Phylogenetic implications

Hot volcanic environments are among the oldest biotopes existing on Earth. What is known about the phylogenetic relationships of the organisms living there? Based on the pioneering work of Carl Woese, the small subunit ribosomal RNA (ss-rRNA) is widely used in phylogenetic studies of prokaryotes (Woese and Fox, 1977; Woese et al., 1990). It consists of approximately 1500 nucleotides and is homologous to its eukaryotic counterpart. On the basis of sequence comparisons, a phylogenetic tree is now available (Figure 7.4). It shows a tripartite division of the living world into the bacterial, archaeal, and eukaryal domains. Within this tree, deep branches are evidence for early separation. The separation of the Bacteria from the stem shared by Archaea and Eukarya represents the deepest and earliest branching point. Short phylogenetic branches indicate a rather slow rate of evolution. In contrast to the eukaryal domain, the bacterial and archaeal domains within the phylogenetic tree exhibit some extremely short and deep branches. Surprisingly,

Eukarya

Arthrop°da Nematode Alveolates i , / Animals

Stramenopiles

Euglenozoa

Bacteria

Amoebo-Flagellates

Microsporidia

Stramenopiles

Euglenozoa

Bacteria

Amoebo-Flagellates

Archaea

Microsporidia

Halobacteriales

Methano-coccales

Thermoplasma

Methanoplanus Methanomicrobium Methanosarcina

Euryarchaeota

Low GC Gram positives (Bacillus, Mycoplasma, Lactobacillus)

Fig. 7.4. Small subunit ribosomal RNA-based phylogenetic tree. The bulky lineages are representing HT.

Archaea

Metallosphaera

Thermofilum Thermoproteus Pyrococcus hermococcus Methanobacterium Methanothermus Methanopyrus Archaeoglobus/Ferroglobus

Halobacteriales

Methano-coccales

Thermoplasma

Methanoplanus Methanomicrobium Methanosarcina

Euryarchaeota

Low GC Gram positives (Bacillus, Mycoplasma, Lactobacillus)

Fig. 7.4. Small subunit ribosomal RNA-based phylogenetic tree. The bulky lineages are representing HT.

those are covered exclusively by hyperthermophiles, which therefore form a cluster around the phylogenetic root (Figure 7.4, bulky lineages). The deepest and shortest phylogenetic branches are represented by the Aquificales and Thermotogales within the Bacteria and the Nanoarchaeota, Pyrodictiaceae, and Methanopyraceae within the Archaea. Long lineages represent mesophilic and moderately thermophilic Bacteria and Archaea (e.g., Gram-positives, Proteobacteria; Halobacteriales; Methanosarcinaceae) indicating their ss-rRNA had experienced a fast rate of evolution. Now, several total genome sequences are available. Phylogenetic trees based on genes involved in information management (for example, DNA replication, transcription, translation) parallel the ss-rRNA tree. Genes involved in metabolism, however, are prone to frequent lateral gene transfer, and a network rather than a tree might reflect their phylogenetic relations (Doolittle, 1999).

To date, approximately 90 species of HT Archaea and Bacteria, which had been isolated from different terrestrial and marine thermal areas in the world, are known. HT are very divergent, both in their phylogeny and physiological properties and are grouped into 34 genera and 10 orders (Boone and Castenholz, 2001).

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