Introduction
The Farm-to-Fork Strategy of the European Green Deal acknowledges the potential of algae to become an important source of alternative low-carbon footprint protein and contribute to improving the sustainability and competitiveness of the aquaculture sector (Gallego et al., 2025) . Within this context, we have developed and demonstrated the integration of fish and algae cultivation in coupled, land-based RAS/IMTA (Recirculating Aquaculture Systems/ Integrated MultiTrophic Aquaculture) systems. This concept aim s to minimize energy and nutrient losses and maximise resource efficiency by closing the nutrient loop .
Material and Methods
In the land-based RAS/IMTA for salmon production connected to microalgae cultivation, we studied the nutrient uptake and microalgae biomass productivity on the RAS effluent water. The nitrogen and phosphorous concentrations of the RAS effluent water change over time and are highly dependent on the operation conditions of the production systems. Moreover, the nutrient concentrations are generally lower than the concentrations used for fertilization at industrial scale microalgae production. Therefore, we explored the possibility of microalgae cultivation systems adaptable to different hydraulic retention times for the water and the algae, i.e. a biofilm reactor and a perfusion system. Perfusion cultivation was tested through the integration of a membrane filtration system . The performance was compared to continuous cultivation with and without additional fertilization. The microalgae species Chlorella vulgaris and Phaeodactylum tricornutum were used for the freshwater experiments and brackish/seawater experiments respectively.
Furthermore, a life cycle assessment was conducted to evaluate the potential environmental impact s for the different microalgae cultivation methods. A preliminary economic evaluation used to determine the differences in capital expenses and operational expenses between the two microalgae cultivation modes.
Results and Discussion
We successfully demonstrated nutrient remediation for RAS effluent water, during the freshwater, brackish water and seawater stages of the salmon production, for both microalgae species Chlorella and Phaeodactylum. Results showed that high microalgae productivities could be achieved (up to 2.15 g L1 d-1), as well as high nutrient uptake rates (up to 188 8 mg NO3 L-1 d-1 and up to 252 mg PO4 L-1 d-1). A steady state microalgae cultivation with stable optical densities and dry weights cultivation was obtained for all experiments. The results from the lab experiments w ere used to update and expand an existing model for 1 ha microalgae cultivation (Böpple et al., 2025; Vázquez-Romero et al., 2022). A life cycle assessment (LCA) and preliminary economic assessment of the upscaled model showed that perfusion cultivation appears to have a lower environmental impact for treating RAS effluent water with low nutrient concentrations , but leads to increased operational (6 %) and capital expenses (up to 60 %) due to additional equipment and energy demands . The infrastructure of the photobioreactor and the electricity needed during cultivation contribute the most to the environmental impacts.
Although t he potential application of microalgae in aquaculture feed could improve the sustainability of fish feed (Ansari et al., 2021), a clearer regulatory framework is needed regarding microalgae cultivated on ’waste streams’ from aquaculture for food and feed production.
References
Ansari, F. A., Guldhe, A., Gupta, S. K., Rawat, I., & Bux, F. (2021). Improving the feasibility of aquaculture feed by using microalgae. Environmental Science and Pollution Research 2021 28:32 , 28(32), 43234–43257. https://doi.org/10.1007/S11356-021-14989-X
Böpple, H., Slegers, P. M., Breuhaus, P., & Kleinegris, D. M. M. (2025). Comparing continuous and perfusion cultivation of microalgae on recirculating aquaculture system effluent water. Bioresource Technology , 418(November 2024), 131881. https://doi.org/10.1016/j.biortech.2024.131881
Gallego, I., Medic, N., Pedersen, J. S., Ramasamy, P. K., Robbens, J., Vereecke, E., & Romeis, J. (2025). The microalgal sector in Europe: Towards a sustainable bioeconomy. New Biotechnology , 86, 1–13. https://doi.org/10.1016/J.NBT.2025.01.002
Vázquez-Romero, B., Perales, J. A., de Vree, J. H., Böpple, H., Steinrücken, P., Barbosa, M. J., Kleinegris, D. M. M., & Ruiz, J. (2022). Techno-economic analysis of microalgae production for aquafeed in Norway. Algal Research , 64, 102679. https://doi.org/10.1016/j.algal.2022.102679