The Family Chloroflexaceae
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The Family Chloroflexaceae SATOSHI HANADA AND BEVERLY K. PIERSON
Introduction The first multicellular filamentous anoxygenic phototrophic bacterium was discovered in thermal environments (Pierson and Castenholz, 1971) and was soon described as a new genus and species with the name of “Chloroflexus aurantiacus” (Pierson and Castenholz, 1974a). The organism showed gliding motility and contained pigment-bearing vesicles called “chlorosomes.” Chlorosomes are generally observed in green sulfur bacteria that are strictly anaerobic phototrophs sensitive to oxygen. The pigmentbearing vesicles in Chloroflexus aurantiacus were very similar, both structurally and functionally, to those in green sulfur bacteria (Schmidt, 1980a). Aside from this pigment resemblance, however, significant differences were found between the filamentous organism and green sulfur bacteria. In addition, this new phototroph apparently differed in many phenotypic characteristics from purple bacteria, which is another type of anoxygenic photosynthetic bacteria. Therefore, the new family Chloroflexaceae was created for this filamentous phototroph containing prominent features (Trüper, 1976). Following the description of Chloroflexus aurantiacus, related mesophilic filamentous phototrophs were found in freshwater habitats: Two genera, Chloronema (Dubinina and Gorlenko, 1975) and Oscillochloris (Gorlenko and Pivovarova, 1977; Keppen et al., 1994), were individually proposed for the isolates. Like Chloroflexus aurantiacus, these mesophiles are morphologically filamentous, show gliding motility, and contain chlorosomes as light-harvesting components. On the other hand, a chlorosome-less filamentous phototroph, Heliothrix oregonensis, was also found in a hot spring (Pierson et al., 1984a). The Chloroflexus-like bacterium is an interesting organism in that it does not contain bacteriochlorophyll (Bchl) c but does contain as its sole chlorophyll species Bchl a. Recently, another chlorosome-less filamentous phototroph was newly isolated from a hot spring and proposed as Roseiflexus castenholzii (Hanada et al., 2002b). Although Roseiflexus castenholzii, like
Heliothrix oregonensis, lacks Bchl c and chlorosomes, genotypic and phenotypic comparisons showed that they were phylogenetically distant from each other. Filamentous morphology and gliding motility are typical features of these organisms, whereas these properties are seldom observed in any other anoxygenic photosynthetic bacteria, i.e., purple bacteria, green sulfur bacteria, and the heliobacteria. The phylogenetic analysis based on 16S rRNA sequence also supported the solitary taxonomic position of this group: Oyaizu et al. revealed that Chloroflexus aurantiacus belonged to a deep branching lineage (phylum) of the domain Bacteria together with the genera Herpetosiphon and Thermomicrobium, which are comprised of nonphototrophic bacteria, and that Chloroflexus aurantiacus was phylogenetically distant from any other photosynthetic group (Oyaizu et al., 1987). The phototrophic group (a
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