Nitrogen fixation is a remarkable prokaryotic skill in which inert atmospheric nitrogen gas (N2) is combined with hydrogen to form ammonia (NH3). This vital process along with nitrification (formation of nitrites and nitrates) and ammonification (formation of ammonia from protein decay) make nitrogen available to autotrophic plants and ultimately to all members of the ecosystem. Although Azolla can absorb nitrates from the water, it can also absorb ammonia secreted by Anabaena within the leaf cavities.