Enhancement of nitrite production via addition of hydroxylamine to partial denitrification (PD) biomass: Functional genes dynamics and enzymatic activities

This study investigated the activity of partial denitrification (PD) biomass/key enzymes, functional gene expressions in response to 0 ~ 50 mg/L hydroxylamine (NH 2 OH) addition. Results indicated that NH 2 OH contributed to nitrite (NO 2 − -N) production, facilitating the maximum increase of nitrate (NO 3 − -N) to NO 2 − -N transformation ratio to 80.47 ± 2.82%, leading to 2.56-fold NO 2 − -N higher than those of control. The observed transient inhibitory effect on NO 3 − -N reduction process was attributed by high-level NH 2 OH (35 ~ 50 mg/L). Enzymatic assays revealed the enhanced activity of both NO 3 − -N and NO 2 − -N reductase while the former showed obvious superiority which led to high NO 2 − -N accumulation. These results were further confirmed by the corresponding functional genes ( nar G, nap A, nir S and nir K). Besides, negative influence of NH 2 OH addition was limited to PD aggregates, due to the increasing secretion of extracellular polymeric substances (EPS) as well as proteins/polysaccharides ratios in tightly-bound structure of EPS.