Sulfidic muds of cold seeps on the Nile Deep Sea Fan (NDSF) are populated by different types of mat-forming sulfide-oxidizing bacteria. The predominant sulfide oxidizers of three different mats were identified by microscopic and phylogenetic analyses as (i) Arcobacter species producing cotton-ball-like sulfur precipitates, (ii) large filamentous sulfur bacteria including Beggiatoa species, and (iii) single, spherical Thiomargarita species. High resolution in situ microprofiles revealed different geochemical settings selecting for the different mat types. Arcobacter mats occurred where oxygen and sulfide overlapped above the seafloor in the bottom water interface. Filamentous sulfide oxidizers were associated with steep gradients of oxygen and sulfide in the sediment. A dense population of Thiomargarita was favored by temporarily changing supplies of oxygen and sulfide in the bottom water. These results indicate that the decisive factors in selecting for different mat-forming bacteria within one deep-sea province are spatial or temporal variations in energy supply. Furthermore, the occurrence of Arcobacter spp.-related 16S rRNA genes in the sediments below all three types of mats, as well as on top of brine lakes of the NDSF, indicates that this group of sulfide oxidizers can switch between different life modes depending on the geobiochemical habitat setting.
A mat-forming population of the giant sulfur bacterium Thiomargarita was discovered at the flank of the mud volcano Amon on the Nile Deep Sea Fan in the Eastern Mediterranean Sea. All cells were of a spherical and vacuolated phenotype and internally stored globules of elemental sulfur. With a diameter of 24-65 µm, Thiomargarita cells from the Eastern Mediterranean were substantially smaller than cells of previously described populations. A 16S rRNA gene fragment was amplified and could be assigned to the Thiomargarita-resembling cells by fluorescence in situ hybridization. This sequence is monophyletic with published Thiomargarita sequences but sequence similarities are only about 94%, indicating a distinct diversification. In the investigated habitat, highly dynamic conditions favour Thiomargarita species over other sulfur-oxidizing bacteria. In contrast to Thiomargarita namibiensis populations, which rely on periodic resuspension from sulfidic sediment into the oxygenated water column, Thiomargarita cells at the Amon mud volcano seem to remain stationary at the sediment surface while environmental conditions change around them due to periodic brine flow.
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