Nitrogen-Fixing Microorganisms

microorganisms that incorporate molecular nitrogen from the air. The nitrogen-fixing bacteria include the genus Rhizobium which live in symbiosis with leguminous plants—peas, lupine, clover, alfalfa, and so on. In each hectare of soil nodules affixed to the roots of these plants bind 100–250 or more kilograms of atmospheric nitrogen per year. Nitrogen-fixing microorganisms also include certain ac-tinomycetes and other microorganisms which form nodules on the roots of nonleguminous plants, such as alder and Elaeagnus. Nitrogen-fixing bacteria form nodes in leaf tissue of a series of tropical plants which cannot develop normally without these nodules.

Active nitrogen fixers are free-living microorganisms living in soil and waterways. They include the anaerobic sporogenous bacterium Clostridium discovered by S. N. Vinogradskii; the aerobic microorganism Azotobacter, which takes first place in nitrogen-fixing activity (to 25 g nitrogen per 1 kg of utilized sugar), but is less widely distributed in soil than is Clostridium. Nitrogen-fixing microorganisms also include oligonitrophils—bacteria which grow well on nitrogenless nutrient media—and some species of Pseudomonas. The ability to bind atmospheric nitrogen has been established in a series of acetone ester bacteria— Bacillus polymyxa and Bacillus macerans. Many species of blue-green algae are also included in the group of active nitrogen fixers—among them, Nostoc and Anabaena; so are some purple sulfur bacteria and green bacteria. Certain species of fungi, yeasts, and spirochetes participate in the fixation of atmospheric nitrogen. Nitrogen-fixing microorganisms have an important place in the circulation of nitrogen in nature and especially in the conservation of usable nitrogen by plants, which cannot obtain it from the air; they receive their nitrogen after mineralization of proteins by the nitrogen-fixing microorganisms.