Pangenome-Scale Mathematical Modelling of ANAMMOX Bacteria Metabolism

Removal of fixed nitrogen compounds such as ammonium and nitrite from wastewater is of critical importance for balancing the nitrogen cycle and protecting aquatic environments from eutrophication. ANaerobic AMMonium OXidising (ANAMMOX) bacteria have recently been employed for fixed nitrogen removal purposes in wastewater treatment processes. These specialised bacteria convert ammonium and nitrite into nitrogen gas anaerobically, thereby reducing the amount of energy required for aeration in conventional wastewater treatment processes. However, slow growth rates of ANAMMOX remain a major obstacle towards their widespread use in industrial wastewater treatment processes. Thus, a pangenome-scale, constraint-based metabolic model, iRB399, of ANAMMOX bacteria has been developed to design strategies for accelerating their growth. The main metabolic limitation was identified in the energy metabolism of these bacteria, concerning the production of ATP. The extremely low efficiency of the electron transport chain combined with very high growth-associated maintenance energy is likely to be responsible for the slow growth of ANAMMOX. However, different ANAMMOX species were found to conserve energy using a variety of different redox couples, and the modelling simulations revealed their comparative advantages under different growth conditions. iRB399 also identified dispensable catabolic reactions that have demonstrably beneficial effects on enhancing the growth rates of ANAMMOX bacteria. Thus, the pangenome-scale model will not only help identify and overcome metabolic limitations of ANNAMOX bacteria, but also provide a valuable resource for designing efficient ANNAMOX-based wastewater treatment processes.