Unique nitrification process of heterotrophic nitrification bacteria and its adaptability in extreme environments
Yuran Yang, Kai He, Liuyi Chen, Tuohong Liu, Zhenlun Li
Abstract
Heterotrophic nitrifying bacteria (HNB) are renowned for their high-efficiency nitrogen removal capabilities and exceptional resilience across a broad spectrum of environmental conditions. Despite this, their significant contribution to the nitrogen cycle has yet to be adequately recognized and emphasized. The heterotrophic nitrification pathways of HNB are distinctly unique, manifested in multiple aspects. Firstly, they possess a wide array of nitrification pathways, which contrasts with the more limited pathways of autotrophic nitrifiers. Secondly, the genetic mechanisms underlying these pathways are complex, involving a large number of genes whose coordinated regulation remains poorly understood. Thirdly, the origin of key nitrification intermediates, such as NH2OH, NO2–, and NO3–, is still uncertain, adding to the complexity of elucidating the nitrification process. Finally, HNB directly integrate ammonium assimilation into their heterotrophic nitrification processes, creating a novel and intertwined metabolic network. Furthermore, the coupling of carbon-nitrogen metabolism and the coupling of nitrification and denitrification further intensifies the complexity of the study of heterotrophic nitrification processes. Meanwhile, HNB demonstrate strong tolerance to environmental factors such as metal ions, extreme temperatures, pH variations, different carbon sources, C/N ratios, etc. This can be attributed to their multi-carbon metabolic pathways, fatty acid metabolism, accumulation of compatible substances, ABC transporters, and cation transmembrane transport systems. In the future, research efforts should be directed toward delving into their nitrogen metabolism mechanisms, further clarifying their role in the ecosystem, and developing efficient biological treatment technologies based on HNB. These endeavors will help to fully utilize the potential of HNB in the nitrogen cycle.