Phylum Thermotogae (Thermotoga and relatives)

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tree

  • Phylum Thermotogae
    • Order Thermotogales
      • Family Thermotogaceae
        • Genus Fervidobacterium
        • Genus Geotoga
        • Genus Marinitoga
        • Genus Petrotoga
        • Genus Thermopallium
        • Genus Thermosipho
        • Genus Thermotoga

General characteristics of the Thermotogae

Diversity

The less than 3 dozen described species in a single Family.

Metabolism

The members of this phylum are thermophilic (65°C to 90°C), anaerobic fermentative organisms that grow on a wide range or organic compounds, using protons or often elemental sulfur as the terminal electron acceptor. Ferric ion (Fe+++) can serve as an electron acceptor in some isolates. The products of the oxidation of organic material are typically CO2 and organic acids such as acetate and lactate. When grown in the absence of sulfur, hydrogen is the waste product of proton reduction, and hydrogen is a strong inhibitor of growth; hydrogen removal is required to maintain the energetic favorability if this reaction.

Morphology

The distinguishing feature of this phylum is the presence of a loose sheath, the “toga”, covering the rod-shaped cells. This sheath is typically snug over the sides of the cells but balloons out at each end of the cell. The toga is the outer membrane of an otherwise typical Gram-negative-type envelop, and so the space captured by the toga is conceptually periplasm. The toga is rich in porin-like proteins, arrayed in a regular pattern over the entire surface of the toga. The function of this unusual toga structure is unknown. Most members of this group are flagellated.

Habitat

These organisms have been isolated primarily from geothermally heated soils and sediments, including solfataras, the soil surrounding hot springs, and hot sediments in the vicinity of deep sea hydrothermal vents.

Example species

Thermotoga maritima

Thermotoga maritima
Thermotoga maritima : K. O. Stetter & R. Rachel, University of Regensburg

By far the best-studied member of this phylum is T. maritima, and for many years it was the only known member of the group. It was isolated from a heated submarine sediment off of the island of Vulcano, Italy. Cells are ca. 0.6 x 2-5μm rods with pronounced togas. It is an extreme thermophile, growing in a temperature range of 50-90°C, optimally at 80°C. This specie is motile via terminal flagella, but many related species of Thermotoga are non-motile. They are able to fix nitrogen (N2), but cannot use nitrate nor sulfur compounds other than elemental sulfur as terminal electron acceptors. T. maritima lacks genes for the electron transport chain, and so must generate its proton motive force (proton gradient) using either ATPase (run in the reverse of the usual reaction) or by a membrane-associated pyrophosphatase proton pump.

T. maritima is easy to grow, and the complete genome sequence was one of the earliest available from a thermophilic bacterium, following only that of Aquifex aeolicus, which is notoriously difficult to handle. Combined with the facts that proteins from extreme thermophiles often form high-quality crystals more readily than do those of mesophiles (perhaps because they are more rigid and so more uniform in structure than are the homologous proteins from mesophilies), and are usually much easier to express in recombinant form in E. coli than are the proteins of Archaea, make this organism an attractive source of proteins for structural examination by X-ray diffraction.

The genome sequence of T. maritima provided the best evidence available for the large-scale horizontal transfer of genes across large phylogenetic distances. It has been argued that up to 1/4th of the genome of T. maritima may have been acquired from an archaeal source recently enough that it can still be identified as foreign; others would revise this figure downward, to about 5%. Regardless, it is clear that this organism has acquired a great deal of genetic potential from outside sources.

Thermosipho africanus

Thermosipho
Thermosipho africanus : Robert Huber & Karl Stetter, University of Regensburg

T. africanus was isolated from hot, sandy sediment off the coast of Obock, Dijouti. This specie is morphologically much like Thermotoga, but grows in chains up to 12 cells long. Cells within chains are connected and separated by their togas. T. africanus is also a thermophile, but grows at lower temperatures than other members of this phyla; 35-77°C, optimally at 75°C.

Fervidobacterium islandicum

Fervidobacterium
Fervidobacterium islandicum : Robert Huber & Karl Stetter, University of Regensburg

F. islandicum was the second member of this phylum to be isolated, after T. maritima, but is much less well-characterized. It was isolated from a solfatara in the Hveragerdi geothermal fields of Iceland. In this specie, and its close relative F. nodusum, the toga forms a large spheroid “nodule” at one end of the cell only. F. islandicum grows at temperatures ranging from 40-80°C, optimally at 65-70°C.