Photonic-chemostat engineering for efficient continuous cultivation of cyanobacteria

Abstract

Optimising continuous phototrophic cultivation remains a major challenge for scalable, energy-efficient cyanobacterial bioprocesses. Here, we combine controlled photophysiology, long-term continuous experimentation, multi-parameter analysis, and batch-derived Monod kinetic modelling to define a precise operational window for Synechocystis sp. PCC 6803 under flat-plate photobioreactor (FP-PBR) illumination. Using a fully calibrated FP-PBR platform, we first quantified intrinsic growth limits (µ_max = 0.081–0.118 day⁻¹) across low, moderate, and high irradiance regimes, establishing the illumination-driven growth ceilings that constrain downstream continuous operation. Guided by these kinetic boundaries, continuous cultivation demonstrated that productive steady-state growth emerges only within a narrow regime governed by light intensity (500–700 µmol photons m⁻² s⁻¹), temperature (32–34 °C), and dilution rate (0.12–0.14 day⁻¹). Single-parameter and 3D interaction analyses revealed strong coupling between photonic supply, thermal sensitivity, and hydraulic residence time, while multi-factor modelling captured these nonlinear constraints and accurately predicted washout boundaries. Translating these insights into sustainability metrics, the optimised regime supports 0.07–0.125 g L⁻¹ day⁻¹ of biomass productivity, equivalent to 8.4–15.0 g biomass day⁻¹ and 176–315 kJ day⁻¹ of chemical energy in a 120 L mini-pilot system. Stoichiometric analysis indicates this corresponds to 15.6–27.6 g CO₂ day⁻¹ sequestered, demonstrating measurable environmental benefit even at a small scale. Together, these results provide a mechanistically grounded, kinetically constrained framework for designing inherently efficient, low-waste, and model-predictive cyanobacterial photobioprocesses aligned with green chemistry and future carbon-neutral manufacturing.