Waste-derived biofertilizers differentially affect antioxidant activity and biochemical composition of Sargassum polycystum (Phaeophyta)

Waste-derived organic fertilizers may provide a resource-recovery approach for modifying the biochemical quality of cultivated seaweeds. This study evaluated the effects of vermicompost biofertilizer, compost biofertilizer, and cassava peel biofertilizer (CPB) on the biochemical composition and functional metabolites of Sargassum polycystum . Seaweeds were cultivated under weekly fertilizer application and evaluated for pigments, fucoxanthin, phenolic compounds, antioxidant activity, proximate composition, amino acids, and fatty acids. Fertilizer treatments were associated with significant differences in the measured biochemical endpoints. At the single equal-mass concentration tested, CPB-treated samples contained the highest concentrations of fucoxanthin, photosynthetic pigments, phenolic compounds, amino acids, and protein and exhibited the lowest ABTS and DPPH IC₅₀ values. In contrast, control samples contained higher lipid and fatty acid concentrations. Because nutrient loading was not standardized, fertilizer characterization was limited to total NPK, and growth performance was not evaluated, these findings represent an initial comparison of endpoint biochemical responses rather than evidence of fertilizer superiority or improved cultivation performance. Further dose–response studies incorporating nutrient-matched treatments, growth measurements, and more comprehensive environmental and fertilizer characterization are required.