The Mechanism of Symbiotic Nitrogen Fixation

Nitrogen is a building block of life. Molecular nitrogen is the relatively inert atmospheric form of this element, and it must be fixed into more biologically accessible forms in order to be used for organic processes. In total, approximately 380 teragrams of nitrogen per year are fixed by atmospheric, biological, and industrial nitrogen fixation processes. Whereas the Haber–Bosch process currently accounts for the majority of the reduced nitrogen that is used agriculturally with the world’s increasing dependence on agriculture to feed its population, the use of reduced nitrogen derived from energy provided by fossil fuels in not likely to be sustainable. Biological nitrogen fixation is mediated by diazotrophic microorganisms that are capable of fixing atmospheric nitrogen using the enzyme nitrogenase. Much of this is carried out as a symbiotic association between plants and some diazotrophic bacteria. The study of symbiotic nitrogen fixation is an area of research that spans both microbiology and plant biology. Since this is an area that has had a great deal of renewed interest, this chapter reviews what is currently understood about the process of symbiotic nitrogen fixation at the molecular and physiological level from both the plant and bacterial perspective.