Nitrogen fixation in cyanobacteria

Cyanobacteria are oxygenic photosynthetic bacteria that are widespread in marine, freshwater and terrestrial environments, and many of them are capable of fixing atmospheric nitrogen. However, 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. Some filamentous cyanobacteria differentiate heterocysts. These cells lack the oxygenic photosystem and possess a glycolipid cell wall that keeps the oxygen concentration sufficiently low for nitrogen fixation to take place. This strategy is known as spatial separation of oxygenic photosynthesis and nitrogen fixation. Nonheterocystous cyanobacteria may temporally separate these processes by fixing nitrogen during the night. Again others use a combination of these strategies.