ERP005584 PRJEB6116 ena-STUDY-RU-15-04-2014-08:44:24:015-90 Interactions between anaerobic ammonium and sulfur-oxidizing bacteria in a laboratory scale model system The recycling of fixed nitrogen from marine systems proceeds via two different nitrogen removal pathways, namely denitrification or anaerobic ammonium oxidation (anammox). Both processes require oxidized N (i.e. nitrate or nitrite) as electron acceptor to ultimately release dinitrogen gas (N2). For a supply with its substrates (ammonium and nitrite) anammox activity depends on the tight coupling with other nitrogen transformations. The aerobic oxidation of ammonium is one of the routes that have been described recently as a source of nitrite. Another possibility is the linkage of the aforementioned N2-releasing processes, a scenario in which denitrification might be providing substrates for anammox. We were especially interested in coupling sulfide-dependent denitrification to anammox, as sulfide is, besides oxygen, the most important reductant at the chemocline of marine anoxic basins and also abundant within sediments. Although sulfide is toxic at µM concentrations and has been shown to have a negative effect on anammox activity, we hypothesize that denitrification converts sulfide and nitrate, thereby allowing anammox to stay active despite an influx of sulfide and supply them with nitrite from partial denitrification. The recycling of fixed nitrogen from marine systems proceeds via two different nitrogen removal pathways, namely denitrification or anaerobic ammonium oxidation (anammox). Both processes require oxidized N (i.e. nitrate or nitrite) as electron acceptor to ultimately release dinitrogen gas (N2). For a supply with its substrates (ammonium and nitrite) anammox activity depends on the tight coupling with other nitrogen transformations. The aerobic oxidation of ammonium is one of the routes that have been described recently as a source of nitrite. Another possibility is the linkage of the aforementioned N2-releasing processes, a scenario in which denitrification might be providing substrates for anammox. We were especially interested in coupling sulfide-dependent denitrification to anammox, as sulfide is, besides oxygen, the most important reductant at the chemocline of marine anoxic basins and also abundant within sediments. Although sulfide is toxic at µM concentrations and has been shown to have a negative effect on anammox activity, we hypothesize that denitrification converts sulfide and nitrate, thereby allowing anammox to stay active despite an influx of sulfide and supply them with nitrite from partial denitrification.