From Manual to Automation: A Comparative Study of Manual, Standard Dilution, and Proportional–Integral Controls for Cultivation of Rhodomonas salina in a 500 L Tubular Alga Photobioreactor

Efficient production of high-quality microalgae is essential for marine aquaculture, yet large-scale cultivation of the cryptophyte Rhodomonas salina remains labor-intensive and technically challenging. In this study, a 500-L tubular photobioreactor (PBR) wasevaluated and compared three control strategies—manual control, a programmed standard dilution control, and aproportional–integral (PI) control—for continuous cultivation of R. salina targeting a setpoint of 1 × 106 cells mL−1. A no-control experiment was first conducted to determine the carrying capacity of the PBR and establish the operational setpoint. The growth, cell yield, pigment content, fatty-acid composition, and inorganic nutrients were quantified across experiments. Incident irradiance washeld constant (105 µmol m−2 s−1), while in-reactor irradiance varied with cell density (26–42 µmol m−2 s−1). Growth rates were comparable across control systems (0.25–0.33 day−1), indicating that automation did not negatively affect microalgal performance. However, daily harvested biomass differed substantially: the PI control system yielded the highest production (1.27 × 1011 cellsday−1), significantly exceeding manual control (5.33 × 1010 cells day−1; p <0:05), while standard dilution control produced the lowestyield (3.28 × 1010 cells day−1). Both automated systems maintained stable cell densities with limited overshoot, though the PIcontroller exhibited moderate oscillations. Pigment (phycoerythrin, chlorophyll [Chl]-a, and Chl-c) and fatty-acid profiles (notablyhigh EPA and DHA content) remained consistent across treatments, indicating no adverse effects of automation on microalgalquality. Nutrient concentrations suggested non-limiting conditions throughout. Overall, automation improved operational consis-tency and reduced manual workload. While both automated systems performed well, the PI controller delivered the highest yieldand maintained culture stability at the desired setpoint. In conclusion the PI-based dilution control is the most promising approachfor large-scale R. salina production, and further improvements for future PBR automation are outlined