固氮作用
Cyanobacteria, commonly known as blue-green algae, inhabit nearly all illuminated environments on Earth and play key roles in the carbon and nitrogen cycle of the biosphere. In general, cyanobacteria can use various inorganic and organic sources of combined nitrogen, such as nitrate, nitrite, ammonium, urea, or some amino acids. Several cyanobacteria strains are also capable of diazotrophic growth, an ability that may have been present in their last common ancestor in the Archean eon. Nitrogen fixation not only naturally occurs in soils but also aquatic systems, including both freshwater and marine. Nitrogen fixation by cyanobacteria in coral reefs can fix twice as much nitrogen as on land—around 660 kg/ha/year. The colonial marine cyanobacterium "Trichodesmium" is thought to fix nitrogen on such a scale that it accounts for almost half of the nitrogen fixation in marine systems globally.