Denitrifying sulfide removal (DSR) process simultaneously removes nitrate, sulfide and organic matters in the same reactor. This study applied Azoarcus sp. NSC3 and Pseudomonas sp. CRS1 mix for DSR tests in autotrophic, heterotrophic and mixotrophic growths. Negligible NO-compounds were noted in heterotrophic or mixotrophic growths, while most cells were damaged and bound with NO-compounds in autotrophic growth. Nitroprusside (SNP) ions were applied as model compound to reveal the formation of nitrososulfide complex (RSNO) by nitroso (NO+) and excess sulfide (S-2 ), rather than the previously proposed mechanism by direct reaction between nitric oxide (NO) and S-2 . We speculated that RSNO was then abiotically decomposed to NO and elemental sulfur in the presence of biological cells. A revised nitrogen cycle considering interactions with sulfur compounds was proposed. We also speculated that SNO and NO were inhibitory to the functional strains, whose efficient removals were essential to reach high-rate DSR performance. (C) 2014 Elsevier Ltd. All rights reserved.