Photonic-chemostat engineering for efficient continuous cultivation of cyanobacteria

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.