Examples of recent HT organisms

Within the Bacteria domain, the deepest phylogenetic branch is represented by the HT Aquifex (Huber et al., 1992). Its type species Aquifex pyrophilus is a motile rod-shaped chemolithoautotroph (Figure 7.6). It is a facultative microaerophilic anaerobe. Under anaerobic conditions, Aquifex pyrophilus grows

Fig. 7.6. Aquifex pyrophilus, dividing cell with a tuft of flagella. Pt-shadowing.

Transmission electron micrograph. Scale bar, 1 ^m.

Fig. 7.6. Aquifex pyrophilus, dividing cell with a tuft of flagella. Pt-shadowing.

Transmission electron micrograph. Scale bar, 1 ^m.

by nitrate reduction with H2 and S0 as electron donors. Alternatively, at very low oxygen concentrations (up to 0.5%, after adaptation), it is able to gain energy by oxidation of H2 and S0, using oxygen as an electron acceptor. Members of Aquifex are found in shallow submarine vents. Aquifex pyrophilus grows up to 95 °C, the highest growth temperature observed within the Bacteria (Table 7.1).

From the walls of a black smoker at the Mid Atlantic Ridge, we had isolated the archaeon Pyrolobus fumarii (Blochl et al., 1997). Cells are lobed cocci, approximately 0.7-2.5 ^m in diameter (Figure 7.7). The species Pyrolobus fumarii is adapted optimally to temperatures of superheated water, exhibiting an optimal growth temperature of 106 °C (Figure 7.8) and an upper temperature border of growth at 113 °C. It is so dependent to high temperatures that it is unable to grow below 90 °C. Cultures of Pyrolobus fumarii, similar to Pyrodictium occultum are able to survive autoclaving for one hour at 121 °C. A very closely related isolate ('strain 121') exhibits the same optimal growth temperature (106 °C), but has been reported to grow slowly even at 121 °C (Kashefi and Lovley, 2003). However, this result has not been confirmed.

Fig. 7.7. Pyrolobus fumarii, lobed coccoid cell. Ultra thin section. Transmission electron micrograph. Scale bar, 0.5 ^m.
Fig. 7.8. Pyrolobus fumarii: Temperature dependence of its doubling time. Optimal growth (about 50 min. doubling time ) occurs between 103 and 106 °C.

From a submarine hydrothermal system situated at the Kolbeinsey Ridge, north of Iceland, we were able to obtain our ultimate hyperthermophilic coccoid isolate Nanoarchaeum equitans, which represents a novel kingdom of Archaea (Huber et al., 2002). With a cell diameter of only 400 nm, it is the smallest living organism known. Cells grow attached to the surface of a specific crenarchaeal

Fig. 7.9. Nanoarchaeum equitans-Ignicoccus hospitalis.: Transmission electron micrographs. (A) Two cells of N. equitans attached on the surface of the (central) Ignicoccus cell. Platinum shadowed. Scale bar, 1 ^m. (B) Freeze-etched cell of Ignicoccus hospitalis (Ig) and attached cells of N. equitans (N) on the surface. Scale bar, 1 ^m. (C) Surface relief reconstruction of N. equitans. Dark: cavity; Bright: elevation. Scale bar, 15 nm. (D) Ultrathin section of a N. equitans cell. Single cell. CM: cytoplasmic membrane; PP: periplasm; SL: S-layer. Scale bar: 0.5 ^m.

Fig. 7.9. Nanoarchaeum equitans-Ignicoccus hospitalis.: Transmission electron micrographs. (A) Two cells of N. equitans attached on the surface of the (central) Ignicoccus cell. Platinum shadowed. Scale bar, 1 ^m. (B) Freeze-etched cell of Ignicoccus hospitalis (Ig) and attached cells of N. equitans (N) on the surface. Scale bar, 1 ^m. (C) Surface relief reconstruction of N. equitans. Dark: cavity; Bright: elevation. Scale bar, 15 nm. (D) Ultrathin section of a N. equitans cell. Single cell. CM: cytoplasmic membrane; PP: periplasm; SL: S-layer. Scale bar: 0.5 ^m.

host, a new member of the genus Ignicoccus (Figure 7.9). Owing to their unusual ss-rRNA sequence, members of Nanoarchaeum equitans remained undetectable by commonly used 'universal' primers in ecological studies based on the polymerase chain reaction. However, two different ss-rRNA genes could be detected in a coculture-derived DNA by Southern blot hybridization, by taking advantage of the generally high sequence homology of all ss-rRNA genes (about 75% similarity). After sequencing of the total genome, the phylogenetic relationships of Nanoarchaeum equitans could be investigated by concatenating and aligning the amino acid sequences of 35 ribosomal proteins (Waters et al., 2003). The species Nanoarchaeum equitans was placed with high support at the most deeply branching position within the Archaea, suggesting that the Nanoarchaeota diverged early within the Archaea. With only 490 885 base pairs, the Nanoarchaeum equitans genome is the smallest microbial genome known to date and also the most compact, with 95% of the DNA predicted to encode proteins and stable RNAs. It harbours the complete machinery for information processing and repair, but lacks genes for lipid, cofactor, amino acid, and nucleotide biosynthesis. The limited biosynthetic and catabolic capacity suggests that the symbiotic relationship of Nanoarchaeum equitans to its Ignicoccus hospitalis host may be parasitic, making it the only known archaeal parasite. Unlike the small genomes of bacterial parasites, however, which are undergoing reductive evolution, the small genome of Nanaoarchaeum equitans has very few pseudogenes and a well-equipped DNA recombination system. Therefore, it may be a very ancient parasite. Alternatively, the Ignicoccus hospitalis-Nanoarchaeum equitans system could be remains from pre-Archaea communities, as postulated earlier (Kandler, 1994). At the molecular level, Nanoarchaeum equitans harbours further unexpected, most likely primitive properties, like separately encoded enzyme modules and - for the first time -1 RNA gene fragments (Waters et al., 2003; Randau et al., 2005). Currently, we possess only rudimentary understanding of the Nanoarchaeum equitans -Ignicoccus hospitalis relationship. The Nanoarchaeota are distributed world-wide within hot environments and had been completely overlooked. Two ss-rRNA sequences from Uzon Caldeira (Kamchatka, Russia) and Yellowstone National Park (USA) exhibited 83% sequence similarity to Nanoarchaeum equitans, and, therefore, represent a distinct group within the Nanoarchaeota (Hohn et al., 2002). Light microscopy and fluorescence in situ staining reveal that these novel Nanoarchaeota are again tiny cocci, approximately the size of Nanoarchaeum equitans, however, attached to a rod-shaped Pyrobaculum-like host. The discovery of the Nanoarchaeota suggests that further unrecognized major groups of microbes may remain undetected by current PCR primers and are waiting for isolation to tell us more about the origins and evolution of life.

Was this article helpful?

0 0

Post a comment