Actinomycetes are filamentous bacteria, inhabitant of soil. The genus Frankia causes formation of nitrogen fixing nodules in alder tree roots (non-legume plants). The genus Screptomyces of Actinomycetes produces a gaseous compound called geosmin, which gives fresh soil its typical musty odour. Streptomyces sp. produces most of our commercial antibiotics. Anabaena is free-living nitrogen fixing cyanobacteria. They contain specialized cells called heterocysts that contain enzymes for nitrogen fixation. Rhizobium is a symbiotic N2 fixing Lacteria. It is symbiont of papilionaceous roots and form root nodules.