Key enzymes, functional genes, and metabolic pathways of the nitrogen removal-related microorganisms

Biological nitrogen removal is a classic technique for removing nitrogen from wastewater that has been studied extensively and gives high nitrogen removal rate, energy efficiency, and is relatively environmentally benign. Using biological nitrogen removal processes to treat the diverse components of wastewater requires a thorough understanding of the microorganisms and pathways involved. However, information regarding the diverse microorganisms participating in nitrogen removal processes remains limited. This review presents the current knowledge of the researches into these microorganisms, including ammonia oxidizing bacteria, archaea, complete ammonia oxidation bacteria, nitrite-oxidizing bacteria, anaerobic ammonium oxidation bacteria, denitrifying bacteria, and heterotrophic nitrification-aerobic denitrification bacteria. The metabolic pathways and enzymatic reactions involved in nitrogen cycle of abovementioned microorganisms are demonstrated. The key enzymes, functional genes, and metabolic pathways of the nitrogen removal-related microorganisms were extensively discussed. Additionally, the advantages of the different microorganisms for the ammonia oxidation, denitrifying, and HN-AD are described. Finally, the limitations and strategies in unraveling the nitrogen metabolic pathways were presented. The aim of the review is to improve our understanding of the principle involved in advanced nitrogen removal processes to allow possible improvements to wastewater treatment processes to be identified.