
An innovative study carried out at Aquacria Piscícolas SA, a Portuguese Senegalese sole (Solea senegalensis) farm belonging to the Sea Eight Group, equipped with RAS technology and a production capacity of 250 tonnes per year, has demonstrated how the integration of macroalgae into the farming system can profoundly transform the aquaculture industry.
Researchers from the University of Porto and CIIMAR demonstrated that integrating two macroalgae species, Ulva sp. and Gracilaria sp., into a recirculating aquaculture system (RAS) can significantly enhance sustainability while generating valuable biomass for multiple industries.
The study, published in the Journal of Applied Phycology, highlights that seaweed, known for its ability to absorb nutrients, can act as natural biofilters, removing excess nitrogen and carbon from aquaculture systems. Among the two, Ulva sp. outperformed Gracilaria sp. in nutrient uptake, achieving a nitrogen absorption rate of 13.0 grams per square metre per week, compared to Gracilaria’s 9.0 grams per square metre per week.
“Macroalgae provide a sustainable nutrient bioremediation solution in Integrated Multi-Trophic Aquaculture Systems (IMTA),” the authors explained. By absorbing nutrients from fish effluents, seaweed helps prevent eutrophication while contributing to resource-efficient aquaculture practices.
In addition to their role as biofilters, the biomass produced by these macroalgae offers noteworthy biotechnological potential. Gracilaria sp., for instance, contained 25.8% protein and was a rich source of phycobiliproteins – compounds valuable for applications in the food, cosmetics, and pharmaceutical industries.
Ulva sp., on the other hand, contained high levels of chlorophyll a (2.51 mg/g dry weight) and carotenoids (0.58 mg/g dry weight), making it suitable for use as a natural colourant or antioxidant in food and cosmetic products.
The researchers observed that seaweed biomass generated in RAS could help diversify revenue streams for aquaculture farms while simultaneously promoting sustainability.
The findings align with the global shift towards sustainable aquaculture practices. RAS is already recognised for its efficient use of water and space, and incorporating seaweed cultivation further enhances the system’s environmental benefits.
“By introducing macroalgae into RAS, we can reduce feed waste, improve water quality, and produce a secondary crop with economic value,” the study concluded.
Future Outlook

The research team emphasised that optimising cultivation conditions – such as stocking density and system flow rates – could further enhance biomass productivity and tailor the seaweed’s biochemical composition to specific market demands.
The study was supported by the Portuguese Foundation for Science and Technology and the Fundo Azul Project.
Reference:
Duarte, A. M., Carvalhal, G., Assunção, J., Guedes, A. C., Sousa Pinto, I., & Costa, I. (2025). Cultivation of Ulva sp. and Gracilaria sp. associated with a recirculating aquaculture system. Journal of Applied Phycology. https://doi.org/10.1007/s10811-024-03424-6