Conversion of the highly stable (“inert”) dinitrogen (N2) molecule to biologically available (“reactive”) nitrogen, a process called “fixation”, is difficult. Fixation is achieved in soil and water by specialised bacteria which can reduce atmospheric dinitrogen to ammonia (NH3) or ammonium (NH4+) (Figure A.1). Converting dinitrogen into NH3/NH4+ is the role of “nitrogen-fixing bacteria”. Microorganisms early on in the Earth's history developed the ability to use enzymes to produce or "fix" NH4+ from dinitrogen, possibly because the availability of nitrogen through abiotic routes were biologically limiting (McRose et al., 2017). Nitrogen-fixing prokaryotes (bacteria and archaea) live in water (e.g. cyanobacteria), in soil (e.g. Azotobacter), in association with plants (e.g. Azospirillum), or in symbiosis with leguminous plants such as peas, clover and soyabeans (e.g. Rhizobium). In the latter case, the prokaryote shares the nitrogen with the plant; in exchange, the plant supplies the prokaryote with a source of carbon and energy for growth.