Role of Nitrification in the Marine Nitrogen Cycle

Many forms of organic and inorganic N can be utiHzed by phytoplankton, and the transformation of NH4+ into N03 by nitrification does not change the absolute inventory of N available for algal nutrition. In soils, the different ionic properties of NH4+ and are important in determining inorganic N availabil- 

Because NH4+ contains N at the oxidation level of proteins, it is readily assimilated by both phytoplankton and bacteria, and is a preferred N source. Ammonia oxidizers may be in competition for NH4+ with other planktonic organisms. The different physiological requirements of phytoplankton and nitrifiers probably play a role in determining exacdy where in the water column NH4+ assimilation and NH4+ oxidation occur. As explained below, most nitrification occurs within or near the base of the euphotic zone in the upper 100 or so meters of the ocean. However, there is usually very little N03 in the surface ocean, due to utilization by phytoplankton, except in high nutrient low chlorophyll regions and when supplied by episodic events such as regional upweUing. The N03 in the deep water of the oceans has accumulated from nitrification because phytoplankton assimilation is essentially zero below the euphotic zone. It is because of nitrifiers 

The that is produced by nitrification serves as a substrate for denitrification 

Nitrification can also be an important sink for oxygen in aquatic environments, is released through the aerobic oxidation of organic matter and nitrification according to the classic Redfield stoichiometry (Redfield et ah, 1963)  

Although most strains of NH3-oxidizing and N02 -oxidizing bacteria have characteristic intracytoplasmic membrane structures, which can be visualized by electron microsocopy, it is not possible to distinguish the otherwise nondescript cells from other bacteria and archaea in water samples using standard microscopic techniques for ceU enumeration, e.g., epifluorescence microscopy with DNA fluorochromes. 

---