Bacterial denitrification

Denitrification results in a loss of bioavailable (fixed) nitrogen in the form of gaseous products such as N2O and N2 (for details on denitrification see the overview article by Devol, 2008):

As can be seen from the denitrification reaction sequence, N2O is an intermediate, with its concentration at any time determined by the balance between

Figure 3.3 Simplified sketch of the oceanic nitrogen cycle Source: Karl et al (2002)
Figure 3.4 Overview of processes that influence the N2O distribution in the ocean

Note:The dashed arrows indicate N2O reduction during N2 fixation. Note that NO is not an obligate intermediate of the nitrification sequence. Source: Bange (2008)

production and consumption to N2. The net accumulation of dissolved N2O depends on the dissolved O2 concentrations (see below). Under extreme O2 depletion (such as found in the intermediate depths of the eastern tropical Pacific Ocean and the Arabian Sea, see above) there is a net N2O consumption during denitrification, resulting in low N2O concentrations. Denitrification is a well-known feature of many different bacteria species in terrestrial and oceanic environments. Denitrifiers are facultative anaerobic bacteria, which can reduce NO3- when oxygen becomes limiting. Thus the occurrence of denitrification is favoured under sub-oxic (0 <O2 <2-10pmol litre-1, Codispoti et al, 2005) conditions. Denitrification does not occur under anoxic conditions (O2 = 0pmol litre 1, hydrogen sulphide present).

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