Prosthecochloris aestuarii strain SK413 T (DSM 271).

The green sulfur bacteria (GSB; Phylum Chlorobi ) are commonly found in illuminated, stratified, and anoxic aquatic environments, sediments, and and other sulfide-rich environments including hot springs (1, 2). Because of unique adaptations of their light-harvesting antennae, these bacteria are capable of growth at light intensities under which no other phototrophs can survive (3). In some aquatic environments, these organisms can account for up to 83% of the total annual productivity, and thus it is clear that these organisms can be the primary contributors of fixed carbon in certain ecological niches. All known GSB are metabolic specialists: all species are strictly anaerobic and obligately photoautotrophic in growth mode, and no known strain is capable of dark respiratory or strictly fermentative metabolism. Most of these bacteria use electrons derived from reduced sulfur compounds in combination with light energy to reduce carbon and nitrogen (4). Many GSB strains can also utilize hydrogen as an electron donor for photoautotrophic growth. Thus, in anoxic environments, Chlorobi are very important compontents of the carbon, nitrogen, and sulfur cycles. Carbon fixation in the GSB occurs by the reverse TCA cycle reactions. The photosynthetic apparatus includes homodimeric Type I reaction centers that are distantly related to Photosystem I reaction centers of higher plants and cyanobacteria. The light harvesting antenna, chlorosomes, are sacs filled with more than 200,000 BChl c/d/e molecules and ~2500 BChl a molecules associated with the protein CsmA (5). Because a GSB cell can contain as many as 250 chlorosomes, a GSB cell may contain more than 50 million BChl molecules! It is these enormous antennae that allow the cells to perform photosynthesis at vanishingly low light conditions (4). Although GSB share some biochemical and metabolic properties with other phototrophs, 16S rRNA analyses indicate that these bacteria are only very distantly related to other phototrophs (cyanobacteria, purple bacteria (proteobacteria), heliobacteria, and filamanetous anoxygenic phototrophs (green non-sulfur bacteria or Chloroflexaceae ). However, the Chlorobi seem to share a common ancestor with the Bacteroidetes and Flavobacteria (2).

Prosthecochloris aestuarii strain SK413 T (= DSM 271 ) is a non-motile, spherical to ovoid GSB (width: 0.3 to 0.6 µm; length 0.5 to 0.8 µm) that forms 10 to 20 non-branching prosthecae per cell (the function of these structures is unknown). Strain SK413 is the type species for the genus Prosthecochloris , and the organism was originally isolated from the hydrogen sulfide-rich mud of a shallow lagoon with elevated salt concentration and described in 1970 by Gorlenko (6). The cells produce BChl c , BChl a , and Chl a as well as chlorobactene and its hydroxylated derivative as the major photosynthetic pigments (7). P. aestuarii can fix nitrogen, lacks gas vesicles, and has an elevated requirement for salt (range: 0.2 to 10% NaCl; optimum 0.5 to 2% NaCl).

References

1. van Gemerden, H. and Mas, J. (1995) Ecology of phototrophic sulfur bacteria. In: Anoxygenic Photosynthetic Bacteria (R. E. Blankenship, M. T. Madigan, and C. E. Bauer, eds.), pp. 49-85. Kluwer Academic Publishers, Dordrecht, The Netherlands.

2. Overmann, J. (2001) Green sulfur bacteria. In: Bergey's Manual of Systematic Bacteriology, 2 nd Edition, Volume I, (D. R. Boone and R. W. Castenholz, eds.), pp. 601-605. Springer-Verlag, New York.

3. Manske, A. K., Glaeser, J., Kuypers, M. M. M., and Overmann, J. 2005. Physiology and phylogeny of green sulfur bacteria forming a monospecific phototrophic assemblage at a depth of 100 meters in the Black Sea. Appl. Environ. Microbiol. 71: 8049-8060.

4. Frigaard, N.-U., Li, H., Gomez Maqueo Chew, A., Maresca, J. A. and Bryant, D. A. 2003. Chlorobium tepidum : insights into the physiology and biochemistry of green sulfur bacteria from the complete genome sequence. Photosynthesis Res. 78: 93-117.

5. Frigaard, N-U. and Bryant, D. A. 2004. Seeing green bacteria in a new light: genomics-enabled studies of the photosynthetic apparatus in green sulfur bacteria and filamentous anoxygenic phototrophic bacteria. Arch. Microbiol. 182: 265-276.

6. Gorlenko, V. M. 1970. A new phototrophic green sulfur bacterium-- Prosthecochloris aestuarii nov. gen. nov. sp. Z. Allg. Mikrobiol. 10: 147-149.

7. Imhoff, J. F. 2003. Phylogenetic taxonomy of the family Chlorobiaceae on the basis of 16S rRNA and fmo (Fenna-Matthews-Olson protein) gene sequences. Int. J. Syst. Evol. Microbiol. 53: 941-951.

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