Nitrogen cycle
Nitrogen is needed to make proteins and DNA.
Nitrogen fixation by bacteria converts atmospheric nitrogen (N2) to the organic form of ammonia (NH3), which then cycles through producers and other organisms in the ecosystem.
Other bacteria complete the cycle through denitrification, restoring the atmospheric nitrogen.
Humans often accelerate the production of organic nitrogen by using fertilizers, some of which escape to aquatic ecosystems, upsetting their nitrogen balance.
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