Abstract
At the Black Sea chemocline, oxygen- and sulfide-rich waters meet and form a niche for thiotrophic pelagic bacteria. Here we investigated an area of the Northwestern Black Sea off Crimea close to the shelf break, where the chemocline reaches the seafloor at around 150–170 m water depth, to assess whether thiotrophic bacteria are favored in this zone. Seafloor video transects were carried out with the submersible JAGO covering 20 km2 on the region between 110 and 200 m depth. Around the chemocline we observed irregular seafloor depressions, covered with whitish mats of large filamentous bacteria. These comprised 25–55% of the seafloor, forming a belt of 3 km width around the chemocline. Cores from the mats obtained with JAGO showed higher accumulations of organic matter under the mats compared to mat-free sediments. The mat-forming bacteria were related to Beggiatoa-like large filamentous sulfur bacteria based on 16S rRNA sequences from the mat, and visual characteristics. The microbial community under the mats was significantly different from the surrounding sediments and enriched with taxa affiliated with polymer degrading, fermenting and sulfate reducing microorganisms. Under the mats, higher organic matter accumulation, as well as higher remineralization and radiotracer-based sulfate reduction rates were measured compared to outside the mat. Mat-covered and mat-free sediments showed similar degradability of the bulk organic matter pool, suggesting that the higher sulfide fluxes and subsequent development of the thiotrophic mats in the patches are consequences of the accumulation of organic matter rather than its qualitative composition. Our observations suggest that the key factors for the distribution of thiotrophic mat-forming communities near to the Crimean shelf break are hypoxic conditions that (i) repress grazers, (ii) enhance the accumulation and degradation of labile organic matter by sulfate-reducers, and (iii) favor thiotrophic filamentous bacteria which are adapted to exploit steep gradients in oxygen and sulfide availability; in addition to a specific seafloor topography which may relate to internal waves at the shelf break.
Highlights
Thiotrophic microbial mats are dense, visible accumulations of microorganisms, dominated by functional groups capable to gain their energy by using reduced forms of sulfur as electron donors, enabling chemoautotrophic biomass production
Dives with the submersible JAGO confirmed our hypothesis that thiotrophic microbial mats are associated with the hypoxic zone of the Black Sea at the outer shelf break
In the outer Western Crimean shelf this zone ranged from ∼140 to 170 m water depth, covering a distance of ca. 6 km perpendicular to the slope, as detected by the oxygen optode mounted to JAGO (Figure 1A)
Summary
Thiotrophic microbial mats are dense, visible accumulations of microorganisms, dominated by functional groups capable to gain their energy by using reduced forms of sulfur as electron donors, enabling chemoautotrophic biomass production. Thiotrophic mats can cover large areas of seafloor in shelf seas and upper continental margins where oxygen depletion leads to high deposition rates of organic matter, such as coastal upwelling regions, oxygen minimum zones and other ecosystems subjected to eutrophication (Jørgensen, 1977; Williams and Reimers, 1983; Schulz and Jørgensen, 2001; Levin, 2003; Mußmann et al, 2003) In these systems, the rapid depletion of oxygen by organic matter remineralization at the seafloor favors high rates of sulfate reduction, and sulfide production, which in turn supports the growth of thiotrophs into dense accumulations (Neira and Rackemann, 1996; Freitag et al, 2003; Lehto et al, 2014). Thiotrophic mats are prominent features of cold seep ecosystems (Grünke et al, 2012), where the anaerobic oxidation of hydrocarbons via sulfate reduction fuels high sulfide fluxes (Boetius et al, 2000; Joye et al, 2004), at sulfide-emitting hydrothermal vents (Jannasch et al, 1989; Meyer et al, 2013; Urich et al, 2014), as well as in caves, profiting from sulfide-rich streams (Macalady et al, 2006)
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