Microbial communities in the chemocline of a hypersaline deep-sea basin (Urania basin, Mediterranean Sea)

The Urania basin is a hypersaline sulfidic brine lake at the bottom of the eastern Mediterranean Sea. Since this basin is located at a depth of simila r to3,500 m below the sea surface, it receives only a small amount of phyto plankton organic carbon. In the present study, the bacterial assemblages at the interface between the hypersaline brine and the overlaying seawater we re investigated. The sulfide concentration increased from 0 to 10 mM within a vertical interval of 5 m across the interface. Within this chemocline, t he total bacterial cell counts and the exoenzyme activities were elevated. Employing 11 cultivation methods, we isolated a total of 70 bacterial strai ns. The 16S ribosomal DNA sequences of 32 of the strains were identical to environmental sequences detected in the chemocline by culture-independent m olecular methods. These strains were identified as flavobacteria, Alteromon as macleodii, and Halomonas aquamarina. All 70 strains could grow chemoorga noheterotrophically under oxic conditions. Sixty-six strains grew on pepton e, casein hydrolysate, and yeast extract, whereas only 15 strains did not u tilize polymeric carbohydrates. Twenty-one of the isolates could grow both chemoorganotrophically and chemolithotrophically. While the most probable n umbers in most cases ranged between 0.006 and 4.3% of the total cell counts , an unsually high value of 54% was determined above the chemocline with me dia containing amino acids as the carbon and energy source. Our results ind icate that culturable bacteria thriving at the oxic-anoxic interface of the Urania basin differ considerably from the chemolithoautotrophic bacteria t ypical of other chemocline habitats.