The marine nitrogen cycle involves the transformation of nitrogen between its various chemical forms, which supports nearly all life in the sea. This biogeochemical cycle mediates the availability of this fundamental element, regulating the productivity of marine ecosystems. Microbial conversions ensure that life can thrive, connecting the vast reservoir of atmospheric nitrogen to the biological demands of the ocean.
The Essential Role of Nitrogen in Marine Life
Nitrogen is a foundational ingredient for all marine organisms. The element is incorporated into amino acids, which are assembled to form proteins, and it is a structural part of nucleic acids like DNA and RNA. For photosynthetic organisms, such as phytoplankton, nitrogen is also a constituent of chlorophyll, the pigment that captures light energy.
In the sunlit surface waters of the open ocean, the availability of biologically usable nitrogen often controls the rate of primary production. Even though nitrogen gas is abundant in the atmosphere and dissolved in seawater, the usable forms, referred to as “fixed” nitrogen, are often scarce. This scarcity means that phytoplankton growth, which forms the base of the marine food web, is frequently limited by the supply of fixed nitrogen compounds like nitrate or ammonium. The cycle’s processes, therefore, determine the maximum biomass the ocean can sustain.
Key Transformations: Converting Atmospheric Nitrogen into Usable Forms
The initial input of new nitrogen into the marine system is governed by nitrogen fixation. This conversion is performed by specialized prokaryotes, known as diazotrophs, using nitrogenase to break the bond of dinitrogen gas (\(\text{N}_2\)). The process converts \(\text{N}_2\) gas into ammonium (\(\text{NH}_4^+\)), a biologically accessible form.
A major group of marine diazotrophs is the cyanobacteria, which includes the filamentous genus Trichodesmium and the single-celled UCYN-A. These microbes thrive in the nutrient-poor, subtropical gyres of the open ocean where fixed nitrogen is depleted. By fixing atmospheric nitrogen, these organisms alleviate the nutrient limitation and support the growth of other non-fixing phytoplankton. The newly fixed ammonium is then rapidly taken up by organisms through assimilation, incorporating the nitrogen into organic matter like proteins and cells.
Internal Cycling: Processing Nitrogen Within the Water Column
Once nitrogen is incorporated into organic compounds, a rapid recycling loop ensures its continued availability. When marine organisms excrete waste or die, decomposition of the organic matter releases nitrogen back into the water as ammonium (\(\text{NH}_4^+\)). This process, called ammonification, regenerates fixed nitrogen, making it available for immediate uptake by phytoplankton. This recycling often fuels the majority of primary production in surface waters, known as regenerated production.
The ammonium can then be oxidized through a two-step process called nitrification, performed by distinct groups of chemoautotrophic bacteria and archaea. In the first step, ammonia-oxidizing organisms convert ammonium (\(\text{NH}_4^+\)) into nitrite (\(\text{NO}_2^-\)). Next, nitrite-oxidizing microbes convert the nitrite into nitrate (\(\text{NO}_3^-\)). Nitrate is a major form of nitrogen used by most phytoplankton, particularly in deeper, nutrient-rich waters. Because nitrification requires oxygen, it typically occurs just below the surface layer where light may inhibit the nitrifying organisms, and it is a crucial link between the recycled ammonium and the more oxidized nitrate pool.
Nitrogen Loss: Removing Nitrogen from the Ocean System
To balance the input from nitrogen fixation, the ocean system has mechanisms to remove fixed nitrogen, primarily by converting it back into \(\text{N}_2\) gas. The principal output process is denitrification, where certain bacteria reduce nitrate (\(\text{NO}_3^-\)) until it forms \(\text{N}_2\) gas, which escapes back to the atmosphere. This process occurs only where oxygen is extremely low or absent, such as anoxic sediments and oxygen minimum zones (OMZs).
Another significant nitrogen-removing process is anaerobic ammonium oxidation, or anammox, which also produces \(\text{N}_2\) gas. Anammox bacteria couple ammonium (\(\text{NH}_4^+\)) directly with nitrite (\(\text{NO}_2^-\)) to yield dinitrogen gas. Anammox is a major pathway for nitrogen loss in the core of OMZs. Burial of organic nitrogen in deep ocean sediments sequesters it from the active cycle for geological time scales. The balance between the gains from fixation and the losses from denitrification and anammox determines the nitrogen inventory of the ocean.