Marine Macroalgae as Sustainable Biofilters: Linking Nitrogen Removal Efficiency with Photosynthetic Performance and Biomass Valorization

Nitrogen pollution and eutrophication are among the most significant environmental challenges affecting coastal ecosystems worldwide, stimulating interest in sustainable nutrient-removal strategies. Marine macroalgae have emerged as promising biological tools for nutrient biofiltration owing to their rapid growth, high nutrient uptake capacity, and ability to convert dissolved nitrogen into valuable biomass. This review examines the mechanisms underlying nitrogen utilization in marine macroalgae and their implications for photosynthetic regulation, biofiltration efficiency, and biomass valorization. Particular emphasis is given to the uptake and assimilation of nitrate, ammonium, urea, and dissolved organic nitrogen, as well as the physiological pathways governing nitrogen metabolism and photosynthetic performance. The interactions between nitrogen availability, chlorophyll synthesis, photochemical efficiency, and electron transport are discussed to highlight the central role of nitrogen in macroalgal productivity. The review further evaluates the application of marine macroalgae in wastewater treatment, integrated multitrophic aquaculture (IMTA), land-based aquaculture effluent remediation, and coastal nutrient management. Key environmental and operational factors influencing biofiltration efficiency, including light, temperature, salinity, water flow, seasonal variability, and species selection, are critically assessed. In addition, the potential for biomass valorization following nutrient recovery is explored, focusing on protein production, nutraceutical compounds, phycocolloids, agricultural biostimulants, and other high-value bioproducts that support circular bioeconomy approaches. Finally, future research directions are identified, with particular attention to climate change impacts, multifunctional cultivation systems, omics-based technologies, physiological modeling, and large-scale implementation challenges. Overall, marine macroalgae represent a sustainable and multifunctional solution for nutrient remediation, resource recovery, and biomass production, offering significant opportunities for advancing environmentally responsible aquaculture, wastewater management, and blue bioeconomy development.