Life on oxidised nitrogen: the biochemistry and physiology of prokaryotic life supported by the nitrate-nitrite redox couple.

In global biogeochemical networks nitrogen transitions between a number of different oxidation states, from +5 to -3. The two most oxidized states are found in the nitrogen oxyanions nitrate (NO₃^-, +5) and nitrite (NO₂^-, +3). These two oxyanions form an electropositive redox couple, with a midpoint redox potential (pH7) of +430 mV, that enables them to serve as both electron acceptor (nitrate) and electron donor (nitrite) in a range of catabolic and anabolic processes. Several enzymatic systems have been identified that can inter-convert the two oxyanions and couple them to a range of electron transport pathways. Recent literature on nitrate reduction and nitrite oxidation by prokaryotes reveals a great number of meta "omics" studies identifying genes, transcripts or peptides functionally related to the nitrate / nitrite redox couple in a wide range of environments. To fully interpret such data in the context of the environment being studied requires a recognition of the different physiological functions the nitrate / nitrite redox couple is able to support. This, in turn, is related to the biochemical diversity of the enzymes that drive this reversible redox couple in nature. This review seeks to define bioenergetically the different enzymes involved in nitrate-nitrite inter-conversion and relate this to the diverse physiological activities that this redox couple supports.