Chlorobium species

Figure 23-28 (A) Model of a light-harvesting chlorosome from green photosynthetic sulfur bacteria such as Chlorobium tepidum and species of Prosthecochloris. The chlorosome is attached to the cytoplasmic membrane via a baseplate, which contains the additional antenna bacteriochlorophylls (795 BChl a) and is adjacent to the trimeric BChl protein shown in (B) and near the reaction center. After Li et al.302 and Remigy et a/.304 (B) Alpha carbon diagram of the polypeptide backbone and seven bound BChl a molecules in one subunit of the trimeric protein from the green photosynthetic bacterium Prosthecochloris. For clarity, the magnesium atoms, the chlorophyll ring substituents, and the phytyl chains, except for the first bond, are omitted. The direction of view is from the three-fold axis, which is horizontal, toward the exterior of the molecule. From Fenna and Matthews.305 See also Li et al.302...

All species contain two different chlorophylls, BChl a and an additional light-harvesting pigment, either BChl c, d ox e [40], As in Chloroflexus these light-harvesting pigments are housed in chlorosomes located adjacent to the cytoplasmic membrane [45,46]. In Chlorobium BChl c is associated with a 7.5-kDa polypeptide. (J.M. Olson and P. Roepstorff, unpublished). A small amount of energytransferring BChl a is also found in the chlorosomes [47,48], but most of the BChl a is found in a water-soluble protein associated with the cytoplasmic membrane [45]. The RC (see Fig. 4 and Ref. 36) is similar to that of PS I in cyanobacteria and chloroplasts. The primary electron donor P-840 = +0.25 V) is a BChl... 

S042-, so S accumulates. This S can be oxidized to S042- by Thiobacillus denitrificans, a chemosynthet-ic bacterium that is unusual for Thiobacillus species in being a facultative anaerobe (a nitrate reducer). The S can also be reduced to S2 for example, by Desuifuromonas acetoxidans, which lives in syn-trophic association with Chlorobium (Kuenen et al. 1985). 

The photosynthetic species, Chlorobium and Chromatium, develop under anaerobic conditions when light is available to provide the source of energy for growth. H2S is used as a reductant in converting CO2 to their cell material. During photosynthesis in muds and lake waters they may oxidize the sulphide and precipitate sulphur on such a scale that it can be ecomomically mined. 

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