Bacteria cyanobacteria, nitrogen-fixing

Only certain prokaryotes can fix atmospheric nitrogen. These include the cyanobacteria of soils and fresh and salt waters, other kinds of free-living soil bacteria such as Azotobacter species, and the nitrogen-fixing bacteria that live as symbionts in the root nodules of leguminous plants. The first important product of nitrogen fixation is ammonia, which can be used by all organisms either directly or after its conversion to other soluble compounds such as nitrites, nitrates, or amino acids. 

In aquatic habitats, cyanobacteria are the principal nitrogen fixers. Rates of nitrogen fixation by cyanobacteria are generally one or two orders of magnitude higher than those by free-living, nonphotosynthetic soil bacteria. Under favorable conditions, cyanobacteria may fix up to 1600 kg of N/ha/year in a rice paddy. 

Asummary of the nitrogen cycle is shown in Fig. 3.17. Nitrogen fixation, which converts N2 into NH4+, is performed by a restricted number of eubacteria and ar-chaebacteria (sometimes in symbiotic associations). Anaerobes such as the photosynthetic bacteria, together with some species of Clostridium and sulphate reducers, can perform the process. However, the most important nitrogen-fixing bacteria are aerobes the cyanobacteria (e.g. Oscillatoria) in aquatic environments and Rhizobium in plant root nodules in soils (free-living soil bacteria such as Azotobacterare... 

Nitrogen-fixing bacteria (symbiotic organisms that form nodules on the roots of leguminous plants, such as beans and alfalfa) and some free-living microbes and cyanobacteria can fix nitrogen. Plants and animals cannot. 

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