Nitrogen removal capability and mechanism of a novel heterotrophic nitrifying–aerobic denitrifying strain H1 as a potential candidate in mariculture wastewater treatment

The nitrogen removal performance and mechanisms of Bacillus subtilis H1 isolated from a mariculture environment were investigated. Strain H1 efficiently removed NH4+-N, NO2--N, and NO3--N in simulated wastewater with removal efficiencies of 85.61%, 90.58%, and 57.82%, respectively. Strain H1 also efficiently degraded mixed nitrogen (NH4+-N mixed with NO2--N and/or NO3--N) and had removal efficiencies ranging from 82.39 to 89.54%. Nitrogen balance analysis revealed that inorganic nitrogen was degraded by heterotrophic nitrification-aerobic denitrification (HN-AD) and assimilation. 15N isotope tracing indicated that N2O was the product of the HN-AD process, while N2 as the final product was only detected during the reduction of 15NO2--N. The nitrogen assimilation and dissimilation pathways by strain H1 were further clarified using complete genome sequencing, nitrification inhibitor addition, and enzymatic activity measurement, and the ammonium oxidation process was speculated as NH4+ → NH2OH → NO → N2O. These results showed the application prospect of B. subtilis H1 in treating mariculture wastewater.