Light-dependent oxygen consumption in nitrogen-fixing Cyanobacteria plays a key role in nitrogenase protection

All colonial diazotrophic cyanobacteria are capable of simultaneously evolving O₂ through oxygenic photosynthesis and fixing nitrogen via nitrogenase. Since nitrogenase is irreversibly inactivated by O₂, accommodation of the two metabolic pathways has led to biochemical and/or structural adaptations that protect the enzyme from O₂. In some species, differentiated cells (heterocysts) are produced within the filaments. PSII is absent in the heterocysts, while PSI activity is maintained. In other, nonheterocystous species, however, a "division of labor" occurs whereby individual cells within a colony appear to ephemerally fix nitrogen while others evolve oxygen. Using membrane inlet mass spectrometry (MIMS) in conjunction with tracer ¹⁸O₂ and inhibitors of photosynthetic and respiratory electron transport, we examined the light dependence of O₂ consumption in Trichodesmium sp. IMS 101, a nonheterocystous, colonial cyanobacterium, and Anabaena flos-aquae (Lyngb.) Bréb. ex Bornet et Flahault, a heterocystous species. Our results indicate that in both species, intracellular O₂ concentrations are maintained at low levels by the light-dependent reduction of oxygen via the Mehler reaction. In N₂-fixing Trichodesmium colonies, Mehler activity can consume ∼75% of gross O₂ production, while in Trichodesmium utilizing nitrate, Mehler activity declines and consumes ∼10% of gross O₂ production. Moreover, evidence for the coupling between N ₂ fixation and Mehler activity was observed in purified heterocysts of Anabaena, where light accelerated O₂ consumption by 3-fold. Our results suggest that a major role for PSI in N₂-fixing cyanobacteria is to effectively act as a photon-catalyzed oxidase, consuming O₂ through pseudocyclic electron transport while simultaneously supplying ATP in both heterocystous and nonheterocystous taxa.