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Abstract
Autotrophic denitrification was measured in the southwestern coastal Black Sea, where the Bosporus Plume injects oxidized chemical species (especially O2 and ) into the oxic, suboxic, and anoxic layers. Prominent oxygen intrusions caused an overlap of and sulfide at the same station where autotrophic denitrification activity was detected with incubation experiments. Several bacteria that have been proposed to oxidize sulfide in other low oxygen environments were found in the Black Sea including SUP05, Sulfurimonas, Arcobacter, and BS-GSO2. Comparison of TRFLP profiles from this mixing zone station and the Western Gyre (a station not affected by the Bosporus Plume) indicate the greatest relative abundance of Sulfurimonas and Arcobacter at the appropriate depths at the mixing zone station. The autotrophic gammaproteobacterium BS-GSO2 correlated with ammonium fluxes rather than with sulfide fluxes and the maximum in SUP05 peak height was shallower than the depths where autotrophic denitrification was detected. Notably, anammox activity was not detected at the mixing zone station, though low levels of DNA from the anammox bacteria Candidatus Scalindua were present. These results provide evidence for a modified ecosystem with different N2 production pathways in the southwest coastal region compared to that found in the rest of the Black Sea. Moreover, the same Sulfurimonas phylotype (BS139) was previously detected on >30 μm particles in the suboxic zone of the Western Gyre along with DNA of potential sulfate reducers, so it is possible that particle-attached autotrophic denitrification may be an overlooked N2 production pathway in the central Black Sea as well.
Highlights
Three processes are responsible for N2 production under low oxygen conditions: (1) heterotrophic denitrification, which converts nitrate to N2 using organic matter as a reductant; (2) anammox, an autotrophic process which reduces nitrite with ammonium to form N2; and (3) autotrophic denitrification, which converts nitrate to N2 using reduced sulfur species as a reductant
The potential for S cycling in suboxic waters without the build up of sulfide has recently been demonstrated in the Chilean Oxygen Minimum Zone (Canfield et al, 2010), and DNA from potential sulfate reducers and sulfide oxidizers were found attached to large particulate matter in the Black Sea suboxic zone (Fuchsman et al, 2011)
When we look at all profiles over five stations instead of just the two depths in 2005, we see that the relative abundances of BS139 from the Sulfurimonas genus of epsilonproteobacteria, BS077 from the SUP05 group of gammaproteobacteria, BS098 from the Arcobacter genus of epsilonproteobacteria, and unidentified MspI peak 91/Mnl peak 254 pair are clearly greater in the mixing zone station (Figures 7 and 8)
Summary
Three processes are responsible for N2 production under low oxygen conditions: (1) heterotrophic denitrification, which converts nitrate to N2 using organic matter as a reductant; (2) anammox, an autotrophic process which reduces nitrite with ammonium to form N2; and (3) autotrophic denitrification, which converts nitrate to N2 using reduced sulfur species as a reductant. In both heterotrophic and autotrophic denitrification, nitrate is reduced using the same pathway with N2O as an intermediate product (Sievert et al, 2008). The potential for S cycling in suboxic waters without the build up of sulfide has recently been demonstrated in the Chilean Oxygen Minimum Zone (Canfield et al, 2010), and DNA from potential sulfate reducers and sulfide oxidizers were found attached to large particulate matter in the Black Sea suboxic zone (Fuchsman et al, 2011)
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