In recent years, seaweed has been attracting attention as a “blue resource.” Seaweeds are classified into brown algae, red algae, and green algae. According to the Food and Agriculture Organization of the United Nations, brown algae account for 76.6% of wild seaweeds. Also, seaweed farming is mainly brown and red algae, and its production reaches 34.7 million tons. From the perspective of production volume and farming, brown algae are an attractive biomass. Many brown algae are composed of polysaccharides such as alginate, fucoidan, cellulose, hemicellulose, and laminarin. Among them, alginate is the most abundant, accounting for about 40% of the composition of some brown algae, depending on the species. In particular, alginate is a polysaccharide not found in terrestrial plants, and its utilization is key to the development of biorefineries.