Sulfide-induced dissimilatory nitrate reduction to ammonia in anaerobic freshwater sediments

Abstract: Different reduced sulfur compounds (H₂S, FeS, S₂O₃^2?) were tested as electron donors for dissimilatory nitrate reduction in nitrate-amended sediment slurries. Only in the free sulfide-enriched slurries was nitrate appreciably reduced to ammonia (     ), with concomitant oxidation of sulfide to S⁰ (     ). The initial concentration of free sulfide appears as a factor determining the type of nitrate reduction. At extremely low concentrations of free S^2? (metal sulfides) nitrate was reduced via denitrification whereas at higher S^2? concentrations, dissimilatory nitrate reduction to ammonia (DNRA) and incomplete denitrification to gaseous nitrogen oxides took place. Sulfide inhibition of NO- and N₂O- reductases is proposed as being responsible for the driving part of the electron flow from S^2? to NH₄⁺.