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.