Design of experiments‐driven optimization of <i>Porphyridium marinum</i> cultivation: impact of operational variables on phycoerythrin and exopolysaccharide production

Abstract Among the Porphyridium genus, Porphyridium marinum exhibits the highest phycoerythrin (PE) content. In this study, the metabolic trade‐off between biomass, PE, and sulfated exopolysaccharide (EPS) production was assessed under varying nitrogen and sulfur availability, light intensity, residence time, and cultivation mode. A Design of Experiments approach was applied to optimize production in a high‐cell‐density membrane photobioreactor, and results were compared with a semicontinuous strategy under industrially relevant conditions. Nitrogen availability had a positive linear effect on both biomass and PE productivity, although it interacted with light intensity in distinct ways. Biomass productivity peaked at 2.68 g L −1 per day under high irradiance (420 μmol m −2 s −1 ), whereas maximum PE productivity (0.45 g L −1 per day) was achieved at a moderate light intensity of 245 μmol m −2 s −1 . Sulfur significantly influences PE productivity, but its effect is modulated by nitrogen availability and varies with light intensity. Under nitrogen limitation and low light, EPS production relative to biomass increased, indicating a metabolic shift favoring extracellular carbon allocation over biomass and PE. This metabolic reallocation was further modulated by residence time: in semicontinuous mode, shorter residence times (3 days) combined with high irradiance (420 μmol m −2 s −1 ) enhanced biomass and PE productivity, while longer residence times and nitrogen limitation under moderate light promoted EPS accumulation relative to biomass. While these trends were consistent across cultivation modes, absolute productivities differed between batch and semicontinuous systems. A reduced quadratic model was validated for batch mode, showing high predictive accuracy, and confirming the feasibility of high‐density cultivation strategies for optimizing target metabolite allocation.