Anammox and beyond.

When looking back and wonder how we did it, I became even more aware of how my wanderings in microbiology are all linked, from the start of my PhD with Hans Veldkamp on sulphur-oxidizing bacteria in chemostats. My interests broadened from obligate chemolithoautotrophic bacteria to facultative organisms and the question about the ecological niches of these different metabolic types. The sulphide oxidizing bacteria also may be used to produce elemental sulphur, which can easily be removed from wastewater. This fitted in a long-standing collaboration with Dimitry Sorokin on the ecophysiology and application of alkaliphilic sulphur bacteria. Then came the denitrifying sulphur-oxidizing bacteria and their application to remove sulphide from wastewater, which lead to our interest in nitrate, nitrite and ammonium removal in general. The big surprise was the serendipitous discovery of the 'anammox'-process, whereby ammonium is anaerobically oxidized to dinitrogen gas with nitrite as electron acceptor. The early days of our anammox research are the main focus of this article, which describes the struggle of growing and identifying the most peculiar bacteria we ever came across. A specialized organelle, the anammoxosome was shown to be responsible for the key ammonium oxidation, whereby a rocket fuel, hydrazine, turned out to be an intermediate. Soon after we became aware that anammox is everywhere and in the marine environment makes up a major portion of the nitrogen cycle. The intense scientific collaboration with Mike Jetten and Mark van Loosdrecht and colleagues led to our further understanding and application of this fascinating process, which is briefly summarized in this article. My broader interest in environmental microbiology and microbial ecology has been a regularly returning theme, taking me all over the world to great collaborations lasting to this very day.