Aquaculture Wastewater as a Feedstock for Functional Seaweed Polysaccharides: Linking Nutrient Recovery, Macromolecular Architecture and Agricultural Valorization

Aquaculture wastewater supplies nutrients for seaweed cultivation, but its suitability for producing functional polysaccharides remains uncertain. This critical review examines the evidence connecting wastewater composition, nutrient assimilation, polysaccharide production, macromolecular architecture, and agricultural performance. Studies of wastewater-grown seaweeds were compared with controlled cultivation, extraction, and agricultural application studies, with attention to nitrogen and phosphorus availability, C:N balance, salinity, and dissolved inorganic carbon. Evidence indicates that nutrient enrichment can improve biomass production without corresponding increases in polysaccharide yield. Measurements linking wastewater chemistry to molecular weight distribution, monosaccharide composition, sulfation, and polymer conformation remain scarce. Differences among species, growth conditions, and nutrient exposure regimes complicate comparisons across studies. Extraction and purification can alter polymer characteristics, limiting the attribution of observed differences to cultivation conditions. Reported agricultural responses may depend on polymer structure and formulation. However, the independent effects of molecular weight and other structural descriptors remain poorly established experimentally. Contaminant transfer from wastewater to purified products and subsequent exposure of soil and crops are inadequately characterized. Future research should combine matched wastewater and conventional cultivation controls, standardized extraction, polymer characterization, contaminant mass balances, and dose–response bioassays. Integrated economic and environmental assessment is also required before product safety, functional equivalence, and practical feasibility can be established.