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) was evaluated and compared three control strategies—manual control, a programmed standard dilution control, and a proportional–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 was held 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 cells day⁻¹), significantly exceeding manual control (5.33 × 10¹⁰ cells day⁻¹; p  < 0.05), while standard dilution control produced the lowest yield (3.28 × 10¹⁰ cells day⁻¹). Both automated systems maintained stable cell densities with limited overshoot, though the PI controller exhibited moderate oscillations. Pigment (phycoerythrin, chlorophyll [Chl]-a, and Chl-c) and fatty-acid profiles (notably high EPA and DHA content) remained consistent across treatments, indicating no adverse effects of automation on microalgal quality. Nutrient concentrations suggested non-limiting conditions throughout. Overall, automation improved operational consistency and reduced manual workload. While both automated systems performed well, the PI controller delivered the highest yield and maintained culture stability at the desired setpoint. In conclusion the PI-based dilution control is the most promising approach for large-scale R. salina production, and further improvements for future PBR automation are outlined.