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

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Abstract
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 (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.

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