Resource-Efficient Photobioreactor Cultivation of Nannochloropsis sp. Using a CO ₂ -Pulsed pH-Stat Strategy

Sustainable microalgal bioprocesses offer a promising route for converting anthropogenic CO2 into high-value bioproducts, yet their resource efficiency is often constrained by suboptimal carbon delivery and energy-intensive operation. In this study, a CO2-pulsed pH-stat (CPPS) cultivation strategy was developed in a stirred-tank photobioreactor to enhance CO2 utilization efficiency, biomass productivity, and EPA-rich lipid formation in Nannochloropsis sp. CCAP 211/78. Systematic evaluation of CO2 concentration (0.04−7.5%) and photoperiod (24:0 to 12:12 h light:dark) identified 5% CO2 and a 16:8 h photoperiod as optimal for promoting resource-efficient carbon assimilation. Dynamic pseudo−steady-state experiments demonstrated that 1 vvm aeration maximized photosynthetic performance and CO2 uptake, yielding a peak oxygen evolution rate of 101.81 ± 5.42 μmolO2 L−1 min−1 and a CO2 supply rate of 57.25 ± 2.34 mg L−1 h−1 under 200 μmol m−2 s−1 illumination. Validation of the optimized CPPS process across nitrate levels revealed robust EPA biosynthesis, achieving a biomass concentration of 2.40 ± 0.02 g L−1 and an EPA titer of 43.25 ± 6.11 mg L−1 at 2 g L−1 nitrate. These findings demonstrate that integrating real-time CO2 dosing with energy- and nutrient-optimized conditions enhances carbon conversion efficiency and supports the resource-efficient production of EPA-rich microalgal biomass.