page_title: Dissimilatory nitrate reduction to ammonium
section_title: Marine microorganisms
hierarchical_section_title: Dissimilatory nitrate reduction to ammonium / Marine context / Marine microorganisms
caption_attribution_description: English: &#160;The oceanic nitrogen cycle with the role of DNRA.&#160;Blue line represents ocean surface, with the atmosphere above. Notice how NH4 produced by DNRA can be up-taken by biota into organic nitrogen, while N2 produced by denitrification is removed from the system, and may only re-enter via nitrogen fixation. It should also be noted that organisms undertaking denitrification and DNRA compete for NO3 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’). &#160;
context_page_description: Dissimilatory nitrate reduction to ammonium, also known as nitrate/nitrite ammonification, is the result of anaerobic respiration by chemoorganoheterotrophic microbes using nitrate as an electron acceptor for respiration. In anaerobic conditions microbes which undertake DNRA oxidise organic matter and use nitrate as an electron acceptor, reducing it to nitrite, then ammonium.
  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.
context_section_description: 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.
  DNRA has been documented in prokaryotes inhabiting the upper layer of marine sediments. 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. These bacteria are able to carry out DNRA using intracellular nitrate stored in vacuoles. The direct reduction of nitrate to ammonium via dissimilatory nitrate reduction, coupled with the direct conversion of ammonium to dinitrogen via Anammox, has been attributed to significant nitrogen loss in certain parts of the ocean; this DNRA-Anammox coupling by DNRA and Anammox bacteria can account for nitrate loss in areas with no detectable denitrification, such as in OMZs off the coast of Chile, Peru, and Namibia, as well as OMZs over the Omani Shelf in the Arabian Sea. While denitrification is more energetically favourable than DNRA, there is evidence that bacteria using DNRA conserve more energy than denitrifiers, allowing them to grow faster. Thus, via DNRA-Anammox coupling, bacteria using DNRA and Anammox may be stronger competitors for substrates than denitrifiers.
  While dissimilatory nitrate reduction to ammonium is more commonly associated with prokaryotes, recent research has found increasing evidence of DNRA in various eukaryotic microorganisms. Of the known DNRA-capable fungal species, one is found in marine ecosystems; an isolate of ascomycete Aspergillus terreus from an OMZ of the Arabian Sea has been found to be capable of performing DNRA under anoxic conditions. Evidence of DNRA has also been found in marine foraminifers.
  More recently, it has been discovered that using intracellular nitrate stores, diatoms can carry out dissimilatory nitrate reduction to ammonium, likely for short-term survival or for entering resting stages, thereby allowing them to persist in dark and anoxic conditions. However, their metabolism is probably not sustained by DNRA for long-term survival during resting stages, as these resting stages often can be much longer than their intracellular nitrate supply would last. 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. Currently, DNRA is known to be carried out by the benthic diatom Amphora coffeaeformis, as well as the pelagic diatom Thalassiosira weissflogii. As diatoms are a significant source of oceanic primary production, the ability for diatoms to perform DNRA has major implications on their ecological role, as well as their role in the marine nitrogen cycle.
caption: The oceanic nitrogen cycle with the role of DNRA. Blue line represents the ocean surface, with the atmosphere above. Notice how NH4 produced by DNRA can be taken up by biota and converted into organic nitrogen, while N2 produced by denitrification is removed from the system, and may only re-enter via nitrogen fixation. Organisms undertaking denitrification and DNRA compete for NO3 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’).
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