N2-Fixing Cyanobacteria: Ecology and Biotechnological Applications

Cyanobacteria were the first organisms capable of oxygenic photosynthesis on Earth. Due to this trait and their wide distribution in freshwater and marine waters, they are important primary producers in aquatic environments. Some cyanobacteria are capable of atmospheric nitrogen gas (N₂) fixation, allowing them to sustain growth under nitrogen-limiting conditions. This chapter provides an introduction to the physiology and genetics underpinning N₂ fixation, the different modes of N₂ fixation realized in different orders, and the phylogeny of nif genes, which encode components of the nitrogenase complex. We provide a brief overview of freshwater species and the important marine cyanobacterium Trichodesmium, then focus on the biological applications of N₂-fixing cyanobacteria for wastewater metal remediation, biofertilizers, bioenergy production (e.g., hydrogen gas, which is a byproduct of N₂ fixation), and other high-value products (e.g., pigments such as phycocyanin and phycoerythrin as antioxidants, exopolysaccharide-derived cosmetics, ultraviolet sunscreens).