Nitrification

Nitrification or ‘ammonium oxidation’ is a two-step respiratory process occurring in sediment (benthic nitrification) or the water column (pelagic nitrification) in which bacteria oxidise ammonium (NH₄) to nitrite (NO₂) and nitrate (NO₃).

(Eq. 1) NH₄⁺ + 1.5O₂ = NO₂^– + H₂O + 2H⁺
(Eq. 2) NO₃^– + 0.5O₂ = NO₃^–

Nitosomonous spp. and Nitrobacter spp. are the main organisms responsible for steps (Eq. 1) and (Eq. 2) respectively. Nitrification exerts an important influence on marine primary productivity because:

• it modifies the form of nitrogen released during the decomposition of organic matter);
• it shunts nitrogen to denitrification pathways; and
• it competes with heterotrophic bacteria for dissolved oxygen supplies [1]. Water temperature is the most important factor regulating benthic and pelagic nitrification rates in estuaries [2].

1. Henriksen, K, and Kemp, W.M. 1988. Nitrification in Estuarine and Coastal Marine Sediments, pp. 207-249 in T.H. Blackburn and J. Sorensen (eds.), Nitrification in Estuarine and Coastal Marine Sediments. Nitrogen Cycling in Coastal Marine Environments, John Wiley and Sons Ltd.
2. Berounsky, V.M. and Nixon, S.W. 1990. Temperature and the annual cycle of nitrification in waters of Narragansett Bay. Limnology and Oceanography 35, 1610-1617.