Effect of culture medium composition and reuse on the growth of Porphyridium cruentum

The aim of this study was to develop a fertiliser-based culture medium to reduce production costs and to enhance the sustainability of producing Porphyridium cruentum, a red marine microalga of commercial interest. Additionally, the impact of water recirculation on microalgal growth was assessed. Overall, the results indicated that the nitrogen source significantly affected biomass growth, with sodium nitrate supporting higher biomass productivity (0.23 g·L^-1·day^-1), surpassing sodium nitrite (0.18 g·L^-1·day^-1) and ammonium chloride (0.14 g·L^-1·day^-1). Urea had a negative impact on growth. A N:P molar ratio of 20:1 increased biomass productivity by approximately 24 % compared to the lower ratio studied (8:1) while also reducing phosphorus demand. The optimal medium composition was: 1.75 g·L^-1 NaNO₃, 0.23 g·L^-1 K₂HPO₄·3 H₂O, 0.04 g·L^-1 CaCl₂, 0.49 g·L^-1 MgSO₄·7 H₂O, and 24.0 mg·L^-1 Karentol®. Lastly, water reutilisation negatively impacted biomass concentration, promoting the accumulation of extracellular organic carbon and bacteria as well as increasing the viscosity and turbidity of the culture.