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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. [41] Without supplies of fixed nitrogen entering the marine cycle, the fixed nitrogen would be used up in about 2000 years. [42] 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]
The marine nitrogen cycle consists of complex microbial transformations which include the fixation of nitrogen, its assimilation, nitrification, anammox and denitrification. [78] Some of these processes take place in deep water so that where there is an upwelling of cold waters, and also near estuaries where land-sourced nutrients are present ...
Trichodesmium is a diazotroph; that is, it fixes atmospheric nitrogen into ammonium, a nutrient used by other organisms. Trichodesmium is thought to fix nitrogen on such a scale that it accounts for almost half of the nitrogen fixation in marine systems globally. [1]
Nitrogen fixation rates in the region have increased since the 1970s, in part fueled by an increase in iron deposition due to expansion of the Sahara Desert. [42] Wind-driven downwelling removes nutrients from the surface, but mesoscale eddies transport some of these nutrients back to the photic zone. [ 37 ]
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]