Dissimilatory nitrate reduction to ammonium

[1][2] In anaerobic conditions microbes which undertake DNRA oxidise organic matter and use nitrate (rather than oxygen) as an electron acceptor, reducing it to nitrite, and then to ammonium (NO3− → NO2− → NH4+).

[8] Dissimilatory nitrate reduction to ammonium is similar to the process of denitrification, though NO2− is reduced farther to NH4+ rather than to N2, transferring eight electrons.

Despite the redox potential of dissimilatory nitrate reduction to ammonium being lower than denitrification and producing less Gibbs free energy, energy yield of denitrification may not be efficiently conserved in its series of enzymatic reactions and nitrate ammonifiers may achieve higher growth rates and outcompete denitrifiers.

[1] As dissimilatory nitrate reduction to ammonium is an anaerobic respiration process, marine microorganisms capable of performing DNRA are most commonly found in environments low in O2, such as oxygen minimum zones (OMZs) in the water column, or sediments with steep O2 gradients.

For example, benthic sulfur bacteria in genera such as Beggiatoa and Thioploca inhabit anoxic sediments on continental shelves and obtain energy by oxidizing sulfide via DNRA.

[14] While dissimilatory nitrate reduction to ammonium is more commonly associated with prokaryotes, recent research has found increasing evidence of DNRA in various eukaryotic microorganisms.

[11] The use of DNRA by diatoms is a possible explanation for how they can survive buried in dark, anoxic sediment layers on the ocean floor, without being able to carry out photosynthesis or aerobic respiration.

[16] Within sediments, the total dissimilatory nitrate reduction to ammonium rate is higher in spring and summer compared to autumn.

[22] DNRA also produces NH4+ (in addition to remineralisation) but from organic matter which has been exported from the photic zone; this may be subsequently reintroduced by mixing or upwelling of deeper water back to the surface, thereby, stimulating primary productivity; thus, in areas where high amounts of DNRA is occurring, f-ratio calculations will not be accurate.

The oceanic nitrogen cycle with the role of DNRA. Blue line represents the ocean surface, with the atmosphere above. Notice how NH 4 produced by DNRA can be taken up by biota and converted into organic nitrogen, while N 2 produced by denitrification is removed from the system, and may only re-enter via nitrogen fixation . Organisms undertaking denitrification and DNRA compete for NO 3 and the balance of the two processes depends on the abiotic and biotic conditions of the ecosystem (see section ‘DNRA in the Marine Context’, subheading ‘Ecological Role’).