Sulfide production by sulfate reducing bacteria (SRB) in wastewater biofilms can induce corrosion in pipes and valves of treatment plants. Nitrate addition has been shown to suppress sulfide production in biofilms with varying success. The detailed effect of nitrate on bacterial activity and community structure was studied in a bench-scale biofilm reactor inoculated with biofilm from a wastewater treatment plant. Biofilm were grown on plastic Kaldnes carriers in anoxic, synthetic wastewater containing sulfate. Once steady-state conditions were reached, nitrate was added continuously to the reactor for 10 days. Gradients of nitrate and sulfide were recorded with microsensors before and after the addition of nitrate, and changes in community composition and biofilm activity were monitored on DNA and RNA level by denaturing gradient gel electrophoresis of 16S rRNA, sulfite reductase(dsrB), and periplasmic nitrate reductase(napA). Addition of nitrate did not inhibit sulfide production although sulfate reduction and dsrB expression were suppressed, suggesting that sulfide production occurred from the reduction of other sulfur compounds. Nitrate induced a community-shift of SRB from Desulfobacter-like SRB to Desulfomicrobium-like species. Fluoresence in situ hybridization of SRB confirmed the shift and furthermore demonstrated that SRB, after nitrate addition, were overgrown by other, presumably nitrate reducing bacteria. Genes and transcripts of the periplasmic nitrate reductase could only be detected after nitrate addition. These results strongly suggest that nitrate serves as an inhibitor of certain species of SRB but does not eliminate sulfide production in wastewater biofilm.
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12th International Symposium on Microbial Ecology: Microbial Diversity - Sustaining the Blue Planet, 2008