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Nitrogen enters the ocean through precipitation, runoff, or as N 2 from the atmosphere. Nitrogen cannot be utilized by phytoplankton as N 2 so it must undergo nitrogen fixation which is performed predominantly by cyanobacteria. [82] Without supplies of fixed nitrogen entering the marine cycle, the fixed nitrogen would be used up in about 2000 ...
The colonial marine cyanobacterium Trichodesmium is thought to fix nitrogen on such a scale that it accounts for almost half of the nitrogen fixation in marine systems globally. [52] Marine surface lichens and non-photosynthetic bacteria belonging in Proteobacteria and Planctomycetes fixate significant atmospheric nitrogen. [53]
2 so it must undergo nitrogen fixation which is performed predominately by cyanobacteria. [43] Without supplies of fixed nitrogen entering the marine cycle, the fixed nitrogen would be used up in about 2000 years. [44] Phytoplankton need nitrogen in biologically available forms for the initial synthesis of organic matter.
Microorganisms preferentially consume oxygen in nitrate over phosphate leading to deeper oceanic waters having an N:P ratio of less than 16:1. From there, the ocean's currents upwell the nutrients to the surface where phytoplankton will consume the excess Phosphorus and maintain a N:P ratio of 16:1 by consuming N 2 via nitrogen fixation. [9]
In the nitrogen cycle, atmospheric nitrogen gas is converted by plants into usable forms such as ammonia and nitrates through the process of nitrogen fixation. These compounds can be used by other organisms, and nitrogen is returned to the atmosphere through denitrification and other processes.
Trichodesmium is the major diazotroph in marine pelagic systems [8] and is an important source of "new" nitrogen in the nutrient poor waters it inhabits. It has been estimated that the global input of nitrogen fixation by Trichodesmium is approximately 60–80 Tg (megatonnes or 10 12 grams) N per year. [1]
Iron plays an important role in the nitrogen cycle, aside from its role as part of the enzymes involved in nitrogen fixation. In anoxic conditions, Fe(II) can donate an electron that is accepted by NO 3 − which is oxidized to several different forms of nitrogen compounds, NO 2 −, N 2 O, N 2, and NH 4 +, while Fe(II) is reduced to Fe(III). [33]
The oceanic nitrogen cycle with the role of DNRA. Blue line represents the ocean surface, with the atmosphere above. Notice how NH 4 produced by DNRA can be taken up by biota and converted into organic nitrogen, while N 2 produced by denitrification is removed from the system, and may only re-enter via nitrogen fixation.