Editing Nitrogen cycle (section)

== Marine nitrogen cycle ==
[[File:Marine Nitrogen Cycle.jpg|thumb|upright=1.8|{{center|'''Marine nitrogen cycle'''}}]]
[[File:Main marine nitrogen cycles.jpg|thumb|upright=1.8| The main studied processes of the N cycle in different marine environments. Every coloured arrow represents a N transformation: {{chem|N|2}} fixation (red), nitrification (light blue), nitrate reduction (violet), DNRA (magenta), denitrification (aquamarine), N-damo (green), and anammox (orange). Black curved arrows represent physical processes such as advection and diffusion.<ref name=Pajares2019>{{cite journal | last1 = Pajares Moreno | first1 = S. | last2 = Ramos | first2 = R. | year = 2019 | title = Processes and Microorganisms Involved in the Marine Nitrogen Cycle: Knowledge and Gaps | url = | journal = Frontiers in Marine Science | volume = 6 | issue = | page = 739 | doi = 10.3389/fmars.2019.00739 | doi-access = free | bibcode = 2019FrMaS...6..739P }} [[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>]]

[[File:Marine nitrogen cycle under future ocean acidification.jpg|thumb|upright=1.8|{{center|'''Marine nitrogen cycle under future ocean acidification'''{{hsp}}<ref name="O'Brien2016" />}}]]

The nitrogen cycle is an important process in the ocean as well. While the overall cycle is similar, there are different players<ref name="Moulton 2016" /> and modes of transfer for nitrogen in the ocean. Nitrogen enters the water through the precipitation, runoff, or as {{chem|N|2}} from the atmosphere. Nitrogen cannot be utilized by [[phytoplankton]] as {{chem|N|2}} so it must undergo nitrogen fixation which is performed predominately by [[cyanobacteria]].<ref name="Miller 2008 60–62" /> Without supplies of fixed nitrogen entering the marine cycle, the fixed nitrogen would be used up in about 2000 years.<ref name="Gruber 2008 1–35" /> Phytoplankton need nitrogen in biologically available forms for the initial synthesis of organic matter.  Ammonia and urea are released into the water by excretion from plankton. Nitrogen sources are removed from the [[Photic zone|euphotic zone]] by the downward movement of the organic matter. This can occur from sinking of phytoplankton, vertical mixing, or sinking of waste of vertical migrators. The sinking results in ammonia being introduced at lower depths below the euphotic zone. Bacteria are able to convert ammonia to nitrite and nitrate but they are inhibited by light so this must occur below the euphotic zone.<ref name="Miller 2008" /> Ammonification or [[Mineralization (geology)|Mineralization]] is performed by bacteria to convert organic nitrogen to ammonia. [[Nitrification]] can then occur to convert the ammonium to nitrite and nitrate.<ref name="Boyes 2011" /> Nitrate can be returned to the euphotic zone by vertical mixing and upwelling where it can be taken up by phytoplankton to continue the cycle. {{chem|N|2}} can be returned to the atmosphere through [[denitrification]].

Ammonium is thought to be the preferred source of fixed nitrogen for phytoplankton because its assimilation does not involve a [[redox]] reaction and therefore requires little energy. Nitrate requires a redox reaction for assimilation but is more abundant so most phytoplankton have adapted to have the enzymes necessary to undertake this reduction ([[nitrate reductase]]). There are a few notable and well-known exceptions that include most ''[[Prochlorococcus]]'' and some ''[[Synechococcus]]'' that can only take up nitrogen as ammonium.<ref name="Gruber 2008 1–35" />

The nutrients in the ocean are not uniformly distributed. Areas of upwelling provide supplies of nitrogen from below the euphotic zone. Coastal zones provide nitrogen from runoff and upwelling occurs readily along the coast. However, the rate at which nitrogen can be taken up by phytoplankton is decreased in [[oligotrophic]] waters year-round and temperate water in the summer resulting in lower primary production.<ref name="Parsons 1997" /> The distribution of the different forms of nitrogen varies throughout the oceans as well.

Nitrate is depleted in near-surface water except in upwelling regions. Coastal upwelling regions usually have high nitrate and [[chlorophyll]] levels as a result of the increased production. However, there are regions of high surface nitrate but low chlorophyll that are referred to as [[HNLC]] (high nitrogen, low chlorophyll) regions. The best explanation for HNLC regions relates to iron scarcity in the ocean, which may play an important part in ocean dynamics and nutrient cycles. The input of iron varies by region and is delivered to the ocean by dust (from [[dust storm]]s) and leached out of rocks. Iron is under consideration as the true limiting element to ecosystem productivity in the ocean.

Ammonium and nitrite show a maximum concentration at 50–80 m (lower end of the [[euphotic zone]]) with decreasing concentration below that depth. This distribution can be accounted for by the fact that nitrite and ammonium are intermediate species. They are both rapidly produced and consumed through the water column.<ref name="Gruber 2008 1–35" /> The amount of ammonium in the ocean is about 3 orders of magnitude less than nitrate.<ref name="Gruber 2008 1–35" /> Between ammonium, nitrite, and nitrate, nitrite has the fastest turnover rate. It can be produced during nitrate assimilation, nitrification, and denitrification; however, it is immediately consumed again.

===New vs. regenerated nitrogen===
Nitrogen entering the euphotic zone is referred to as new nitrogen because it is newly arrived from outside the productive layer.<ref name="Miller 2008 60–62" /> The new nitrogen can come from below the euphotic zone or from outside sources. Outside sources are upwelling from deep water and nitrogen fixation. If the organic matter is eaten, respired, delivered to the water as ammonia, and re-incorporated into organic matter by phytoplankton it is considered recycled/regenerated production.

New production is an important component of the marine environment. One reason is that only continual input of new nitrogen can determine the total capacity of the ocean to produce a sustainable fish harvest.<ref name="Parsons 1997" /> Harvesting fish from regenerated nitrogen areas will lead to a decrease in nitrogen and therefore a decrease in primary production. This will have a negative effect on the system. However, if fish are harvested from areas of new nitrogen the nitrogen will be replenished.

===Future acidification===
As illustrated by the diagram on the right, additional [[carbon dioxide]] ({{CO2}}) is absorbed by the [[ocean]] and reacts with water, [[carbonic acid]] ({{chem|H|2|CO|3}}) is formed and broken down into both [[bicarbonate]] ({{Chem2|HCO3-}}) and hydrogen ({{chem|link=Hydronium|H|+}}) ions (gray arrow), which reduces bioavailable [[carbonate]] ({{Chem2|CO3(2-)}}) and decreases ocean [[pH]] (black arrow). This is likely to enhance nitrogen fixation by [[diazotroph]]s (gray arrow), which utilize {{chem|H|+}} ions to convert nitrogen into bioavailable forms such as [[ammonia]] ({{chem|NH|3}}) and [[ammonium]] ions ({{chem2|NH4+}}). However, as pH decreases, and more ammonia is converted to ammonium ions (gray arrow), there is less [[Redox|oxidation]] of ammonia to [[nitrite]] (NO{{su|b=2|p=–}}), resulting in an overall decrease in nitrification and denitrification (black arrows). This in turn would lead to a further build-up of fixed nitrogen in the ocean, with the potential consequence of [[eutrophication]]. Gray arrows represent an increase while black arrows represent a decrease in the associated process.<ref name="O'Brien2016">{{cite journal |title = Implications of Ocean Acidification for Marine Microorganisms from the Free-Living to the Host-Associated |year = 2016|doi = 10.3389/fmars.2016.00047|doi-access = free|last1 = O'Brien|first1 = Paul A.|last2 = Morrow|first2 = Kathleen M.|last3 = Willis|first3 = Bette L.|last4 = Bourne|first4 = David G.|journal = Frontiers in Marine Science|volume = 3| page=47 | bibcode=2016FrMaS...3...47O }} [[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>

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