what are commonly called biological nitrogen fixers
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The biological nitrogen fixation, carried out by prokaryotes, leads to the reduction of molecular nitrogen to ammonia subsequently assimilated in amino acids. This is an event of capital importance allowing for the recovery of nitrogen irreversibly lost in ecosystems due to bacterial activities. The nitrogen fixation provides Earth’s ecosystems with about 200 million tons N per year. This article describes the overall reaction of nitrogen fixation carried out by the oxygen-intolerant complex of nitrogenase. The different nitrogen-fixing prokaryotes occurring as free-living organisms or in symbiosis with plants are indicated. The following symbiotic associations are more extensively treated: (1) Symbiosis between the aerobic bacteria rhizobia and roots of Leguminosae. Bacterium–host specificity, development of root nodule, and its nitrogen-fixing activity are described. (2) Symbiosis between aerobic diazotrophic actinomicetes of the genus Frankia and roots of actinorhizal plants, including, in this case, root nodule formation and mechanisms of nitrogen fixation. Finally, the associations between cereals and endophytic diazotrophic bacteria are considered. Among freeliving organisms, attention is paid to oxygenic photosynthetic cyanobacteria that have to combine photosynthetic O2 production with O2-poisoned nitrogen fixation. The solutions based on spatial segregation of nitrogen fixation in specialized cells or on nitrogen fixation during the night through a rhythmic nitrogenase activity are documented.
Some bacteria and blue green algae are able to fix nitrogen from the atmosphere to enrich soil with nitrogen and increase its fertility.These microbes are commonly called biological nitrogen fixers.