Restoration of Spirulina growth in recycled medium via a combined strategy of nutrient supplementation and flocculation treatment.
Spirulina is recognized as a nutrient-dense superfood. Recirculation of cultivation medium offers a sustainable and cost-effective approach for microalgae production. However, the present study revealed that the Spirulina biomass yield in the second recycled medium (R2 batch) in shake flasks was 70.8% lower than that in fresh medium (R0 batch), while protein and phycocyanin contents decreased by 37.0% and 69.8%, respectively. Nutrient depletion and accumulation of inhibitory organic compounds, particularly extracellular polymeric substances (EPS), were identified as the primary limiting factors in recirculated medium. To promote Spirulina biomass production in recycled medium, a combined strategy involving nutrient supplementation and flocculation with polyaluminum chloride (PAC) and polyacrylamide (PAM) (designated as SM-F strategy) was developed. This strategy reduced EPS levels by over 40% and restored the biomass yield of the R2 batch to 1.59 g/L in flasks. Moreover, the SM-F strategy was applied to medium recirculation in a 100 L column photobioreactor. Owing to the SM-F strategy, the Spirulina biomass yield, protein content and phycocyanin content of the R7 batch reached 0.88 g/L, 70.03% and 126.10 mg/g, which were comparable to those of the R0 batch. The chemical costs of the SM-F treatment were less than one-tenth of the fresh medium preparation. Therefore, the SM-F strategy effectively overcame inhibitory effects in recycled cultivation medium, providing a scalable, low-cost and water-saving solution for Spirulina production.