The Order Haloanaerobiales
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The Order Haloanaerobiales AHARON OREN
Introduction Bottom sediments of hypersaline lakes and lagoons may be expected to support a rich community of anaerobic halophilic bacteria, as the solubility of oxygen in hypersaline brines is low and the amounts of organic matter available are often high (Oren, 1988). Therefore, it is surprising that the first records of obligatory anaerobic fermentative bacteria growing at salt concentrations of 10–20% and higher appeared only in the early 1980s, when Haloanaerobium praevalens was isolated from the bottom sediments of the Great Salt Lake in Utah (Zeikus, 1983; Zeikus et al., 1983) and Sporohalobacter lortetii and Halobacteroides halobius were discovered in Dead Sea sediments (Oren, 1983; Oren et al., 1984b). Haloanaerobium praevalens may be similar to “Bacteroides halosmophilus,” isolated by Baumgartner, 1937 from solar salt and from salted anchovies, but unfortunately no cultures of that isolate have been preserved. The fermentative obligatory anaerobic halophilic Bacteria appear to be a phylogenetically coherent group (Rainey et al., 1995). The order Haloanaerobiales was created to accommodate these halophilic anaerobes. At the time of writing (January 1999), 19 species had been described and classified in two families, the Haloanaerobiaceae (Oren et al., 1984a; Rainey et al., 1995) and the Halobacteroidaceae (Rainey et al., 1995). This chapter presents these species and discusses their properties. The group was earlier reviewed by Lowe et al., 1993, Ollivier et al., 1994, and Oren, 1986a, Oren, 1990, Oren, 1993a, Oren, 1993b.
Phylogeny Phylogenetic analysis based on 16S rRNA gene sequences has shown that the 19 obligatory anaerobic halophilic bacteria validly described thus far, may be classified in a single order, the Haloanaerobiales (Rainey et al., 1995), with two families, the Haloanaerobiaceae (Oren et al., 1984a) and the Halobacteroidaceae (Rainey et al., 1995) (Table 1, Fig. 1). Figures 2–3 present
micrographs and electron micrographs of selected species. All isolates share a low content of G+C in their DNA. With the exception of the thermophilic Halothermothrix orenii, which has a G+C content of 39.6 mol%, all species have G+C contents between 27 and 36.9 mol%. Based on 16S rRNA gene sequences, the halophilic anaerobic bacteria should be classified in the domain Bacteria within the phylum of the Gram-positive bacteria. They form a coherent cluster close to the bifurcation point that separates the Actinomycetes subphylum and the Bacillus/Clostridium subphylum. Sequences of the halophilic anaerobes contain all of the few signature nucleotides that have been defined as characteristic of members of the Bacillus/ Clostridium subphylum, while they lack any of the Actinomycetes-specific nucleotides. Therefore, the halophilic anaerobes were suggested to be in the Bacillus/Clostridium subphylum (Patel et al., 1995; Rainey et al., 1995; Tourova et al., 1995). The phylogenetic affiliation of Haloanaerobium praevalens with the Bac
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