The microbial nitrogen-cycling network

Nitrogen-transforming microorganisms shape global biogeochemical nutrient cycles. In this Review, Kuypers, Marchant and Kartal explore the vast diversity of these microorganisms and their enzymes, highlighting novel pathways, and discuss how nitrogen-transforming microorganisms form complex nitrogen-cycling networks in different environments. Nitrogen is an essential component of all living organisms and the main nutrient limiting life on our planet. By far, the largest inventory of freely accessible nitrogen is atmospheric dinitrogen, but most organisms rely on more bioavailable forms of nitrogen, such as ammonium and nitrate, for growth. The availability of these substrates depends on diverse nitrogen-transforming reactions that are carried out by complex networks of metabolically versatile microorganisms. In this Review, we summarize our current understanding of the microbial nitrogen-cycling network, including novel processes, their underlying biochemical pathways, the involved microorganisms, their environmental importance and industrial applications.