Figure 1. The universally phylogenetic tree with the extreme thermophilic and hyperthermophilic lineages in bold. Examples are: 1, Thermoanaerobacter spp.; 2, Thermotoga spp.; 3, Desulfurococcus spp.; 4, Sulfolobus spp.; 5, Thermophilum spp.; 6, Thermoproteus spp.; 7, Aquifex spp.; 8, Methanopyrus spp.; 9, Methanococcus jannaschii; 10, Thermococcus spp.

Figure 2. The universal phylogenetic tree and the position of some representative extreme thermophiles.

Figure 3. Dictyoglomus thermophilium strain Rt8N2 (Patel et al., 1987, isolated from a hot spring in New Zealand (light microscopy, phase contrast; bar, 10 m). (a)–(e) Aggregation and lysis of cells during growth cycle. (a) Single cells at the early exponential growth phase. (b) Beginning of cell aggregation. (c) Coiling of cells forming a shape of a bowl. Notice that at this stage no common membrane is visible. (d) Sphere with cells arranged around the periphery. Note the visible common membrane/envelope around the cells at this stage. (e) At the stationary growth phase there is a loss of distinct cell appearance, which makes the common membrane/envelope clearly visible. Unpublished results of H. Morgan and J. Wiegel. Reproduced courtesy of H. Morgan and J. Wiegel.

Figure 4. Thermosipho, an anaerobic extreme thermophile, a bacterium that exhibits one of the highest growth temperatures among the bacteria and the special morphological feature of a toga (electron microscope preparation shadowed with platinum). Several cells (the cell bodies are marked by arrows) are enclosed in one long sheath, the toga, surrounding the cells. The toga (marked by arrowheads) becomes visible between the cells. Bar, 5 m. Reproduced courtesy of Drs Reinhard Rachel and Karl O. Stetter, University of Regensburg.

Figure 5. Thermotoga maritima, an anaerobic extreme thermophile. The electron micrograph depicts a cell shadowed with platinum. The cell is under division and therefore possess (already) two flagella. The elongated cell body is marked by an arrow; the toga surrounding the cell at both ends (or cell poles?) is marked by arrowheads. Bar, 1 m. Reproduced courtesy of Drs Reinhard Rachel and Karl O. Stetter, University of Regensburg.

Figure 6. Thermotoga maritima. Electron micrograph of an ultrathin section of a cell: the cell body is densely stained (marked by a white arrow) and intimately surrounded by a cytoplasmic membrane, which is not visible as a layered structure in this section. The cell body is loosely surrounded by the ‘toga’, which in fact is an outer membrane (marked by arrowheads). Bar, 0.5 m. Reproduced courtesy of Drs Reinhard Rachel and Karl O. Stetter, University of Regensburg.

Figure 7. Thermoanaerobacter ethanolicus. (a)–(c) Light microscopic and electron microscopic observations of strain JW 200. (a) Normal size (arrow) and longer filamentous cells that may divide to give long chains of bacteria (arrow heads; light microscopy). (b) Coccoid cell (scanning electron micrograph). The cells contain DNA and thus are not equivalent to the E. coli minicells. (c) Uneven length of dividing cells (electron micrograph); notice the incomplete cross wall formation (arrow). (d) and (e) Protuberances or autoplast type of cells (light microscopy, Normaski technique). (f) Single cell with flagella. The insert on the right shows the hexagonal pattern of the outer cell wall layer (‘S-layer’) and the insertion point of the flagellum; the insert on the left shows pili (arrows) and flagella. Bars, 0.1 m. (g) Ultrathin section showing cell envelope layers (electron micrograph). (h) Higher magnification electron micrograph of the cell envelope layers: CM, cytoplasmic membrane: D, dense layer; O, outer cell wall layer. Bar, 25 nm. (i) Electron micrograph of a cell wall from a lysed cell showing three layers: outer cell wall layer (O), dense layer (D) and an inner cell wall layer (I). Bar, 25 nm. Reproduced with permission from Wiegel and Ljungdahl 1981. Copyright © 1981 Springer-Verlag.