The denitrification ability and nitrogen metabolism pathway of aerobic denitrifier Marinobacter alkaliphilus SBY-1 under low C/N ratios.

Mariculture tail water is characterized as the low C/N ratios and thus blocks the conventional heterotrophic denitrification process due to insufficient carbon source. Therefore, oligotrophic marine bacteria with heterotrophic nitrification and aerobic denitrification (HN-AD) are urgently required to bioaugment aerobic biological filter. In this study, Marinobacter alkaliphilus SBY-1 was isolated and confirmed optimal nitrate removal capacity at a rate of 716 mg/L·d without ammonia production or nitrite accumulation under initial nitrate concentration of 800 mg/L, pH 7, salinity 20 ‰, sodium acetate as the carbon source, and low C/N ratios of 3.6. SBY-1 also demonstrated heterotrophic nitrification capability with a maximum ammonia removal rate reaching 69.21 % when ammonia was used as the nitrogen source. The enzymes involved in the HN-AD process including ammonia monooxygenase (AMO), nitrate reductase (NR), and nitrite reductase (NIR) were all detected in SBY-1 with superior activity observed for NR and NIR. Additionally, analysis of EPS and auto-aggregation revealed that SBY-1 exhibited excellent auto-aggregation ability under high influent nitrogen concentration conditions, making it more suitable for biofilm formation and further application in biofilm-based denitrification process. Genome analysis identified genes associated with Nar, Nap, Nas, Nir, Nif, Nrt, Nrf, Nor, Nos which confirmed that SBY-1 possessed a complete HN-AD pathway for nitrogen metabolism. The predicted nitrogen metabolism pathway of SBY-1 was NO₃^--N → NO₂^--N → NO→N₂O → N₂. These findings provide new insights into the efficient removal of nitrate by SBY-1 under lower C/N conditions.