native feathers from chickens, ducks or geese (Leuschner and Antranikian, 1994). For enrichment of G. petraea, G. subterranea and P. miotherma, alpha-glycerophosphate and starch should be added to petroleum reservoir brines (Davey et al., 1993a). Marinitoga piezophila can be selectively enriched heterotrophically under a hydrostatic pressure of 30 MPa (Alain et al., 2002).

Table 2. Physiological properties of the described species within the Thermotogales.

Table 3. Substrate specificity of the Thermotogales type strains.

The enrichment bottles are incubated at the appropriate temperatures (45–85°C; Table 2) and monitored for growth over a certain time period (e.g., two weeks) by phase contrast microscopy. When rod-shaped cells with an outer sheath-like structure or with a single terminal bleb become visible, the enrichments are serially diluted. The isolates are obtained by plating under anaerobic conditions and at an incubation temperature between 45 and 80°C (Table 2). For plating, a stainless steel anaerobic jar (Balch et al., 1979) and plates, solidified with Gelrite (0.7–1.0%; Huber and Stetter, 2001a) or agar (0.8–3%), can be used. Other methods for isolation are agar shake tubes (2%) or phytagel roll tubes (4%). For T. neapolitana, mean plating efficiencies up to 84% on 0.7% Gelrite have been reported. For growth, the plates were incubated at 77°C in a glass canning jar containing a packet of palladium pellets as a catalyst (Childers et al., 1992; Vargas and Noll, 1994). Colonies of the Thermotogales are uniformly round with a diameter of about 1–2 mm. Colonies of the genera Thermotoga, Fervidobacterium, Geotoga and Petrotoga appear whitish, while colonies of Thermosipho are colorless or brownish.

From liquid cultures, Thermotogales relatives can be also isolated by a plating-independent, newly developed isolation procedure (selected cell cultivation technique) based on optical trapping of single cells using a strongly focused infrared laser beam (“optical tweezers”; Huber et al., 1995; Beck and Huber, 1997; Huber, 1999; Huber and Stetter, 2001a).

In the future, a new isolation strategy might be used to isolate specifically Thermotogales relatives from their biotopes (Huber et al., 1995; Huber et al., 2000). This method combines 16S rRNA gene sequence analysis in situ, specific whole-cell hybridization within enrichments, and the cultivation of cells selected by the use of the laser microscope (Huber et al., 1995).

Identification

Cells of Thermotogales representatives are rod-shaped, about 1.0–50.0 µm long and 0.4–1.0 µm in diameter. They form an outer sheath, ballooning over the ends (‘toga’), visible in all phases of growth (Figs. 4 and 5). Usually, they grow singly and in pairs (Fig. 4); members of the genus Thermosipho form short chains of up to 12 individuals surrounded by a sheath (Fig. 6). The majority of the Fervidobacterium cells form a characteristic terminal bleb (‘spheroids’) on one end of the cells, which occurs during all stages of growth (diameter 1.0–4.0 µm; Fig. 7). Furthermore, growth in short chains is observed, and F. islandicum frequently forms aggregates of up to 50 cells. Besides spheroids, Fervidobacterium species form spheres (diameter 5.0–8.0 µm; Fig. 8), membrane-bound structures containing one to seven individual cells. All members of the Thermotogales exhibit a Gram-negative staining reaction, but diamino-pimelic acid, typical for Gram-negative bacteria, is absent.

Fig. 4.

Platinum-shadowed, flagellated cell of Thermotoga maritima. Bar, 1 µm.

Fig. 5.

Ultrathin section of Thermotoga maritima. Bar, 1 µm.

Fig. 6.

Four rod-shaped cells of Thermosipho africanus in a tube-like sheath; platinum-shadowed. Bar, 1 µm.

Fig. 7.

Ultrathin section of Fervidobacterium islandicum. Bar, 1 µm.

Fig. 8.

Ultrathin section of Fervidobacterium nodosum. Single sphere, containing several cells. Bar, 1 µm. (Courtesy of H. W. Morgan.)

Thermotogales species either are immotile or possess flagella and are motile (Huber and Stetter, 1999; Huber and Stetter, 2001b; Fig. 4). Motility of T. maritima was observed at 60–90°C after heating the microscopic slide. At the optimal growth temperature of 80°C, the maximum speed was approximately 3 mm/min. Furthermore, a thermotactic response to temporal temperature changes was reported for T. maritima and thermostable chemotaxis proteins have been identified (Gluch et al., 1995; Swanson et al., 1996). Recently, the crystal structure of CheA, a signal-transducing histidine kinase, and the crystal structures of the middle and C-terminal domain of the flagellar rotor protein FliG from T. maritima have been published (Bilwes et al., 1999; Brown et al., 2002).

So far, there is only one report of film formation within the Thermotogales. During continuous cultivation of T. maritima, exopolysaccharides were produced under optimal growth conditions. Concomitantly, a thin and white, but not further specified film below the liquid level of the medium was observed (Rinker and Kelly, 2000). Very recently, we found biofilm formation by Thermotoga sp. SG1-L2A, produced under physiological growth conditions in batch cultures at the end of the logarithmic growth phase. The cells formed free-floating, white-colored biofilms in the medium with a length of up to 1 cm (Fig. 9).

Fig. 9.

Free-floating biofilm of Thermotoga sp. SG1-L2A. Bar, 0.5 cm.

The ultrastructure of T. maritima (Huber et al., 1986) and related species, as determined by transmission electron microscopy by freeze-etching and in ultrathin sections, can only be interpreted as Thermotoga being a true Gram-negative bacterium (Rachel et al., 1990). A sheath-like outer membrane and overballooning ends give the cells their characteristic appearance. The porin of the outer membrane has been purified and its specific functional properties characterized after insertion into artificial lipid bilayers (Engel et al., 1993); in addition, its secondary structure contains predominantly beta-sheets and the amino acid composition is characteristic for porins (for a review, see Schirmer, 1998). Outer membrane protein (Omp) alpha is a rod-shaped spacer that spans the periplasm, connecting the outer membrane to the inner body (Engel et al., 1992; Lupas et al., 1995). So far, it is not clear how Omp alpha is connected to the peptidoglycan or the cytoplasmic membrane. It is likely that the COOH-terminal hydrophobic tail of the protein is associated with the porin layer in the “balloon” portion of the cell wall. Thermotoga cells have an unusual thin and labile murein layer, presumably owing to a low degree of cross-linking; it is not synthesized in the presence of penicillin G or ampicillin and is sensitive to lysozyme. After addition of lysozyme, the rod-shaped cells of T. maritima round up within some minutes, which was also observed for Ts. africanus and F. islandicum. The murein of T. maritima consists of muramic acid, N-acetylglucosamine, glutamic acid, alanine and lysine (molar ratio = 0.41:0.69:1.00:1.43:0.89). D- and L-Lysine are present and not found so far in Gram-negative bacteria.

Recently, the crystal structure of the cell division protein FtsA and of the cell division inhibitor MinC was reported from T. maritima (van den Ent and Löwe, 2000; Cordell et al., 2001). Biochemical and structural investigations gave evidence that the T. maritima MreB protein is the bacterial homolog to the physiological polymer of the eucaryotic F-actin (van den Ent et al., 2001).

Electron microscopic studies showed that the cell envelope of Fervidobacterium is composed of two layers. The outer layer protrudes to form spheres (Patel et al., 1985a). In F. nodosum, the two layers have an irregular convoluted structure and are connected with regular junctions, and the outer layer is susceptible to lysis by SDS. The cells are able to grow and multiply within the spheres (Patel et al., 1985a).

About 50% of the total polar lipids of T. maritima are two amphipathic monopolar glycolipids with a very rare alpha-(1-4) diglucosyl structure (Manca et al., 1992). Unusual long-chain dicarboxylic fatty acids are present in the core lipids of the Thermotogales (Huber et al., 1986; Huber et al., 1989a; Huber et al., 1990; Jeanthon et al., 1995). In T. maritima, a total of 37 different fatty acids, including the novel 13,14-dimethyloctacosanedioic acid (Carballeira et al., 1997) and a new ether core lipid, 15,16-dimethyl-30-glyceroloxytriacontanoic acid (DeRosa et al., 1988), have been identified. Comparative analysis showed that this core lipid is characteristic for the genus Thermotoga (Huber et al., 1986; DeRosa et al., 1988; Windberger et al., 1989; Jeanthon et al., 1995; Fig. 10).

Fig. 10.

The new 15,16-dimethyl-30 glyceryloxytriacontanoic acid from Thermotoga maritima (courtesy A. Gambacorta).

Members of the Thermotogales differ in their sensitivity to antibiotics (Table 4). For T. elfii and F. pennivorans, no information on antibiotic sensitivity was reported. Thermotogales representatives are significantly different in their sensitivity to the antibiotic rifampicin, which blocks transcription initiation (Huber and Stetter, 1992a; Table 4). Growth of some species is inhibited by only 1 µg/ml (e.g., T. thermarum), while other species (e.g., T. maritima and T. neapolitana) are resistant even against 100 µg/ml. Unusual for bacteria, the purified RNA polymerase of T. maritima is resistant to rifampicin (1 µg/ml), and only 80% of its activity is inhibited by 200 µg/ml (Huber et al., 1986). Growth of T. maritima is also not inhibited by 10 µg/ml of the aminoglycoside antibiotics paromomycin, neomycin, streptomycin, gentamicin and kanamycin (Huber et al., 1986). Unprecedented for bacteria, the purified ribosomes of T. maritima are resistant to these antibiotics, too (Londei et al., 1988). Another unusual feature of T. maritima is the resistance of the elongation factor EF-G to fusidic acid (Huber and Stetter, 1992a).

Table 4. Influence of antibiotics on growth of the Thermotogales species.

Cultivation

Owing to the anaerobic nature of the Thermotogales, anaerobic media must be prepared for their cultivation. First, the medium (e.g., 1 liter volume) is flushed with nitrogen for about 20 min to get rid of oxygen. Afterwards, the medium is chemically reduced by the addition of unsterile sodium sulfide and/or cysteine-HCl or after dispension in small portions (see Culture Media) from a sterile, anaerobic stock (see Culture Media). After pH-adjustment, the media are dispensed in 10–20 ml portions in an anaerobic chamber into the culture bottles. They are closed by rubber stoppers and the gas atmosphere is changed to the desired gas mixture (see Culture Media). Thermotogales relatives grow well using 28-ml serum tubes (borosilicate glass; Schott, Mainz, Germany) or 120-ml soda-lime-silicate bottles (Stute GmbH, Rheinbreitbach, Germany). All media were autoclaved for 20 min at 121°C, and the stock-solutions were filter-sterilized.

At large scale, members of the Thermotogales are routinely grown in batch cultures, using enamel-protected fermentors with an operating volume up to 300 liters (HTE, Bioengineering, Wald, Switzerland; Huber et al., 1986). The inhibitory hydrogen gas produced during growth should be removed by continuously flushing the fermentor with nitrogen or argon gas (Huber et al., 1986; Huber et al., 2000).

Culture Media

MSH-Medium for T. maritima and T. neapolitana

KH2PO4	0.5 g
(NH4)2SO4	0.5 g
NaHCO3	0.1 g
Yeast extract (BD DifcoTM)	0.5 g
Starch	5 g
Trace mineral solution (see 10 ml Trace Mineral Solution)
Resazurin	1 mg
Artificial sea water (see 250 ml Artificial Sea Water)
Na2S · 9H2O	0.5 g
H2O, double distilled	700.0 ml

Adjust pH to 7.0 with 25% H₂SO₄ and volume to 1 liter with double-distilled water. The headspace of the culture bottles consisted of N₂ (300 kPa).

Trace Mineral Solution (modified; Balch et al., 1979)

Nitrilotriacetic acid	1.5 g
MgSO4 · 7H2O	3.0 g
MnSO4 · H2O	0.5 g
NaCl	1.0 g
FeSO4 · 7H2O	0.1 g
CoSO4	0.1 g
CaCl2 · 2H2O	0.1 g
CuSO4 · 5H2O	0.01 g
AlK(SO4)2	0.01 g
H3BO3	0.01 g
ZnSO4	0.1 g
Na2WO4 · 2H2O	0.01 g
Na2MoO4 · 2H2O	0.01 g
Na2SeO4	0.01 g
(NH4)2Ni(SO4)2	0.2 g
H2O, double distilled	950.0 ml

Adjust pH of the solution to 6.5 with 5N KOH and volume to 1 liter with double-distilled water. Store at 4°C in the dark.

Artificial Sea Water (Huber et al., 1990)

NaCl	27.7 g
MgSO4 · 7H2O	7.0 g
MgCl2 · 6H2O	5.5 g
KCl	0.6 g
NaBr	0.1 g
H3BO3	0.03 g
SrCl2 · 6H2O	0.015 g
KI	0.05 mg
CaCl2 · 2H2O	1.5 g
H2O, double distilled	950.0 ml

Adjust pH to 5.5–6.0 and volume to 1 liter with double-distilled water.

LA3 Medium for T. thermarum

NaCl	3.46 g
MgSO4 · 7H2O	0.88 g
MgCl2 · 6H2O	0.69 g
KH2PO4	0.5 g
CaCl2	0.09 g
KCl	0.08 g
NaBr	12.50 mg
H3BO3	3.75 mg
SrCl2 · 6H2O	1.9 mg
KI	0.006 mg
(NH4)2Ni(SO4)2	3 mg
Trace mineral solution (see 15 ml Trace Mineral Solution)
EDTA · Na4-salt	0.9 g
Starch	5 g
Yeast extract (BD DifcoTM)	0.5 g
Resazurin	1 mg
Na2S · 9H2O	0.5 g
H2O, double distilled	950.0 ml

Adjust pH to 7.0 with 25% H₂SO₄ and volume to 1 liter with double-distilled water. The headspace of the culture bottles consisted of N₂ (300 kPa).

Trace Mineral Solution (Balch et al., 1979)

Nitrilotriacetic acid	1.5 g
MgSO4 · 7H2O	3.0 g
MnSO4 · 2H2O	0.5 g
NaCl	1.0 g
FeSO4 · 7H2O	0.1 g
CoCl2	0.1 g
CaCl2 · 2H2O	0.1 g
ZnSO4	0.1 g
CuSO4 · 5H2O	0.01 g
AlK(SO4)2	0.01 g
H3BO3	0.01 g
Na2MoO4 · 2H2O	0.01 g
H2O, double distilled	950.0 ml

Adjust pH of the solution to 6.5 with 5N KOH and the volume to 1 liter with double-distilled water. Store at 4°C in the dark.

MB Medium for T. elfii

NH4Cl	1.0 g
K2HPO4	0.3 g
KH2PO4	0.3 g
MgCl2 · 6H2O	0.2 g
CaCl2 · 2H2O	0.1 g
NaCl	10.0 g
KCl	0.1 g
Sodium acetate	0.5 g

Trace mineral solution (see 10 ml LA3-medium)
Yeast extract (BD DifcoTM)	2.0 g
Bio-Trypcase (bioMérieux)	2.0 g
Resazurin	1 mg
Cysteine · HCl	0.5 g
Na2S · 9H2O	0.4 g
Na2CO3	2.0 g
H2O, double distilled	950.0 ml

Dissolve all components except the Na₂S · 9H₂O and Na₂CO₃.Adjust pH to 8.0 with 10M KOH and volume to 1 liter with double-distilled water. Autoclave (121°C, 20 min) the medium, then add the Na₂S · 9H₂O and Na₂CO₃ (from sterile, anaerobic stocks). Store or incubate medium in a gas phase of N₂/CO₂ (80:20 v/v; 200 kPa).

Medium for T. subterranea

NaCl	12.0 g
MgSO4 · 7H2O	0.5 g
PIPES (piperazine-N, 3.4 g N′-bis[ethanesulfonic acid])
KCl	2.0 g
NH4Cl	0.1 g
CaCl2 · 2H2O	25 mg
K2HPO4	20 mg
Trace mineral solution (see 10 ml LA3 medium)
Vitamin solution (see {Vitamin Solution}]Vitamin Solution)	10 ml
Yeast extract	0.5 g
Peptone	1.0 g
Resazurin	1 mg
Na2S · 9H2O	0.5 g
H2O, double distilled	950.0 ml

Add the Na₂S · 9H₂O from a sterile, anaerobic stock to the autoclaved (121°C, 20 min) solution of all other components. Adjust the pH to 7.0 with 25% H₂SO₄ and the volume to 1 liter with double-distilled water. Store or incubate medium in an atmosphere of N₂ (100 kPa).

Vitamin Solution (Balch et al., 1979)

Biotin	2 mg
Folic acid	2 mg
Pyridoxine · HCl	10 mg
Thiamine · HCl	5 mg
Riboflavin	5 mg
Nicotinic acid	5 mg
DL-Calcium pantothenate	5 mg
Vitamin B12	0.10 mg
p-Aminobenzoic acid	5 mg
Lipoic acid	5 mg
H2O, double distilled	950.0 ml

Adjust volume to 1 liter with double-distilled water, filter-sterilize, and store in the dark at 4°C.

Modified MB Medium for T. hypogea

NaCl	10.0 g
NH4Cl	1.0 g
K2HPO4	0.3 g
KH2PO4	0.3 g
MgCl2	0.5 g
CaCl2	0.1 g
KCl	0.2 g
Trace mineral solution (see 10 ml LA3 Medium)

Yeast extract (BD DifcoTM)	2.0 g
Bio-Trypcase (bioMérieux)	2.0 g
Xylan	10.0 g
Resazurin	1 mg
Na2CO3	2.0 g
Na2S2O3 · 5H2O	20 mM
Cysteine · HCl	0.5 g
Na2S · 9H2O	0.4 g
H2O, double distilled	950.0 ml

Add the Na₂CO₃, Na₂S₂O₃ · 5H₂O, and Na₂S · 9H₂O (from sterile, anaerobic stocks) to the autoclaved (121°C, 20 min) solution of the remaining components. Adjust pH to 8.0 with 10M KOH and the volume to 1 liter with double-distilled water. Store or incubate medium in an atmosphere of N₂/CO₂ (80:20 v/v; 100 kPa).

Modified DSMZ Medium No. 664 for T. lettingae

NH4Cl	1.0 g
K2HPO4	0.3 g
KH2PO4	0.3 g
MgCl2 · 6H2O	0.2 g
CaCl2 · 2H2O	0.1 g
KCl	0.1 g
NaCl	10.0 g
Na2CO3	2.0 g
Na2S2O3 · 5H2O	5.0 g
Yeast extract	0.5 g
Glucose	4.0 g
Trace mineral solution (see 10.0 ml Trace Mineral Solution (DSMZ Medium 141))
Resazurin	0.5 mg
Cysteine · HCl	0.5 g
Na2S · 9H2O	0.4 g
NaHCO3	2.0 g
H2O, double distilled	950.0 ml

Add the Na₂CO₃, Na₂S₂O₃ · 5H₂O, glucose, Na₂S · 9H₂O, and NaHCO₃ (from a sterile, anaerobic stocks) to an autoclaved (121°C, 20 min) solution of the remaining components. Adjust the pH to 7.0 with 10M KOH and the volume to 1 liter with double-distilled water. Store or incubate medium in an atmosphere of N₂/CO₂ (80:20 v/v; 100 kPa).

Trace Mineral Solution (DSMZ Medium 141)

Nitrilotriacetic acid	1.5 g
MgSO4 · 7H2O	3.0 g
MnSO4 · 2H2O	0.5 g
NaCl	1.0 g
FeSO4 · 7H2O	0.1 g
CoSO4 · 7H2O	0.18 g
CaCl2 · 2H2O	0.1 g
ZnSO4 · 7H2O	0.18 g
CuSO4 · 5H2O	0.01 g
AlK(SO4)2 · 12H2O	0.02 g
H3BO3	0.01 g
Na2MoO4 · 2H2O	0.01 g
NiCl2 · 6H2O	0.025 g
Na2SeO3 · 5H2O	0.3 g
H2O, double distilled	950.0 ml

Prior to adding nitrilotriacetic acid to the other solution components, adjust its pH to 6.5 with 5N KOH. Adjust pH to 7.0 with 10 M KOH and volume to 1 liter with double-distilled water.

YE Medium for T. petrophila and T. naphthophila

Yeast extract	2.0 g
Na2S · 9H2O	0.4 mM
Artificial sea water (ASW; see 950.0 ml Artificial Sea Water)

Add the Na₂S · 9H₂O (from a sterile, anaerobic stock) to an autoclaved (121°C, 20 min) solution of the remaining components. Adjust pH to 7.0 with 6M HCl and the volume to 1 liter with artificial sea water. Store or incubate medium in an atmosphere of N₂ (100kPa).

Artificial Sea Water (ASW; Jannasch et al., 1995)

NaCl	20.0 g
MgCl2 · 6H2O	3.0 g
MgSO4 · 7H2O	6.0 g
(NH4)2SO4	1.0 g
CaCl2 · 2H2O	0.3 g
KH2PO4	0.2 g
KCl	0.5 g
NaBr	0.05 g
H3BO3	0.025 g
SrCl2 · 6H2O	0.02 g
Ferric ammonium citrate	0.01 g
Bis-tris-propane	2.25 g
Trace mineral solution (see 10 ml Trace Mineral Solution)
Vitamin solution (see Vitamin 10 ml Solution)
Resazurin	0.6 mg
H2O, double distilled	950.0 ml

Adjust volume to 1 liter with double-distilled water.

Trace Mineral Solution (Wolin et al., 1963)

Nitrilotriacetic acid	1.5 g
MgSO4	3.0 g
MnSO4	0.5 g
NaCl	1.0 g
FeSO4	0.1 g
CoCl2	0.1 g
CaCl2	0.1 g
ZnSO4	0.1 g
CuSO4	0.01 g
AlK(SO4)2	0.01 g
H3BO3	0.01 g
Na2MoO4	0.01 g
H2O, double distilled	950.0 ml

Adjust volume to 1 liter with double-distilled water.

Vitamin Solution (Bazylinski et al., 1989)

Niacin	10 mg
Calcium pantothenate	10 mg
p-Aminobenzoic acid	10 mg
Thiamine	10 mg
Riboflavin	10 mg
Pyridoxine	10 mg
Cobalamin	10 mg
Thioctic (alpha-lipoic) acid	10 mg
Folic acid	4 mg
Biotin	4 mg
H2O, double distilled	950.0 ml

Filter sterilize and store solution in the dark at 4°C.

MG Medium for Ts. africanus

NaCl	18.0 g
MgSO4 · 7H2O	3.45 g
MgCl2 · 6H2O	4.30 g
KCl	0.34 g
NH4Cl	0.25 g
CaCl2 · 2H2O	0.14 g
K2HPO4 · 3H2O	0.14 g
(NH4)2Fe(SO4)2 (0.2%)	1.0 ml
(NH4)2Ni(SO4)2 (0.2%)	1.0 ml
Sodium acetate · 3H2O	1.0 g
Trace mineral solution (see: MSH Medium)	10 ml
Vitamin solution (see: T. subterranea Medium)	10 ml
Yeast extract (BD DifcoTM)	2.0 g
Peptone (pancreatic digest of casein; Merck) (pankreatisch verdaut verwendet)	2.0 g
Resazurin (0.1%)	0.5 ml
Na2S · 9H2O	0.5 g
H2O, double distilled	950.0 ml

Add the trace mineral solution (from a sterile, anaerobic stock) to an autoclaved (121°C, 20 min) solution of the remaining components.Adjust pH to 6.5 with 25% formic acid and volume to 1 liter with double-distilled water. Store or incubate medium in an atmosphere of N₂ (300 kPa).

DSMZ Medium No. 343 for Ts. melanesiensis

NaCl	20.0 g
KH2PO4	0.5 g
NiCl2 · 6H2O	2.0 mg
Trace mineral solution (see 15.0 ml T. lettingae Medium)
Yeast extract (BD DifcoTM)	0.5 g
Starch	5.0 g
Resazurin	1.0 mg
Na2S · 9H2O	0.5 g
Artificial sea water (see: MSH 250 ml Medium)
H2O, double distilled	700.0 ml

Adjust pH to 6.5 with 25% formic acid and volume to 1 liter with double-distilled water. Store or incubate medium in an atmosphere of N₂ (100 kPa).

MJDYP Medium for Ts. japonicus

Yeast extract	2.0 g
Trypticase peptone	2.0 g
Resazurin	1.0 mg
Na2S · 9H2O	0.5 g
MJD synthetic water (see 1000.0 ml MJD Synthetic Water)

Adjust pH to 7.5 with 6N HCl. Store or incubate medium in an atmosphere of N₂ (200 kPa).

MJD Synthetic Water

NaCl	30.0 g
K2HPO4	0.14 g
CaCl2 · 2H2O	0.14 g
MgSO4 · 7H2O	3.4 g
MgCl2 · 6H2O	4.18 g
KCl	0.33 g
NiCl2 · 6H2O	0.5 mg

Na2SeO3 · 5H2O	0.5 mg
Fe(NH4)2(SO4)2 · 6H2O	0.01 g
Trace mineral solution (see 10 ml LA3 Medium)
DHV mineral solution (see 1 ml DHV Mineral Solution)
H2O, double distilled	950.0 ml

Adjust volume to 1 liter with double-distilled water.

DHV Mineral Solution

Na2SiO3 · 9H2O	2.3 g
SrCl2 · 6H2O	1.1 g
CoSO4 · 7H2O	1.1 g
Na2MoO4 · 2H2O	0.97 g
MnSO4 · 2H2O	0.96 g
NiCl2 · 6H2O	0.95 g
Na2WO4 · 2H2O	0.66 g
ZnSO4 · 7H2O	0.58 g
CuSO4 · 5H2O	0.50 g
VSO4 · xH2O	0.25 g
Na2SeO3	0.17 g
LiSO4 · H2O	0.13 g
H2O, double distilled	950.0 ml

Adjust volume to 1 liter with double-distilled water.

Medium for Ts. geolei

NaCl	15.0 g
MgCl2 · 6H2O	0.5 g
PIPES	3.4 g
KCl	0.2 g
NH4Cl	1.0 g
CaCl2 · 2H2O	0.1 g
K2HPO4	0.35 g
KH2PO4	0.35 g
Sodium acetate · 3H2O	2.72 g
Yeast extract	2.0 g
Bio-Trypcase (bioMérieux)	2.0 g
Resazurin	1.0 mg
Maltose	2.0 g
Na2S · 9H2O	1.0 g
H2O, double distilled	950.0 ml

Add the maltose and Na₂S · 9H₂O (from sterile, anaerobic stocks) to an autoclaved (121°C, 20 min) solution of the remaining components. Adjust pH to 7.0 with 5M HCl and volume to 1 liter with double-distilled water. Store or incubate medium in an atmosphere of N₂ (100 kPa).

TYEG Medium for F. nodosum, F. islandicum and F. gondwanense

NH4Cl	0.9 g
MgCl2 · 6H2O	0.2 g
KH2PO4	0.75 g
K2HPO4	1.5 g
FeSO4 · 7H2O (10%)	0.03 ml
Trace mineral solution (see 9.0 ml Trace Mineral Solution)
Vitamin solution (see 5.0 ml T. subterranea Medium)
Yeast extract (BD DifcoTM)	3.0 g
Peptone (from tryptically digested 10.0 g meat; Merck)
Glucose	5.0 g
Resazurin (0.2%)	1.0 ml
Na2S · 9H2O	1.0 g
H2O, double distilled	950.0 ml

Add the vitamin solution and glucose (from a sterile, anaerobic stock) to an autoclaved (121°C, 20 min) solution of the remaining components. Adjust pH to 7.2 with 25% H₂SO₄ and the volume up to 1 liter with double-distilled water. Store or incubate medium in an atmosphere of N₂ (300 kPa).

Trace Mineral Solution (Patel et al., 1985a)

Nitrilotriacetic acid	12.5 g
FeCl3 · 4H2O	0.2 g
MnCl2 · 4H2O	0.1 g
CoCl2 · 6H2O	0.017 g
CaCl2 · 2H2O	0.1 g
ZnCl2	0.1 g
CuCl2	0.02 g
H3BO3	0.01 g
Na2MoO4 · 2H2O	0.01 g
NaCl	1.0 g
Na2SeO3	0.02 g
H2O, double distilled	950.0 ml

Prior to adding to the trace mineral solution, adjust pH of the nitrilotriacetic acid to 6.5 with 5N KOH. Adjust pH of the trace mineral solution to 6.5 with 5N KOH and the volume of the trace mineral solution to 1 liter with double-distilled water. Store medium at 4°C in the dark.

TF Medium for F. pennivorans

NH4Cl	0.50 g
MgSO4 · 7H2O	0.16 g
K2HPO4	1.6 g
NaH2PO4 · H2O	1.0 g
CaCl2 · 2H2O	0.06 g
Trace mineral solution (see: 10 ml LA3 Medium)
Vitamin solution (see 10 ml T. subterranea Medium)
Yeast extract	2.0 g
Trypticase	2.0 g
Resazurin	1 mg
Na2S · 9H2O	0.3 g
Cysteine · HCl	0.3 g
Glucose or starch	3.0 g
H2O, double distilled	950.0 ml

Add the CaCl₂ · 2H₂O (from a sterile, anaerobic stock) to an autoclaved (121°C, 20 min) solution of the remaining components. Adjust pH to 6.8 with 10N KOH and the volume up to 1 liter with double-distilled water. Store or incubate medium in an atmosphere of N₂.

MSH Medium for P. miotherma, G. petraea and G. subterranea

See MSH Medium for T. maritima and T. neapolitana

Medium for P. mobilis

NaCl	30.0 g
MgSO4 · 7H2O	7.0 g
KCl	0.34 g
NH4Cl	0.25 g
CaCl2 · 2H2O	0.14 g
KH2PO4	0.14 g
Yeast extract (BD DifcoTM)*	0.2 g
Trace element solution SL-10 (see 1.0 ml Trace Element Solution SL-10)
Vitamin solution (see 10 ml T. subterranea Medium)*

Resazurin (0.02%)	0.5 ml
Na2S · 9H2O (0.5 M)	4 ml
H2O, double distilled	950.0 ml

Add the yeast extract and the vitamin solution (from sterile, anaerobic stock) to an autoclaved (121°C, 20 min) solution of the remaining components. Adjust pH to 6.5 with 6M HCl and the volume up to 1 liter with double-distilled water. Store or incubate medium in an atmosphere of argon (200 kPa).

Trace Element Solution SL-10 (Widdel et al., 1983)

HCl (25% w/w, 7.7 M)	10 ml
FeCl2 · 4H2O	1.5 g
CoCl2 · 6H2O	190 mg
MnCl2 · 4H2O	100 mg
ZnCl2	70 mg
H3BO3	6 mg
Na2MoO4 · 2H2O	36 mg
NiCl2 · 6H2O	24 mg
CuCl2 · 2H2O	2 mg
H2O, double distilled	950.0 ml

Adjust volume up to 1 liter with double-distilled water.

Medium for P. olearia and P. sibirica

NaCl	25.0 g
MgCl2 · 6H2O	0.5 g
PIPES	3.4 g
KCl	0.2 g
NH4Cl	1.0 g
CaCl2 · 2H2O	0.1 g
K2HPO4	0.35 g
KH2PO4	0.35 g
Peptone	1.5 g
Tryptone	1.5 g
Yeast extract	1.5 g
Maltose	2.0 g
Resazurin	1.0 mg
Na2S · 9H2O	1.0 g
H2O, double distilled	950.0 ml

Add the maltose and Na₂S · 9H₂O (from sterile, anaerobic stocks) to an autoclaved (121°C, 20 min) solution of the remaining components. Adjust pH to 7.0 with 5M HCl and the volume up to 1 liter with double-distilled water. Store or incubate medium in an atmosphere of N₂ (100 kPa).

YPCS Medium for M. camini

Yeast extract (BD DifcoTM)	0.5 g
Peptone (BD DifcoTM)	1.0 g
Cellobiose (Sigma-Aldrich)	5.0 g
Sea salt (Sigma-Aldrich)	30.0 g
PIPES buffer (Sigma-Aldrich)	6.05 g
Resazurin	1.0 mg
Na2S · 9H2O	0.5 g
H2O, double distilled	950.0 ml

Adjust pH to 7.5 with 5M HCl and the volume up to 1 liter with double-distilled water. Store or incubate medium in an atmosphere of N₂/H₂/CO₂ (90:5:5 v/v/v, 100 kPa).

RC+ Medium for M. piezophila

NaCl	30.0 g
MES (2-[N-morpholino]ethanesulfonic 10 mM acid); Sigma-Aldrich)
NH4Cl	1.0 g

K2HPO4	0.3 g
MgCl2 · 6H2O	0.2 g
CaCl2 · 2H2O	0.1 g
KCl	0.1 g
Sodium acetate · 3H2O	0.83 g
Yeast extract (BD DifcoTM)	5.0 g
Bio-Trypcase (BD DifcoTM)	5.0 g
Maltose (Sigma-Aldrich)	20 mM
L-Cystine	12.0 g
Resazurin	1.0 mg
Na2S · 9H2O	0.5 g
H2O, double distilled	950.0 ml

Add the L-cystine and Na₂S · 9H₂O (from a sterile, anaerobic stocks) to an autoclaved (121°C, 20 min) solution of the remaining components. Adjust pH to 6.0 with 5M HCl and the volume up to 1 liter with double-distilled water. Store or incubate medium in an atmosphere of N₂/H₂/CO₂ (90:5:5 v/v/v, 100 kPa up to 40 MPa).

Preservation

Under anaerobic conditions, members of the Thermotogales can be stored for several months at 4°C. For long-term preservation, storage at −140°C in liquid nitrogen in the presence of 5% dimethylsulfoxide is recommended (Malik, 1999). No loss of cell viability of Thermotoga, Thermosipho and Fervidobacterium was observed after storage over a period of more than ten years. Thermotoga subterranea was stored at –80°C in the same medium containing 20% (w/v) glycerol (Jeanthon et al., 1995).

Physiology

Members of the Thermotogales are strictly anaerobic, extremely thermophilic or hyperthermophilic bacteria with optimal growth in the neutral pH range (Huber and Stetter, 1999; Table 2). With an optimal growth temperature around 80°C and a maximum growth temperature of 90°C, the hyperthermophilic species T. maritima and T. neaplitana represent the organisms with the highest growth temperatures within the order. Growth of some Thermotogales species is restricted to low salinity, while other species exhibit a broad salt tolerance, reflecting their adaptation to the natural biotope (Huber and Stetter, 1992a; Huber and Stetter, 1999; Table 2). Detailed physiological properties of the Thermotogales are listed in Table 2.

All members of the Thermotogales are strict organotrophs, fermenting preferentially simple and complex carbohydrates or complex organic matter (Table 3). When grown on defined carbon sources, the addition of yeast extract and/or peptone/bio-trypticase to the medium is often stimulatory or required for growth (Table 3; see also Culture Media). In addition, cell homogenates of bacteria (e.g., Lactobacillus bavaricus) and archaea (e.g., Pyrodictium brockii) can be used as substrates (Huber et al., 1986). Furthermore, T. lettingae is able to degrade methanol, but only in the presence of yeast extract, which is required for growth (Balk et al., 2002).

With glucose as growth substrate, Thermotogales representatives form L(+)-lactate, acetate, ethanol, L-alanine, carbon dioxide, and hydrogen as major final products. Some of the isolates form traces of isovaleric acid, isobutyric acid, alpha-aminobutyrate, hydroxy-phenylacetate and phenylacetate in addition. The L-alanine production of members of the Thermotogales is a trait in common with members of the archaeal order Thermococcales. Therefore, it has been proposed that L-alanine production from sugar fermentation is a remnant of an ancestral metabolism (Ravot et al., 1996a). The pathway of arginine synthesis is the only biosynthetic reaction studied in the Thermotogales so far. Thermotoga maritima generates arginine from glutamate via N-acetylated intermediates in an eight-step pathway, as in mesophilic bacteria (Van de Casteele et al., 1990).

Thermotoga maritima degrades glucose mainly via the Embden-Meyerhof glycolytic pathway and, to a lesser extent, via the Entner-Doudoroff pathway (Schröder et al., 1994; Selig et al., 1997). In T. neapolitana, it was shown that D-glucose is taken up via an active transport system, energized by an ion gradient. This gradient is generated by ATP, derived from substrate-level phosphorylation (Galperin et al., 1996). The role of this gradient is not known since glucose (and maltose) periplasmic binding proteins appear to be involved in transport (Nanavati et al., 2002).

Hydrogen, which accumulates during fermentation processes, can be a potent inhibitor of growth of the Thermotogales. This inhibition can be overcome by the addition of sulfur or inorganic sulfur-containing compounds, depending on the species. Under these culture conditions, H₂S is formed as final product. Therefore, H₂S formation may be a kind of detoxification reaction of H₂ (Huber et al., 1986; Huber et al., 1992a). A general trait of different Thermotogales is the production of H₂S when grown in the presence of thiosulfate, except for Ts. geolei, M. piezophila, P. olearia and P. sibirica (Ravot et al., 1995; L’Haridon et al., 2001; L’Haridon et al., 2002; Alain et al., 2002). In addition to removal by sulfurous compounds, hydrogen can be removed by gassing the media with nitrogen or argon (Huber et al., 1986; Huber and Stetter, 1999) or by interspecies hydrogen transfer during cocultivation of the Thermotogales with hydrogen-consuming hyperthermophiles (e.g., Methanococcus, Methanopyrus, Archaeoglobus or Ferroglobus; Huber et al., 2000).

Recently, respiratory growth of T. maritima had been reported, with hydrogen as electron donor and Fe(III) as the electron acceptor, forming Fe(II) as final product (Vargas et al., 1998). In contrast, growth of T. maritima with similar doubling times and final cell densities, in the absence of both hydrogen and Fe(III) in the same culture medium, was observed (Huber and Stetter, 2001a). Furthermore, when T. maritima was cultivated in the presence of hydrogen (100 kPa) and Fe(III), Fe(II) was formed without obvious growth stimulation (Huber and Stetter, 2001a). These results indicate that T. maritima may not gain energy by iron-respiration but may use Fe(III) in place of sulfur as an additional electron sink to get rid of inhibitory hydrogen during fermentation (Huber et al., 1986; Schröder et al., 1994).

Different low-molecular-weight organic compounds, serving as compatible solutes, are present in the different members of the Thermotogales (Martins et al., 1996; Ramakrishnan et al., 1997). However, the accumulation of solutes in response to either salt stress or supraoptimal temperatures is restricted to Thermotogales species that grow in saline media containing over 0.4% sodium chloride (Martins et al., 1996).

An enormous variety of enzymes (and proteins) from the order Thermotogales, especially T. maritima, has been isolated and intensively studied since the late 1980s. In general, these enzymes exhibit a high thermostability, with denaturation temperatures often above the optimum growth temperature of the bacterium. Despite the strict anaerobic nature of T. maritima, these enzymes are apparently not oxygen sensitive. A comprehensive overview of the biochemistry and biophysics of these “hyperthermophilic enzymes and proteins” was published recently in volumes 330, 331 and 334 of Methods in Enzymology (2001).

Genetics

So far, the only plasmids discovered in the Thermotogales are in Thermotoga sp. RQ7, an isolate obtained from the hot sea-floor of Ribeira Quente (the Azores), and in Thermotoga maritima MC24, an isolate from hydrothermal vents near the Kuril Islands (Huber et al., 1986; Harriott et al., 1994; Akimkina et al., 1999). These small cryptic miniplasmids, pRQ7 and pMC24, are very closely related and were shown to be the smallest natural replicon so far described (846 bp). Both were shown to be negatively supercoiled (Harriott et al., 1994; Akimkina et al., 1999). The plasmid pRQ7 replicates by the rolling-circle mechanism (Yu and Noll, 1997). With pRQ7, a vector system was developed and a genetic transformation of T. maritima and T. neapolitana spheroplasts was achieved using cationic liposomes (Yu et al., 2001).

For genetic studies, auxotrophic and antimetabolite-resistant mutants of T. neapolitana have been isolated by the use of mutagenic agents (Vargas and Noll, 1994).

The rather small, 1.8-Mb genome from T. maritima has been sequenced at The Institute for Genomic Research (TIGR; Rockville, MD, USA; Nelson et al., 1999). It is a single circular chromosome and contains 1877 predicted cod-ing regions. Fifty-four percent of these coding regions have functional assignments, while 46% are of unknown function (Nelson et al., 1999; Nelson et al., 2001). Genome analysis has identified metabolic pathways involved in the degradation of sugar and plant polysaccharides. About 7% of the predicted codon sequences in the genome are involved in the metabolism of simple and complex sugars, a percentage more than twice that seen in the genomes of other bacteria or archaea (Nelson et al., 2001). The metabolism and transport in T. maritima (Fig. 11) were elucidated from the genome sequence data.

Fig. 11.

Overview of metabolism and transport in Thermotoga maritima. Pathways for energy production and the metabolism of organic compounds, acids and aldehydes are shown. (From Nelson et al., 1999, with permission from Nature [{http://www.nature.com}].)

Comparison of total genome sequences of different organisms showed that about 24% of the T. maritima genes are of archaeal origin (Nelson et al., 1999). Therefore, the authors have claimed that hyperthermophilic archaea and bacteria have often exchanged genes, particularly by lateral gene transfer (Aravind et al., 1998; Nelson et al., 1999; see chapter on Aquificales in this Volume). On the other hand, it was argued that if Thermotoga (and Aquifex; see chapter on Aquificales in this Volume) are truly deep branching, it is equally parsimonious to suppose that some of the genes are primitive features shared by these hyperthermophilic bacteria with archaea. They have retained ancestral genes that have since been lost in most bacteria as they adapted to mesophilic environments (Kyrpides and Olson, 1999; Logsdon and Faguy, 1999). On the basis of recent phylogenetic analyses of archaeal genes in different bacteria, it was hypothesized that lateral gene transfer in hyperthermophiles may be as much the consequence as the cause of adaptation to hyperthermophily (Nesbø et al., 2001). By using suppressive subtractive hybridization, extensive genomic diversity in T. maritima was reported recently and frequent lateral gene transfer within the order Thermotogales was suggested (Nesbø et al., 2002).

Very recently, a high-throughput structural genomics pipeline and its application to the proteome of T. maritima were designed and implemented. By using this pipeline, 1376 of the predicted 1877 genes (73%) were successfully cloned, their expression was attempted, and the crystallization conditions for 432 proteins (23% of the T. maritima proteome) were identified (Lesley et al., 2002).

Biotechnology

Enzymes active at high temperatures are sought for industrial processes. Intra- and extracellular enzymes of Thermotogales are highly thermostable and might be of great interest as biocatalysts, e.g., in the chemical or food industry. A recombinant xylanase from T. maritima was shown to be active for several hours at 100°C and efficient in releasing lignin from kraft pulp (Chen et al., 1997). Therefore, such xylanases have a high potential for use in the pulp and paper industry. Highly thermostable amylases might be used in starch processing and high-temperature glucose isomerases in the production of corn syrup. From T. maritima, the recombinant Ultma^TM DNA polymerase with proofreading activities is commercially available. The 70-kDa enzyme is highly thermostable and has a half-life of 40–50 min at 97.5°C. In addition, the sequencing of the total genome of T. maritima makes available the use of its genes as tools for a variety of biotechnological applications (Nelson et al., 2001).

Acknowledgments

The authors thank K. O. Stetter for continuous support. Furthermore, we thank C. Rudolph and C. Moissl for phylogenetic analysis and R. Rachel for electron microscopy. Furthermore, we are grateful to K. M. Noll for critical reading of the manuscript.