The Family Chlorobiaceae
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The Family Chlorobiaceae JÖRG OVERMANN
Introduction Green sulfur bacteria (Chlorobiaceae) represent a phylogenetically coherent and isolated group within the domain Bacteria (see “Phylogeny”). As a distinct cytological feature, Chlorobiaceaecontain special light-harvesting complexes, so-called chlorosomes which harbor bacteriochlorophylls and carotenoids. Furthermore, green sulfur bacteria differ from most other phototrophic organisms with respect to the chemical structure of the antenna bacteriochlorophylls. Similar antenna complexes have only been found in the phylogenetically distant bacterial family Chloroflexaceae. Green sulfur bacteria are obligate photolithotrophs and are similar to the Chloroflexaceae, Chromatiaceae, and phototrophic species of the a- and b-proteobacteria, such that water cannot serve as electron donating substrate of the photosynthetic reaction center and molecular oxygen is not generated during anoxygenic photosynthesis. Instead, the electrons which are ultimately required for the assimilatory reduction of CO2 originate from sulfur compounds with low redox potential or from molecular hydrogen (see The Phototrophic Way of Life in Volume 2). Because of their limited physiological flexibility, the ecological niche of green sulfur bacteria is rather narrow. All known species of the green sulfur bacteria are typical aquatic microorganisms and occur where light reaches anoxic water layers of lakes or littoral sediments. In some of these ecosystems green sulfur bacteria play a key role in the transformations of carbon and sulfur compounds. Another phenotypic trait of ecological significance is the adaptation to low light intensities. Compared to any other phototrophic microorganism, green sulfur bacteria are able to colonize the lowermost water or sediment layers in oxic/anoxic ecosystems (see “Ecology”). The cells of most species of green sulfur bacteria are morphologically rather inconspicuous members of natural bacterial communities (see “Identification”). Exceptions are the phototrophic consortia, stable associations of green
sulfur bacteria with chemotrophic bacteria which are found in many freshwater habitats. At the present state of knowledge, these phototrophic consortia represent the most highly evolved symbiosis in the prokaryotic world.
Phylogeny Relatedness to Other Bacterial Phyla Based on a recent phylogenetic analysis (Overmann and Tuschak, 1997b), green sulfur bacteria as a group are phylogenetically isolated within the eubacterial radiation. The comparison of 16S rRNA gene sequences thus fully confirmed earlier results of ribosomal oligonucleotid cataloguing (Gibson et al., 1985; Woese et al., 1985). The closest relative presently in pure culture is Cytophaga sp. strain BD1-15 (77.9% similarity, Knuc = 0.26). Green sulfur bacteria appear to be the sister taxon of the Bacteroides/Flavobacterium phylum (Stackebrandt et al., 1996), which is also indicated by sequence homology of the recA gene of both groups (Gruber et al., 1998b). Green sulf
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