Molecular Microbiology Research 2024, Vol.14, No.4, 198-207 http://microbescipublisher.com/index.php/mmr 200 3.2 Denitrification and Anammox Processes 3.2.1 Pathways and enzymatic mechanisms Denitrification and anaerobic ammonium oxidation (anammox) are key processes that contribute to nitrogen loss in marine environments. Denitrification involves the reduction of nitrate to nitrogen gas (N2) through intermediate steps involving nitrite, nitric oxide (NO), and nitrous oxide (N2O) (Figure 1). This process is facilitated by a variety of denitrifying bacteria that possess specific enzymes such as nitrate reductase, nitrite reductase, nitric oxide reductase, and nitrous oxide reductase (Wu et al., 2021). Anammox bacteria, such as those from the genus Candidatus Brocadia, oxidize ammonium (NH4 +) using nitrite as an electron acceptor to produce nitrogen gas (N2) (Kartal et al., 2007; Wang et al., 2019). These bacteria possess unique enzymes, including hydrazine synthase and hydrazine dehydrogenase, which facilitate the anammox reaction (Harb et al., 2021). Figure 1 The main studied processes of the N cycle in different marine environments (Adopted from Pajares and Ramos, 2019) Image caption: Every colored arrow represents a N transformation: N2 fixation (red), nitrification (light blue), nitrate reduction (violet), DNRA (magenta), denitrification (aquamarine), N-damo (green), and anammox (orange). Black curved arrows represent physical processes such as advection and diffusion (Adopted from Pajares and Ramos, 2019)
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