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. 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% Furthermore, "Chromatiaceae" have been found in chemocline environments over 20 m depths. 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, while the less dense population (found in the Black Sea (0.068–0.94 μg BChle/dm−3), scattered over an interval of 30 m. 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 1.5–5 mm of the sediments. At the same time, biomass densities of 900 mg bacteriochlorophyll/dm−3 can be attained in these latter systems.