Insights into the nitrogen transformation mechanism of Pseudomonas sp. Y15 capable of heterotrophic nitrification and aerobic denitrification

Excessive nitrogen (N) discharged in water is a major cause of eutrophication and other severe environmental issues. Biological N removal via heterotrophic nitrification and aerobic denitrification (HN-AD) has drawn particular attention, owing to the merit of concurrent nitrification and denitrification inside one cell. However, the mechanisms underlying N transformation during HN-AD remain unclear. In the present study, the HN-AD Pseudomonas sp. Y15 was isolated to explore the N distribution and flow, based on stoichiometry and energetics. The total N removal efficiency of Y15 increased linearly with C/N ratio (in the range of 5–15) to ∼96.8%. Of this, ∼32.2% and ∼64.6% were transformed into gas-N and biomass-N, respectively. The observation of a new intracellular N metabolic bypass (NO → NO2) addressed the substantial gaseous N production during HN-AD. Regarding energetics, a large portion of the biomass-N is ascribed to the synthesis of amino acids, which consumes low energy. Finally, two novel stoichiometric equations for different N sources were proposed, to describe the overall HN-AD process. This study deepens the fundamental knowledge on HN-AD bacteria and enlightens their use in treating N-contaminated wastewater.