The process by which N₂ is reduced to NH⁴⁺ is called nitrogen fixation. Nitrogenase enzyme catalyzes this reduction. It is only carried out by prokaryotic microorganisms. Principle N₂-fixers include certain free-living cyanobacteria in symbiotic associations with fungi lichens or with ferns, mosses, and liverworts, and by bacteria or other microbes associated symbiotically with roots, especially those of legumes. About 15 percent of the nearly 20,000 species in the Fabaceae (leguminosae) family have been examined for N₂ fixation, and approximately 90 percent of these have root nodules in which fixation occurs. So without active nitrogenase enzyme, there will be no N₂ fixation in legumes.