Flv3A facilitates O ₂ photoreduction and affects H ₂ photoproduction independently of Flv1A in diazotrophic Anabaena filaments

Summary The model heterocyst‐forming filamentous cyanobacterium Anabaena sp. PCC 7120 ( Anabaena ) is a typical example of a multicellular organism capable of simultaneously performing oxygenic photosynthesis in vegetative cells and O 2 ‐sensitive N 2 ‐fixation inside heterocysts. The flavodiiron proteins have been shown to participate in photoprotection of photosynthesis by driving excess electrons to O 2 (a Mehler‐like reaction). Here, we performed a phenotypic and biophysical characterization of Anabaena mutants impaired in vegetative‐specific Flv1A and Flv3A in order to address their physiological relevance in the bioenergetic processes occurring in diazotrophic Anabaena under variable CO 2 conditions. We demonstrate that both Flv1A and Flv3A are required for proper induction of the Mehler‐like reaction upon a sudden increase in light intensity, which is likely important for the activation of carbon‐concentrating mechanisms and CO 2 fixation. Under ambient CO 2 diazotrophic conditions, Flv3A is responsible for moderate O 2 photoreduction, independently of Flv1A, but only in the presence of Flv2 and Flv4. Strikingly, the lack of Flv3A resulted in strong downregulation of the heterocyst‐specific uptake hydrogenase, which led to enhanced H 2 photoproduction under both oxic and micro‐oxic conditions. These results reveal a novel regulatory network between the Mehler‐like reaction and the diazotrophic metabolism, which is of great interest for future biotechnological applications.