An extremely low‐light adapted phototrophic sulfur bacterium from the Black Sea

Five strains of a brown phototrophic sulfur bacterium (Chlorobium phaeobacteroides) were isolated from the chemocline of the Black Sea (80‐m depth). All contain bacteriochlorophyll e as the main photosynthetic pigment. The strains revealed extreme low‐light adaptation of growth compared to 12 other green and purple sulfur bacterial strains. At very low light intensities (<4 µEinst m‒2 s‒1), the Black Sea strain MN 1 oxidized sulfide faster than the type strain 2430; the latter reached three times higher oxidation rates at light saturation. Low‐light adaptation is achieved by an increase of light‐harvesting pigments (175% compared to the type strain) and a very low maintenance energy requirement. The efficiency of energy transfer (59%) within light‐harvesting structures (chlorosomes) is comparable in other green sulfur bacteria and, therefore, appears to be limited by the molecular organization of the chlorosomes. From data in the literature, a light transmission of 0.0005% of surface irradiance was calculated for the chemocline of the Black Sea. Extrapolation of our laboratory data revealed that anoxygenic photosynthesis could account for 4% of total sulfide oxidation under average light conditions in situ and for 13% at maximal surface irradiance in summer.