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Ecology
According to the most recent studies, the ecology of purple sulfur bacteria appears to focus mostly on habitats, distribution, and various ecological niches.


 * Ecological niches

Quantity and quality of light

Several studies have shown that a strong accumulation of phototrophic sulfur bacteria has been observed between 2 and 20 meters deep (in some cases even 30 m) of pelagic environments (Herbert et al., 2005). This is due to the fact that in some environments the light transmission for various populations of phototrophic sulfur bacteria varies with a density from 0.015 to 10% (van Gemerden and Mas, 1995). Furthermore, Chromatiaceace have been found in chemoclin environments over ≤20 m. depth. The correlation between anoxygenic photosynthesis and the availability of solar radiation suggests that light is the main factor controlling all the activities of phototrophic sulfur bacteria. The density of pelagic communities of phototrophic sulfur bacteria extends beyond a depth range of 10 cm (Overmann et al., 1991), while the less dense population (found in the black sea (0.068–0.94 μg BChle.l− 1), is scattered over an interval of 30 m (Manske et al., 2005). Communities of phototrophic sulfur bacteria located in the coastal sediments of sandy, saline or muddy beaches live in an environment with a higher light gradient, limiting growth to the highest value between 1.5-5mm (van Gemerden and Mas, 1995). At the same time, biomass densities of 900 mg bacteriochlorophyll.dm − 3 can beattained in these latter systems (van Gemerden et al., 1989)

Temperature and salinity

Purple sulfur bacteria (such as green sulfur bacteria) typically form blooms in non-thermal aquatic ecosystems, some members have been found in some hot springs (Castenholz et al., 1990). For example Chlorobaculum tepidum can only be found in some hot springs in New Zealand at a ph value between 4.3 and 6.2 and at a higher temperature of 56 ° C.

Another exemple are Thermochromatium tepidum, has been found in several hot springs in western North America at temperatures above 58 ° C and may represent the most thermophilic protobacteria that exists (Castenholz and Pierson, 1995). Of the purple sulfur bacteria, many members of the Chromatiaceae family are often found in freshwater and marine environments. About 10 species of Chromatiaceae only halophilic (Imhoff, 2005a).

Syntrophy and Symbioses

Like green sulfur bacteria, purple sulfur bacteria are also capable of symbiosis. In fact, it was noted that they rapidly creating stable associations (Warthmann et al., 1992) between other purple sulfur bacteria and sulfur- or sulfate-reducing bacteria. These associations are based on a cycle of sulfur but not carbon compounds. Thus, a simultaneous growth of two bacteria partners takes place, which are fed by the oxidation of organic carbon and light substrates. Experiments with Chromatiaceae have pointed out that cell aggregates consisting of sulfate-reducin Proteobacterium Desulfocapasa thiozymogenes and small cells of Chromatiaceae have been observed in the chemocline of an alpine mermocitic lake (Tonolla et al., 2000).

It was also noted that the purple sulfur bacteria Chromatium weissei often establish commensal associations with epibiotic bacteria (Clark et al., 1993). An unidentified epibiont attaches to healthy Chromatium cells by performing a lysis process on host cells, a methodology very similar to that of Vampirococcus (Guerrero et al., 1987a). It seems that the epibiont grows chemotrophically on carbon compaunds excreted by the purpule sulfur bacteria.