Resource-Efficient Photobioreactor Cultivation of Nannochloropsis sp. Using a CO2-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 μmol_O2 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.