N-cycle scheme. NO3- is used as nitrogen source for growth under aerobic conditions using an assimilatory NO3- reductase, while it acts as an electron acceptor to eliminate excess of reductant power through dissimilatory NO3- reduction. Dissimilatory NO3- reduction, NO3- respiration or denitrification are often used equivalently in the literature. However, dissimilatory pathway makes reference to non-assimilatory reactions that are not directly coupled to generation of proton-motive force. In some Enterobacteriaceae, NO2- is reduced to NH4+ which is then excreted; this process is known as NO3-/NO2- ammonification. Specialised organisms are able to oxidize either NH4+ or NO2- by using a pathway called nitrification, while other organisms such as some planctomycetes oxidize NH4+ and utilize NO2- as respiratory electron acceptor in a pathway named anammox. Finally, (di)nitrogen fixation allows several bacteria and archaea to reduce N2 to NH4+ to provide N-requirements.