We love to study the hidden world of microbes and are particularly excited to investigate microbes directly in their natural environment. My team is interested in many aspects of the nitrogen cycle. Bioavailable nitrogen is essential for all organisms and is the main limiting nutrient for life on our planet. The process of nitrification — the oxidation of ammonia to nitrate by way of nitrite — links the gain and loss of bioavailable nitrogen and thus plays a central part in the nitrogen cycle. Since the first description of nitrifying microbes more than 100 years ago by Sergei Winogradsky nitrification was thought to be conducted by the joint activity of two groups of microorganisms. We recently discovered that complete nitrifiers exist that can oxidize as single microorganisms ammonia to nitrate. These so-called Comammox (complete ammonia oxidizers) microorganisms are members of the genus Nitrospira and are widespread in terrestrial and freshwater habitats. We continue to develop innovative single cell tools for investigating the identity and function of these individual microbial cells within their natural habitats, and to characterize in detail the biochemistry, (eco)physiology, and ecology of these fascinating microbes.