How do cyanobacteria fix nitrogen?

Cyanobacteria fix atmospheric nitrogen by transforming gaseous nitrogen into ammonia, a bioavailable form that living cells can utilize for growth.12

Many filamentous species of cyanobacteria achieve this through the use of specialized, thick-walled cells known as heterocysts. These cells serve as essential, oxygen-free (anaerobic) environments because the enzymes responsible for nitrogen fixation are sensitive to and can be inhibited by oxygen. By maintaining this specialized internal environment, heterocysts allow the nitrogen-fixing process to occur efficiently.2

In addition to these free-living forms, some cyanobacteria have evolved into endosymbiotic relationships. For example, the cyanobacterium Candidatus Atelocyanobacterium thalassa (UCYN-A) has integrated with a marine alga to form a nitroplast, an organelle that enables its host to fix nitrogen. Other cyanobacterial endosymbionts, such as those found in certain diatoms, are known as spheroid bodies.3