Valorization of Indoor CO2 into High-Density Leptolyngbya sp. Biomass: Spectral Optimization and Continuous Cultivation for Circular Biorefining

Elevated indoor carbon dioxide (CO2) represents an underutilized biogenic carbon feedstock. Coupling indoor atmospheric remediation with biological sequestration requires overcoming mass-transfer limitations and culture collapse under high carbon loads. This study empirically evaluated a direct-injection, 2.0 L filamentous Leptolyngbya sp. photobioreactor for continuous indoor CO2 valorization. A fortified nitrogen medium (4.5 g L−1 NaNO3) was utilized to sustain high-density proliferation. Initial optimization confirmed that narrow-band Red LEDs (10,000 lux) maximized the Apparent Quantum Efficiency (ΦCO2) at 1.49 × 10−3 mol/mol. During a 20-day continuous trial under 5000 ppm CO2 sparging, the EPS-rich cyanobacterial matrix resisted acidification, achieving a stationary biomass density of 3.30 g L−1. The system maintained a steady bio-mitigation velocity of ~2120 ppm day−1. Intracellular chlorophyll a peaked at 26.0 mg L−1 on Day 10, before severe optical self-shading induced a 47% photoprotective degradation, defining Day 10 as the optimal biorefinery harvesting threshold. At this peak, dynamic efficiency reached 2.07 × 10−3 mol/mol. Projecting these kinetics onto a 50 m3 occupied room confirms that capturing ambient CO2 requires a working volume of 85–90 L and a spatial footprint of <0.25 m2, validating Leptolyngbya sp. as a highly resilient catalyst for decentralized, commercial-scale indoor bioprocessing.