Composition and activities of microbial communities involved in carbon, sulfur, nitrogen and manganese cycling in the oxic/anoxic interface of the Black Sea

Own and literature data on the structure and functional activities of microbial communities in the Black Sea chemocline are reviewed. Bacterial numbers in the oxic/anoxic interface increase by an order of magnitude compared to above-lying waters. The dark carbon dioxide fixation rate increases too and often does not correspond to the maximum in total cell numbers. Carbon isotope measurements of particulate organic carbon indicate that bacterial chemosynthesis (rates are between 9.6 and 25 mmol C m(-2) d(-1)) is the main source of organic matter in the Black Sea chemocline and accounts for 20-50% of total primary production. The increased dark CO2 fixation rates in the chemocline reflect a mixed signal derived from CO2 fixation of a number of lithoautotrophic bacteria involved in sulfide oxidation, but also with methanotrophs, methanogens and with sulfate reducers. Chemolithoautotrophic bacteria related to Thiobacillus and Thiomicrospira and heterotrophic sulfur oxidizing Rhizobiaceae strains are probably the main sulfide oxidizing bacteria. Shewanella species using Mn (Fe) oxyhydroxides as an alternative electron acceptor to dissolved oxygen may play a role in sulfide oxidation too. Anoxygenic photosynthesis mediated by green sulfur bacteria related to Chlorobium accounts for not more than 13% of the total sulfide flux [Overmann and Manske, this volume]. 'Anammox' bacteria together with denitrifiers may be mainly responsible for the inorganic nitrogen loss in the interface. Below the interface, sulfate reduction, methane oxidation and methanogenesis co-occur. Highest sulfate reduction rates are observed below the interface down to 300 m; however this process is detected throughout the entire water column. Several lines of evidence (isolates, rate measurements, molecular fingerprinting and biomarkers) suggest that both, aerobic and anaerobic methane oxidation occur at the Black Sea oxic/anoxic interface and in anoxic waters. ANME-1 and ANME-2 groups of Archaea in association with delta-Proteobacteria are possible candidates mediating anaerobic methane oxidation in the water column. However neither consortia present in marine sediments nor direct rates of anaerobic methane oxidation in the water column are known. Literature data suggest a presence of an abundant community of protozoa, mostly ciliates, in the Black Sea redox zone. Many of these microorganisms harbor bacteria as ecto- or endosymbionts. A protozoa-bacteria food web can play an important role in the organic matter transfer between the oxic and anoxic waters and can also have a significant impact on functional characteristics of bacterial communities.