Heterocyst-specific flavodiiron protein Flv3B enables oxic diazotrophic growth of the filamentous cyanobacterium Anabaena sp. PCC 7120

Significance Cyanobacterial flavodiiron proteins (FDPs) comprise a protein family with unique modular structure and photoprotective functions in an oxygenic environment. It is conceivable that FDPs have made the development of oxygenic photosynthesis possible in cyanobacteria. Here, we report the ability of specific FDPs to reduce O 2 directly to water in heterocyst-forming filamentous cyanobacteria, not only to support the photosynthetic machinery, but also to prevent oxidative damage of the N 2 -fixing enzyme nitrogenase. Whilst in the ancient environment, N 2 fixation was secured from O 2 inhibition, the later increase of atmospheric O 2 may have initiated an important role for FDP-mediated protection of nitrogenase in maintaining the N 2 -fixing activity of cyanobacteria.