Nitrogen metabolic characteristics and adaptive mechanisms of Paracoccus sp. QD-21 under complex nitrogenous...

A novel strain of Paracoccus sp. QD-21, which is capable of simultaneous heterotrophic nitrification and aerobic denitrification, was isolated and investigated for the potential in removal of nitrogen in wastewater treatment. The strain exhibited nitrogen removal rates of 5.55, 3.35, and 2.78 mg/(L·h) for NH₄⁺-N (100 mg/L), NO₂^--N (100 mg/L), and NO₃^--N (100 mg/L), respectively. Notably, QD-21 maintained substantial nitrogen removal efficiency under high concentrations of inorganic nitrogen, highlighting its remarkable tolerance to complex nitrogenous conditions. Optimal nitrogen removal occurred with sodium succinate as carbon source, C/N 7:1, pH 8.41, 140 rpm, 38.41 °C, and inoculum size 4.56% (v/v). Analysis using molecular biology techniques revealed the presence of genes associated with the nitrification process, such as amo and hao, in QD-21. This confirms the nitrification pathway of strain: NH₄⁺-N → NH₂OH → NO₂^--N → NO₃^--N. Additionally, the presence of nirK, norB, and nosZ confirms the denitrification pathway in QD-21: NO₃^--N → NO₂^--N → NO → N₂O → N₂. Meanwhile, the presence of nirBD, nark, glnL, glnA, gltB, and nasA indicates that a portion of nitrogen is assimilated into biomass through the ammonia assimilation pathway, which supports cellular biosynthesis and growth at the expense of metabolic energy. Furthermore, in practical wastewater tests QD-21 achieved removal efficiencies of 75.5% for NH₄⁺-N and 55.8% for COD. Such findings demonstrate the great potential of strain QD-21 in treating nitrogen pollution from diverse sources.