The discovery of anammox bacteria in the 1990s changed our understanding of the global nitrogen cycle (1). These extraordinary microorganisms with unusual morphology derive their energy from the oxidation of ammonium coupled with nitrite reduction, which relies on highly toxic intermediates such as hydrazine and nitric oxide (2). Approximately 50% of the dinitrogen gas released is produced by anammox bacteria. In biotechnology, the Anammox process is being used as a sustainable alternative to current wastewater treatment systems for the removal of nitrogen compounds.We have made significant contributions to elucidating the nature of the catabolic pathway and characterizing the key soluble enzymes. Central to the harvesting of energy from hydrazine is the hydrazine dehydrogenase complex, which converts hydrazine to dinitrogen gas, releasing four extremely low-potential electrons (-750 mV) (3). In addition, anammox bacteria obtain additional reducing equivalents from the oxidation of nitrite to nitrate, catalyzed by a nitrite oxidoreductase (NXR) (4). However, little is known about the membrane complexes at the heart of this bioenergetic process. Nature of the membrane complexes will be elucidated using cryo-electron tomography, sub-tomogram averaging and segmentation. This technique will provide insights into the native landscape at molecular resolution and the bioenergetics of this largely unexplored organism. In particular, protein complexes in the respiratory chain can be differentiated and their organization studied in situ.

(1) Kuypers, M. M. M., et al. The microbial nitrogen-cycling network. Nat. Rev. Microbiol. 16, 263-276 (2018)

(2) Kartal, B., et al. (2013) How to make a living from anaerobic ammonium oxidation. FEMS Microbiol. Rev.37, 428-461

(3) Akram, M., et al. (2019) A 192-heme electron transfer network in the hydrazine dehydrogenase complex. Sci Adv 5, eaav4310

(4) Chicano, T. M., et al. (2021) Structural and functional characterization of the intracellular filament-forming nitrite oxidoreductase multiprotein complex. Nat. Microbiol. 6, 1129-1139