Flavodiiron proteins are known to have crucial and specific roles in photoprotection of photosystems I and II in cyanobacteria. The filamentous, heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120 contains, besides the four flavodiiron proteins Flv1A, Flv2, Flv3A, and Flv4 present in vegetative cells, two heterocyst-specific flavodiiron proteins, Flv1B and Flv3B. Here, we demonstrate that Flv3B is responsible for light-induced O-2 uptake in heterocysts, and that the absence of the Flv3B protein severely compromises the growth of filaments in oxic, but not in microoxic, conditions. It is further demonstrated that Flv3B-mediated photosynthetic O-2 uptake has a distinct role in heterocysts which cannot be substituted by respiratory O-2 uptake in the protection of nitrogenase from oxidative damage and, thus, in an efficient provision of nitrogen to filaments. In line with this conclusion, the Delta flv3B strain has reduced amounts of nitrogenase NifHDK subunits and shows multiple symptoms of nitrogen deficiency in the filaments. The apparent imbalance of cytosolic redox state in Delta flv3B heterocysts also has a pronounced influence on the amounts of different transcripts and proteins. Therefore, an O-2-related mechanism for control of gene expression is suggested to take place in heterocysts.