Dissimilatory nitrate reduction to ammonium (DNRA), also known as nitrate/nitrite ammonification, is the result of anaerobic respiration by chemoorganoheterotrophic microbes using nitrate (NO3−) as an electron acceptor for respiration. In anaerobic conditions microbes which undertake DNRA oxidise organic matter and use nitrate (rather than oxygen) as an electron acceptor, reducing it to nitrite, then ammonium (NO3−→NO2−→NH4+). Dissimilatory nitrate reduction to ammonium is more common in prokaryotes but may also occur in eukaryotic microorganisms. DNRA is a component of the terrestrial and oceanic nitrogen cycle. Unlike denitrification, it acts to conserve bioavailable nitrogen in the system, producing soluble ammonium rather than unreactive dinitrogen gas. Background and process Cellular process Dissimilatory nitrate reduction to ammonium is a two step process, reducing NO3− to NO2− then NO2− to NH4+, though the reaction may begin with NO2− directly. Each step is mediated by a different enzyme, the first step of dissimilatory nitrate reduction to ammonium is usually mediated by a periplasmic nitrate reductase. The second step (respiratory NO2− reduction to NH4+) is mediated by cytochrome c nitrite reductase, occurring at the periplasmic membrane surface. Despite DNRA not producing N2O as an intermediate during nitrate reduction (as denitrification does) N2O may still be released as a byproduct, thus DNRA may also act as a sink of fixed, bioavailable nitrogen. DNRA's production of N2O may be enhanced at higher pH levels. Denitrification 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. Both denitrifiers and nitrate ammonifiers are competing for NO3− in the environment. 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. This is may be especially pronounced when NO3− is limiting compared to organic carbon, as organic carbon is oxidised more 'efficiently' per NO3− (as each molecule NO3− is reduced farther). The balance of denitrification and DNRA is important to the nitrogen cycle of an environment as both use NO3− but, unlike denitrification, which produces gaseous, non-bioavailable N2 (a sink of nitrogen), DNRA produces bioavailable, soluble NH4+. Marine context Marine microorganisms As dissimilatory nitrate reduction to ammonium is an anaerobic respiration process, marine microorganisms capable of performing DNRA are most commonly