Cyanobacteria are oxygenic photosynthetic bacteria that are widespread in marine, freshwater and terrestrial environments and many of them are capable of fixing atmospheric nitrogen. But ironically, nitrogenase, the enzyme that is responsible for the reduction of N2, is extremely sensitive to O2. Therefore, oxygenic photosynthesis and N2 fixation are not compatible. Hence, cyanobacteria had to evolve a variety of strategies circumventing this paradox allowing them to grow at the expense of N2, a ubiquitous source of nitrogen.