Denitrification - Denitrification By Rhizobia

Denitrification By Rhizobia

Rhizobia are soil bacteria with the unique ability to establish a N₂-fixing symbiosis on legume roots. When faced with a shortage of oxygen some rhizobia species are able to switch from O₂-respiration to using nitrates to support respiration. This denitrification pathway comprises the sequential reduction of nitrate or nitrite to dinitrogen, via the gaseous intermediates nitric oxide and nitrous oxide. The enzymes involved in denitrification are nitrate-, nitrite-, nitric oxide- and nitrous oxide reductase, encoded by nar/nap, nir, nor and nos genes, respectively. In recent years it has emerged that many rhizobia species have genes for enzymes of some or all of the four reductase reactions for denitrification. In fact, denitrification can be readily observed in many rhizobia species, in their free-living form, in legume root nodules, or in isolated bacteroids.