Two kinds of nitrogen fixers are recognized: free-living (non-symbiotic) bacteria, including the cyanobacteria (or blue-green algae) Anabaena and Nostoc and such genera as Azotobacter, Beijerinckia, and Clostridium; and mutualistic (symbiotic) bacteria such as Rhizobium, associated with leguminous plants, and Spirillum
