Other primitive thermophiles

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There are a number of other groups of primitive thermophiles that are even less well-understood than are the Aquificae and Thermotogae. Most are known only from 16S rRNA sequences obtained directly from thermal environments, and have not been cultivated. Others are either know from single isolates, or their placement in the tree is uncertain (or both).

Thermodesulfobacterium

Thermodesulfobacterium
Thermodesulfobacterium hydrogeniphilum : Jeanthon...Prieur IJSEM 52:765-772

The phylum Thermodesulfobacteriales consists of only six named isolates in three genera: Geothermobacterium, Thermodesulfatator, and Thermodesulfobacterium. The best (bit still poorly) characterized member of this group is Thermodesulfobacterium hydrogenophilum (pictured above). T. hydrogenophilum is a thermophilic (50-80°C, optimum 75°C) anaerobic sulfate reducer isolated from the a deep sea hydrothermal vent in the Guaymus Basin. It is a small (0.4-0.5 x 0.5-0.8μm) motile rod with a single, polar flagellum. In stationary-phase culture, cell tend to form short chains of longer cells, and sometimes cyst-like cells appear. This organism grows by sulfate reduction to sulfide, using hydrogen (H2) as the only electron donor (other species in this genus use organics acids). Unlike other species of this genus, T. hydrogenophilum cannot use sulfite or thiosulfate as an electron acceptor. It is an obligate chemolithoautotroph.

Thermomicrobium

The phylum Thermomicrobia consists of only a single characterized specie, Thermomicrobium roseum, a potential second uncharacterized isolate, and some environmental sequences. This phylum may be a distant but specific relative of the Chloroflexi. T. roseum was isolated from Toadstool Spring in Yellowstone National Park, and grows optimally at 75°C. It forms irregular to pleomorphic short rods ca. 1.5 x 3-6μm in size, is non-motile, and pink colonies on plates. It is an obligately aerobic heterotroph, but it’s respiratory pathway is unknown and, as it is cyanide resistant, probably unique.

Chloroflexi and Deinococcus-Thermus

Although these phyla will be discussed later, they are also largely thermophilic and perhaps deeply branching as well. However, these phyla do contain many mesophilic species, and, except perhaps for Thermus and its relatives, are not strikingly primitive. It is not clear whether or not these should be included amongst the primitive thermophiles and/or the ancestral thermophiles.