N,-fixing organisms

The cyanobacteria are considered the main N-fixing organisms in both fresh water and marine ecosystems. Some estimates have shown that these species are... 

The cyanobacteria are considered the main N-fixing organisms in both freshwater and marine ecosystems. Some estimates have shown that these species are responsible for up to 80% of total fixed nitrogen in freshwaters. Cyanobacteria are widespread so that significant nitrogen fixation is possible on local, regional and global scales. 

N-fixing organisms have been obtained under tropical or continental... 

However, in many cases the numbers of N-fixing organisms still seem very low, the mechanism of transfer of N to the plant is not defined, and the energetics of the process are obscure. 

Nitrification is the process whereby ammonium (NH4+) is oxidized to nitrite (N02 ) and then to nitrate (N03 ). It thus links the most oxidized and most reduced components of the nitrogen (N) redox cycle and helps determine the overall distributions of these important nutrients. Ammonium rarely occurs at significant concentrations in oxygenated habitats. It is recycled rapidly between heterotrophic and N2 fixing organisms (which excrete NH4+ directly or release organic N that is microbiaUy degraded to NH4+) and many heterotrophic and photosynthetic plankton (which utilize NH4+ as a N source) in the surface ocean. 

Table 2.1 Mass of phosphorus (P), fixed nitrogen (N), and organic carbon (C) in mmol m-2 in 1 m of the water column above the benthos, in the living benthos (autotrophic and heterotrophic), and in the top-most 1 m of sediments (solid and dissolved phases)...

Luperco. [Atochem N. Am.] Organic peroxide compds. ciosslinking s ent, initiator fix dastomers, thermoplastic... 

Dark periods are characterized by H2 production, which appears to result principally from fermentation activities [65,214-216]. The H2 production is, in all determined cases, distinctly sensitive to O2, which supports either a fermentative or N-fixing mechanism. However, in the heterocystous mats examined, nitrogen fixation occurred almost exclusively during the light period, while H2 flux occurred principally in the dark [214]. Similarly, H2 production occurred in non-heterocystous mats whether or not nitrogen fixation was active [65,217]. Additionally, organic fermentation products were liberated simultaneously with H2 in at least two mat systems [218]. Thus, fermentation... 

Molybdenum nitrogenase. As mentioned above N. consists of two proteins, both of which are required for activity. One of these proteins contains iron, molybdenum and acid-labile sulfur (Mo-Fe protein, component I, molybdoferredoxin, azofermo) and the other contains iron and labile sulfur (Fe-protein, component II, azoferredoxin. azofer). The two components are present in the ratio 1 Mo-Fe protein 2 Fe-proteins. Both protein components have been isolated and characterized from various nitrogen fixing organisms, and the separated components can be reconstituted to active N. 

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