What is the process by which bacteria convert nitrogen gas into ammonia?

Nitrogen fixation is the process by which bacteria convert atmospheric nitrogen gas into ammonia, making nitrogen available for biological use. The reaction is difficult because nitrogen gas contains a very strong triple bond, but specialized microbes use the enzyme complex nitrogenase to break and reduce it.

In nature, nitrogen-fixing bacteria include free-living organisms such as Azotobacter and symbiotic rhizobia living in root nodules of legumes. Cyanobacteria can also fix nitrogen, often inside specialized cells called heterocysts that help protect nitrogenase from oxygen. The ammonia produced may be incorporated into amino acids or converted into other nitrogen compounds in the soil.

This process is easy to confuse with nitrification, denitrification and ammonification. Nitrification converts ammonia into nitrite and nitrate; denitrification generally returns nitrate-derived nitrogen to the atmosphere as nitrogen gas; ammonification releases ammonia from organic nitrogen. Nitrogen fixation can also occur without bacteria: archaea, lightning and industrial chemistry contribute to nitrogen fixation, although the quiz wording correctly points to the biological process carried out by bacteria.