Nitrogenase is an enzyme complex that is able to reduce elementary, molecular nitrogen (N2) and thus transform it into a biologically available form. This process is called nitrogen fixation. Nitrogenases occur in various bacteria as well as some archaea. Nitrogen fixing actinomycetes are as well-known as cyanobacteria (e.g. anabaena) and proteobacteria (e.g. azotobacter).