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

Five strains ofa 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 PEinst mm2 s’), the Black Sea strain MN1 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. Striking changes in the vertical distribution of oxygen and sulfide in the Black Sea were reported recently (Murray et al. 1989). Since 1969 the sulfide-containing zone of this largest anoxic basin in the world has moved upward by 30 m to a depth of 74 m (center of west basin, Repeta et al. 1989). Above the sulfide zone the RV Knorr cruises in 1988 revealed a 20-30-m-thick water layer free of oxygen and sulfide. At the upper boundary of the sulfide zone, maxima of sulfide oxidation and carbon assimilation rates were observed, but the roles of microorganisms in these processes remained unclear (Jorgensen et al. 199 1). Earlier investigators did not find any viAcknowledgments We thank Holger Jannasch for inviting H.C. on the RV Knorr cruise and A. Angerhofer and J. Greis for cooperation with energy transfer efficiency measurements. able phototrophic bacteria in the free water column of the Black Sea (Hashwa and Triiper 1978). Recently, a peak of bacteriochlorophyll (BChl) e was detected in the chemoCline at 74 m (station BS2-2), indicating anoxygenic photosynthesis. BChl e-containing water layers extended from 68 to 98 m deep (Repeta et al. 1989). Anoxygenic photosynthesis could not be demonstrated in chemocline water, possibly because of cell damage during manipulation of the samples or inappropriate laboratory incubation conditions (Jorgensen et al. 199 1). Therefore we examined the physiological properties of phototrophic bacterial strains isolated from the Black Sea and evaluated their possible roles in sulfide oxidation and primary productivity under in situ conditions. Samples from the chemocline (77-83 m, station BS2-2 42”50’N, 32”OO’E; center of west basin) were obtained in May 1988 during RV Knorr cruise 134. After filtration of water samples through membrane filters (0.2 pm), the latter were transferred to chemocline water and stored under nitrogen in the dark in rubber-stoppered serum bottles. Five strains of a rod-shaped, brown bacterium were isolated after 10 times repeating a deep agar dilution series (Pfennig 1978). Although various modifications of the basal growth medium (Pfennig and Triiper 1989) were tried (various concentrations of dithionite between 0 and 1,000 PM, pH values between 6.8 and 7.2, addition of various vitamins and growth factors), the cultures still contained low numbers (< 2%) of a colorless bacterium, which was isolated and identified as a sulfate-reducing strain. Bacteriochlorophylls were identified from absorbance spectra of whole cells and acetone extracts (Shimadzu UV-300 spectrophotometer) and carotenoids by HPLC analysis (Perkin-Elmer with column Spherisorb ODS1, eluent acetonitrile/methanol/ isopropanol 90/7/3, photodiode array detector Waters 994). The pigment composition resembled that of other brown-colored