Nitrogen fixation is the process wherein N2 is converted to ammonium, or NH4+. This is the only way that organisms can attain nitrogen directly from the atmosphere; the few that can do this are called nitrogen-fixing organisms. Certain bacteria, including those among the genus Rhizobium, are able to fix nitrogen (or convert it to ammonium) through metabolic processes, analogous to the way mammals convert oxygen to CO2 when they breathe. Nitrogen-fixing bacteria often form symbiotic relationships with host plants. This symbiosis is well-known to occur in the legume family of plants (e.g., beans, peas, and clover). In this relationship, nitrogen-fixing bacteria inhabit legume root nodules and receive carbohydrates and a favorable environment from their host plant in exchange for some of the nitrogen they fix. There are also nitrogen-fixing bacteria that exist without plant hosts, known as free-living nitrogen fixers. In aquatic environments, blue-green algae (really a bacteria called cyanobacteria) are an important free-living nitrogen fixer.
In addition to nitrogen-fixing bacteria, high-energy natural events such as lightning, forest fires, and even hot lava flows can cause the fixation of smaller, but significant, amounts of nitrogen. The high energy of these natural phenomena can break the triple bonds of N2 molecules, thereby making individual N atoms available for chemical transformation.