Nitrogen in the atmosphere is not readily available for organisms to use because it exists in the form of nitrogen gas (N2), which is a very stable molecule. In order to be used by living things, nitrogen must be converted into a form that is more reactive and easier to incorporate into biological molecules. This process, called nitrogen fixation, is carried out by certain bacteria and archaea, which convert atmospheric nitrogen into ammonia (NH3). Ammonia can then be converted into other nitrogen-containing compounds, such as nitrates (NO3-) and nitrites (NO2-), which can be taken up and used by plants and other organisms.