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). Currently, using single-particle cryo-electron microscopy and cryo-electron tomography, we are working on uncovering the complete atomic structure of the Surface-layer protein (SLPs), which form a para-crystalline layer covering the entire anammox cell. SLPs are the most abundant macromolecules in anammox bacteria and play several roles such as membrane scaffolding and external cell protection. In the context of anammox bacteria, we believe that SLPs also play a role in nutrient uptake by sequestering ammonium and nitrite ions inside the cells, which are essential for the anaerobic ammonium oxidation process.