The nitrogen (N) cycle is a fundamental biogeochemical process that regulates the availability of nitrogen in ecosystems, with implications for nitrogen retention, greenhouse gas emissions, and overall ecosystem health. The major microbial processes involved in nitrogen transformation are discussed, with particular emphasis on nitrate reduction, nitrite reduction, and nitrite oxidation. The pathways of nitrate and nitrite reduction, including denitrification, dissimilatory nitrate reduction to ammonium (DNRA), and anaerobic ammonium oxidation (anammox) are examined in detail, highlighting the enzyme structures involved, their biochemical mechanisms, environmental significance, and microbial diversity. The role of nitrite-oxidizing bacteria (NOB) in nitrite oxidation - a crucial step in nitrification - is explored, emphasizing its contribution to nitrogen cycling and ecosystem stability. Recent advancements in genomics, metagenomics, and structural biology have significantly enhanced our understanding of these microbial processes, leading to the discovery of novel enzymes and previously unidentified microbial players, along with the discovery of alternative nitrite transformation mechanisms. This growing body of knowledge reveals an intricate network of interconnected pathways that regulate nitrogen fluxes in the environment, and transformed the ‘black box’ problem into a well-defined system, where the players are structurally defined and mechanistic approaches can be discussed. However, the regulation and control of these complex microbial pathways remain a significant research challenge.
In summary, nitrate reduction and nitrite oxidation are biochemically distinct yet functionally interconnected processes essential for maintaining nitrogen balance. The physiological significance of NO₃⁻/NO₂⁻/NO pathways is also acknowledged across a wide range of life forms, including higher plants and bacteria.
References
1. Maia L.B; Moura I.; Moura J.J.G. in Molybdenum and Tungsten Enzymes: Biochemistry, RSC Metalobiology Series No. 5. Eds Hille, Schulzke, Kirk, RSC, 1-80, 2017. 2. Maia L.; Moura J.J.G. Chem. Rev. 2014, 114, 5273. 3. Maia L.; Moura I.; Moura J.J.G. Inorg. Chim. Acta 2017, 455, 350. 4. Maia L.B; Moura I.; Moura J.J.G, Series on Chemistry, Energy and the Environment Synthesis and Applications in Chemistry and Materials, pp. 3-37 (2024), World Scientific Pub. Company.
Acknowledgements
This work was supported by the Associate Laboratory for Green Chemistry - LAQV, which is financed by national funds from FCT, MCTES (UID/QUI/50006/2019).