For over a century it was believed that ammonium could only be oxidized by microbes in the presence of oxygen. The possibility of anaerobic ammonium oxidation (anammox) was considered impossible. However, about 10 years ago the microbes responsible for the anammox reaction were discovered in a wastewater plant. This was followed by the identification of the responsible bacteria. Recently, the widespread environmental occurrence of the anammox bacteria was demonstrated leading to the realization that anammox bacteria may play a major role in biological nitrogen cycling. The anammox bacteria are unique microbes with many unusual properties. These include the biological turn-over of hydrazine, a well known rocket fuel, the biological synthesis of ladderane lipids, and the presence of a prokaryotic organelle in the cytoplasma of anammox bacteria. The aim of this project is to obtain a fundamental understanding of the metabolism and ecological importance of the anammox bacteria. Such understanding contributes directly to our environment and economy because the anammox bacteria form a new opportunity for nitrogen removal from wastewater, cheaper, with lower carbon dioxide emissions than existing technology. Scientifically the results will contribute to the understanding how hydrazine and dinitrogen gas are made by the anammox bacteria. The research will show which gene products are responsible for the anammox reaction, and how their expression is regulated. Furthermore, the experiments proposed will show if the prokaryotic organelle in anammox bacteria is involved in energy generation. Together the environmental and metabolic data will help to understand why anammox bacteria are so successful in the biogeochemical nitrogen cycle and thus shape our planets atmosphere. The different research lines will employ state of the art microbial and molecular methods to unravel the exceptional properties of these highly unusual and important anammox bacteria.