Nitrogen Cycle

Nitrogen plays a crucial role for nearly all living organisms in the Earth system. Changes in  the marine nitrogen cycle not only alter the marine biota, but will also have an impact on the marine carbon cycle and, in turn, on climate  due to the close coupling of the carbon-nitrogen cycle. The understanding of processes and controls of the marine nitrogen cycle is therefore a prerequisite to reduce uncertainties in the prediction of future climate.

Nitrogen is one of the most important growth-limiting nutrients for marine primary producers.  Its availability controls the biologically induced carbon transport to the ocean interior, i.e. the biological carbon pump. Deoxygenation and thermal stratification of the ocean as projected for future might alter the global oceanic nitrogen inventory and the surface nitrogen concentrations and hence the marine carbon uptake.

The whole complexity of the marine nitrogen cycle is still not fully understood. Nitrogen in the ocean is present in different oxidation states from oxygen reduced forms like ammonium (NH4) and dinitrogen (N2) to fully oxidized nitrate (NO3). For many different organisms assimilatory and dissimilatory biological transformations of nitrogen serve as energy source:  ammonium and nitrate are relevant for primary producers, the fixing of N2 enables cyanobacteria to grow in N-depleted surface waters, and in oxygen depleted water bacteria convert ammonium and nitrite (NO2) to N2, a process called anaerobic  ammonium oxidation (anammox).

Our goal is the representation of a comprehensive nitrogen cycle within the framework of a global Earth System Model  to investigate the different N-pathways and to evaluate their importance in today’s and future’s ocean (Figure 1).  Our main focus is on these three aspects:

  • N2 fixing organisms:  the activity of these bacteria adds new bioavailable nitrogen to the ocean and might compensate the N-loss due to processes as denitrification or anammox.  N2-fixer contribute to the opaqueness of sea water and might therefore alter the oceanic heat budget (more).
  • Nitrogen loss due to dissimilatory processes: in low oxygenated water nitrate serves as oxygen donator for bacteria to remineralize organic material. This stepwise nitrate reduction produces nitrite and ammonium.  At very low oxygen concentrations nitrite might be reduced to dinitrogen by denitrifiers. However, anammox bacteria can also use nitrite as elector acceptor to oxidize ammonium to N2. This process might contribute to a fairly significant proportion to the marine nitrogen loss without the need of organic material.
  • Nitrification in suboxic water:  In oxygen rich water it is known that nitrifying bacteria oxidize ammonium and nitrite to nitrate. Recent studies now have shown that nitrifying bacteria are also active in suboxic water. Nitrite resulting from the first step of dissimilatory nitrate reduction is oxidized back to nitrate and, thus, a potential N-loss is prevented by this pathway.

 Contact: Katharina Six,Hanna Paulsen