Microbial manganese and sulfate reduction in Black Sea shelf sediments

The microbial ecology of anaerobic carbon oxidation processes was investiga ted in Black Sea shelf sediments from mid-shelf with well-oxygenated bottom water to the oxic-anoxic chemocline at the shelf-break At all stations, or ganic carbon (C-org) oxidation rates were rapidly attenuated with depth in anoxically incubated sediment. Dissimilatory Mn reduction was the most impo rtant terminal electron-accepting process in the active surface layer to a depth of similar to 1 cm, while SO42- reduction accounted for the entire C- org oxidation below. Manganese reduction was supported by moderately high M n oxide concentrations. A contribution from microbial Fe reduction could no t be discerned, and the process was not stimulated by addition of ferrihydr ite. Manganese reduction resulted in carbonate precipitation, which complic ated the quantification of C-org oxidation rates. The relative contribution of Mn reduction to C-org oxidation in the anaerobic incubations was 25 to 73% at the stations with oxic bottom water. In situ, where Mn reduction mus t compete with oxygen respiration, the contribution of the process will var y in response to fluctuations in bottom water oxygen concentrations. Total bacterial numbers as well as the detection frequency of bacteria with fluor escent in situ hybridization scaled to the mineralization rates. Most-proba ble-number enumerations yielded up to 10(5) cells of acetate-oxidizing Mn-r educing bacteria (MnRB) cm(-3), while counts of Fe reducers were <10(2) cm- 3. At two stations, organisms affiliated with Arcobacter were the only type s identified from 16S rRNA clone libraries from the highest positive MPN di lutions for MnRB. At the third station, a clone type affiliated with Peloba cter was also observed. Our results delineate a niche for dissimilatory Mn- reducing bacteria in sediments with Mn oxide concentrations greater than si milar to 10 pmol cm(-3) and indicate that bacteria that are specialized in Mn reduction, rather than known Mn and Fe reducers, are important in this n iche.