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

Different reduced sulfur compounds (H2S, FeS, S2O32-) 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 (0.30 mu mol NH4+/mu mol NO3-), with concomitant oxidation of sulfide to S-0 (0.55 mu mol S-0/mu mol S-2(-)). The, initial concentration of free sulfide appears as a factor determining the type of nitrate reduction. At extremely low concentrations of free S2- (metal sulfides) nitrate was reduced via denitrification whereas at higher S2- 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 S2- to NH4+.