The largest reservoir for nitrogen on earth is the atmosphere that contains 78 percent nitrogen gas. Until now the only known biological process interacting with elemental nitrogen is the bacterial reduction of nitrogen to ammonia for the build up of biomass (nitrogen fixation). This reaction requires energy and is only carried out in the absence of other nitrogen sources, such as ammonia or nitrate. Thermodynamically, the oxidation of nitrogen to nitrate with oxygen releases reasonable amounts of energy, but no bacterium using this redox couple has been known until today. We have isolated a marine bacterium, which is capable of growing in the dark with nitrogen gas as electron donor and oxygen as electron acceptor while forming nitrate. As this microorganism can also use carbondioxide as a carbon source it basically lives of air. While oxidizing atmospheric nitrogen gas the bacterium releases large amounts of nitrate and thereby enhances the amount of fixed nitrogen available for other organisms. At the moment the apparent flux of elemental nitrogen to the ocean by bacterial nitrogen fixation is much smaller than the loss of nitrogen through bacterial denitrification, suggesting that we are missing a major input of nitrogen. This newly discovered physiology of nitrogen oxidation could close this large gap in our understanding of the nitrogen cycle. The amount of biological available nitrogen determines the amount of biomass that can be build up by living organisms. Therefore, it is crucial to know the nitrogen flux into the biosphere, to understand the balances in the carbon cycle. In this project I propose to study this new bacterial physiology in order to understand, which factors control the activity of nitrogen oxidizing bacteria. We need to know how widespread these bacteria are, to estimate their influence on the global nitrogen cycle, and I propose to investigate the interactions between nitrogen oxidizers and other relevant bacteria of the nitrogen cycle.