Expanding photosynthesis through biosolar oxic N₂ fixation: toward a sustainable BioNitrogen Economy

Atmospheric dinitrogen (N₂) is far more abundant than carbon dioxide (CO₂), yet photosynthesis has been studied and harnessed predominantly for photosynthetic carbon fixation (PCF), underpinning the global biocarbon economy. In contrast, biological N₂ fixation-the natural gateway for atmospheric nitrogen into the biosphere-remains comparatively underexploited. Modern agriculture and industry consequently depend heavily on the fossil-fuel-intensive Haber-Bosch process, with substantial energy and environmental costs. Diazotrophic cyanobacteria offer a fundamentally different paradigm by coupling oxygenic photosynthesis to nitrogenase-mediated N₂ fixation. We term this process photosynthetic nitrogen fixation (PNF), which uses solar energy to convert water and atmospheric N₂ into bioavailable nitrogen. Beyond its ecological significance, PNF expands the functional scope of photosynthesis beyond carbon fixation and provides a biological foundation for sustainable nitrogen production, biofertilization, and N₂-based biosolar manufacturing. Recent advances in cyanobacterial physiology, ecology, evolution, synthetic biology, and AI-enabled multi-omics are accelerating efforts to harness this potential. This review examines the diversity of diazotrophic cyanobacteria, their strategies for protecting oxygen-sensitive nitrogenase, and emerging approaches for genetic engineering cyanobacteria as programmable platforms for nitrogen production and crop-associated nitrogen delivery. We further propose a roadmap toward a solar-powered BioNitrogen Economy encompassing bioprospecting, synthetic biology, genomic engineering, crop integration, and scalable agricultural and industrial deployment. By expanding photosynthesis from carbon fixation to the solar-powered capture of both carbon and atmospheric nitrogen, PNF could help drive the transition from a fossil-fuel-dependent chemo-nitrogen economy toward a sustainable BioNitrogen Civilization.