Diazotrophs
Diazotrophs are a group of prokaryotic organisms with the ability to fix atmospheric nitrogen into ammonium or ammonia, forms usable to plants. There are two types of terrestrial diazotrophs: those free living in the soil, and those that form symbiotic relationships with plants.
Nitrogen makes up the majority of the Earth’s atmosphere, but is mostly found in a form unusable to organisms. Atmospheric nitrogen (N2) is found as two nitrogen atoms held together by a triple bond. In this form, nitrogen is inaccessible. Diazotrophs have the ability to split, or “fix” these bonds, freeing the nitrogen molecules.
Free Living Diazotrophs
Symbiotic Diazotrophs
The enzymes that are needed to fix nitrogen are easily damaged by oxygen, so some diazotophs form symbiotic relationships with plants. In this relationship, diazaotophs are protection from oxygen's damaging properties, while also being supplied carbon, and energy. In exchange for this, plants are able to benefit from the nitrogen.
The Rhizosphere
Nod Factor
Bacteria from the genus Rhizobium form this relationship through the signaling of the nodulation, or nod, factor.
References
[1] Streng, Arend, et al. “Evolutionary Origin of Rhizobium Nod Factor Signaling.” Plant Signaling & Behavior, vol. 6, no. 10, 2011, pp. 1510–1514., doi:10.4161/psb.6.10.17444.