Assessment of water reuse potential for the cultivation of the red microalga Galdieria sulphuraria

The red microalga Galdieria sulphuraria has emerged as a promising biotechnological platform for large-scale cultivation and production of high-value compounds, such as the blue pigment phycocyanin. However, its extremophilic nature requires cultivation in an appropriately acidified culture medium, resulting in strongly acidic wastewater exceeding legal limits for industrial wastewater release. Additionally, the high freshwater usage, together with the supply of the necessary nutrients, is among the main costs associated with large-scale algae cultivation. This study investigates the possibility of recycling water and metals for multiple G. sulphuraria’s cultivation cycles to reduce costs and make microalgae cultivation more sustainable. Firstly, 25% water recycling showed no significant differences in consecutive cycles compared to the control, reaching the same final biomass concentration. Further, recycling the whole permeate derived from the harvesting process with the only re-integration of nitrogen and phosphate sources, did not affect G. sulphuraria growth in a first cycle of water recycling. Finally, the study evaluated the potential of G. sulphuraria’s phycocyanin (C-PC) accumulation achievable at high cells densities within the flat panel PBR, with values between the highest reported in the literature (10.8 % w/w). No negative effects of the recycled water on the C-PC accumulation were observed, suggesting that this approach could lead to a more sustainable and cost-effective strategy for large-scale G. sulphuraria cultivation and production of high-value compounds.