Ethanol Fermentation-based Valorization of Alginate Extraction Residue from Pelagic Brown Seaweed Sargassum siliquosum.

Brown seaweeds are the primary industrial source of alginates and are increasingly valued as marine-based feedstocks in integrated biorefineries. However, the substantial solid residue generated in the alginate extraction industry remains underexplored as a secondary source of carbon and energy. This study investigated the potential of the alginate-extracted residue (AER) from Sargassum siliquosum biomass collected from the southwestern coast of Taiwan as a substrate for bioethanol production. The alkaline extraction method yielded 39.00% alginate and 33.80% residue on a dry weight basis. Comprehensive characterization of the AER revealed that the solid fraction is enriched in carbohydrates (33.00%), especially the cellulose fraction, with a monosaccharide composition of glucose, fucose, and xylose in addition to guluronic acid. Elemental analysis revealed that the residue is enriched in carbon (35.29%) and shows notable reductions in ash, sulfur, and phenolic compounds. Fourier-transform infrared spectroscopy (FT-IR) analysis confirmed the selective removal of mannuronic and guluronic acid residues in AER compared to biomass, while cellulose-associated functional groups have been preserved. Enzymatic hydrolysis with commercial fungal cellulase released 2.90 g/L glucose from biomass and 5.10 g/L glucose from AER, respectively. Fermentation of the hydrolysates with Saccharomyces cerevisiae BCRC 22458 yielded ethanol titers of 0.89 and 1.20 g/L from biomass and AER hydrolysate, respectively. Concentration of the hydrolysates to increase the sugar concentration increased the ethanol titer to 2.47 g/L (yield 0.098 g/g biomass; productivity 0.33 g/L/h) for AER and 1.18 g/L for biomass. These results demonstrated that S. siliquosum AER is a promising secondary feedstock for bioethanol fermentation within a cascading biorefinery approach for brown seaweeds.