Nitrogen removal characteristics and auto-aggregation capacity of the heterotrophic nitrification–aerobic denitrification bacterium, Bacillus amyloliquefaciens N8

At present, the main factors restricting the development of high-salinity recirculating aquaculture systems are high costs and the control of inorganic nitrogen pollutants, but only a few studies have reported on salt-tolerant HN-AD strains and their denitrification mechanisms and quantification capabilities. Strain N8 was obtained from seawater biofilter and identified as Bacillus amyloliquefaciens based on the analysis of 16S rRNA gene sequence. Strain N8 has a good ability to remove ammonia nitrogen, nitrite and nitrate at three different concentrations (20, 50, 100 mg L⁻¹) with little accumulation of intermediates, and the maximum removal rate of ammonia nitrogen (50 mg L⁻¹), nitrite (100 mg L⁻¹), and nitrate (100 mg L⁻¹) were 96.56 %, 99.20 % and 90.20 % after 72 h incubation. Through inhibitor and nitrogen balance experiments, it was found that the first step of degradation of ammonia nitrogen by strain N8 was to oxidize it to hydroxylamine, and 72.28 % of ammonia nitrogen and 46.31 % of nitrate nitrogen were eventually converted into intracellular nitrogen. Two denitrification related genes, nirK and nirS, were successfully amplified from strain N8. The ideal conditions for growth and nitrogen removal of strain N8 were sodium succinate as the carbon source, C/N of 20, and salinity between 0 and 30 ‰. Furthermore, the highest auto-aggregation rate of the strain was above 25 % under different nitrogen sources, and the biofilm growth amount of the strain was 3.04 after 24 h culture. All results demonstrate that strain N8 has great practical application potential in nitrogen removal in high-salinity recirculating aquaculture systems.