The bacteria colonize plant cells within root nodules, where they convert atmospheric nitrogen into ammonia using the enzyme nitrogenase and then provide organic nitrogenous compounds such as glutamine or ureides to the plant. The plant, in turn, provides the bacteria with organic compounds made by photosynthesis. This mutually beneficial relationship is true of all of the rhizobia, of which the genus Rhizobium is a typical example. Rhizobium is also capable to solubilize phosphorus.
Rhizobium forms a symbiotic relationship with certain plants such as legumes, fixing nitrogen from the air into ammonia, which acts as a natural fertilizer for the plants. Current research is being conducted by Agricultural Research Service microbiologists to discover a way to use Rhizobium's biological nitrogen fixation. This research involves the genetic mapping of various rhizobial species with their respective symbiotic plant species, like alfalfa or soybean. The goal of this research is to increase the plants' productivity without using fertilizers.
In molecular biology, Rhizobium has also been identified as a contaminant of DNA extraction kit reagents and ultrapure water systems, which may lead to its erroneous appearance in microbiota or metagenomic datasets. The presence of nitrogen-fixing bacteria as contaminants may be due to the use of nitrogen gas in ultra-pure water production to inhibit microbial growth in storage tanks.
The genus Rhizobium comprises the following species:
^ abSilva C, Vinuesa P, Eguiarte LE, Souza V, Martínez-Romero E (November 2005). "Evolutionary genetics and biogeographic structure of Rhizobium gallicum sensu lato, a widely distributed bacterial symbiont of diverse legumes". Molecular Ecology. 14 (13): 4033-50. doi:10.1111/j.1365-294X.2005.02721.x. PMID16262857. S2CID16668742.
^Xu L, Zhang Y, Deng ZS, Zhao L, Wei XL, Wei GH (March 2013). "Rhizobium qilianshanense sp. nov., a novel species isolated from root nodule of Oxytropis ochrocephala Bunge in China". Antonie van Leeuwenhoek. 103 (3): 559-65. doi:10.1007/s10482-012-9840-x. PMID23142858. S2CID18660422.