[in Japanese]

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Seaweeds have long been utilized as food and medicinal resources, and recent advances in the understanding of their functional components have positioned them as important subjects in food science and healthcare research. In particular, seaweed polysaccharides, such as fucoidan, alginate, and laminarin, exhibit diverse biological activities owing to their structural diversity and biocompatibility. These include antioxidant, anti-inflammatory, immuno-modulatory, and antitumor effects, with molecular mechanisms increasingly being elucidated. Fucoidan has attracted particular attention for its multifunctional bioactivities mediated through cellular signaling pathways and apoptosis, while other polysaccharides, including laminarin, are associated with immune activation and metabolic regulation. In addition, their indigestibility allows them to function as prebiotics through interactions with the gut microbiota, contributing to immune homeostasis and reduced risks of metabolic disorders. These characteristics highlight their potential applications in functional foods and health-related products, as well as their value as sustainable biomass resources. This special issue, based on a symposium held in 2025 in Kumamoto, Kyushu, Japan, and presents recent advances and perspectives from academic and industrial re-searchers, with the aim of promoting interdisciplinary research and future applications of seaweed polysaccharides.

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“Sea Vegetables First” is the dietary practice of beginning daily meals with seaweed consumption, developed to prevent lifestyle-related diseases and ameliorate symptoms. This review explains effects of this practice elucidated from epidemiological data and multiple hu-man intervention studies. The main finding was that regular consumption of seaweed promotes salt excretion, reducing estimated daily salt intake, particularly in individuals with high salt intake. It can also lower blood pressure (especially systolic blood pressure) and im-prove facial and eye swelling and leg fatigue. Studies have also confirmed suppression of postprandial blood glucose levels, increased GLP-1 concentrations, and a second-meal effect (suppression of blood glucose levels after the next meal). It also suppresses postprandial blood lipid levels, suggesting its potential to prevent dyslipidemia. A large-scale cohort study has also shown that people who regularly eat seaweed have a lower risk of developing ischemic heart disease. This study is sup-ported by the presence of seaweed in the diets of long-lived Japanese individuals. Because seaweed consumption is declining, elucidating its health benefits and promoting its intake are crucial.

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Fucoidan is a sulfated polysaccharide from brown algae with diverse biological activities, including antitumor effects. Low molecular weight fucoidan (LMF) has attracted attention because of its improved intestinal absorption and bioavailability. This review focuses on LMF derived from Cladosiphon species, widely consumed in Japan as Okinawa mozuku, and summarizes its antitumor effects and underlying mechanisms. Evidence from in vitro and in vivo studies shows that Cladosiphon-derived LMF selectively suppresses the proliferation of various cancer cells with minimal effects on normal cells, mainly by inducing apoptosis. LMF also enhances the anticancer efficacy of chemotherapeutic agents such as cisplatin while reducing their cytotoxicity in normal cells. In addition, LMF inhibits tumor angiogenesis and invasion by downregulating vascular endothelial growth factor expression and matrix metalloproteinase-2/9 activity, thereby modulat-ing the tumor microenvironment. Recent findings further suggest that LMF reduces PD-L1 expression in cancer cells and may enhance antitumor immune responses. These findings highlight LMF as a multifunctional polysaccharide with potential as a functional food ingredient and complementary strategy in cancer therapy.

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Marine algae are used not only in food and medicine but also in livestock feed and fertilizer. Worldwide aqua-culture production of marine algae has increased annually to meet the increased demand for valuable algae-derived polysaccharides such as alginate, fucoidan, agarose, and carrageenan. In the field of nutrition, these polysaccharides are considered dietary fibers because they are not digestible in the human gut. Recently, marine algae have attracted considerable attention for their role in “blue carbon,” specifically as a means of fixing the increasing amount of carbon dioxide produced by burning fossil fuels. Representative polysaccharides such as alginate, ascophyllan, and porphyran contain high levels of carboxyl and sulfate groups in their structures and are acidic in aqueous solutions. These acidic polysaccharides isolated from marine algae not only function as dietary fiber but also exhibit other attractive biological activities, including immunopotentiating, anti-cancer, antioxidant, and anti-inflammatory activities. In this review, the author summarizes the various biological activities of alginate, ascophyllan, and porphyran.

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In February 2021, large-scale blade discoloration of Undaria pinnatifida occurred around the Naruto Strait, coinciding with a period when dissolved inorganic nitrogen (DIN) remained at or below the threshold level of 2 µmol L^-1 for one and a half months. In this study, we collected blades, including healthy individuals, from eight cultivation grounds and examined blade length, upper and lower blade thickness, and SPAD values as an indicator of chlorophyll content. Discolored thalli exhibited significantly shorter blades compared with healthy ones, whereas no differences were found in blade thick-ness. However, SPAD values showed a clear decreasing trend in discolored individuals. In healthy thalli, SPAD values were higher in the upper parts than in the lower parts, whereas in extremely discolored thalli, SPAD values in the lower parts became higher. ANOVA of SPAD values in thalli from eight cultivation grounds revealed that threshold values between healthy and discolored thalli were 8.9 in the lower parts and 12.3 in the upper parts. Because discoloration begins in the lower parts and recovery also starts from the lower parts, identifying these thresholds and measuring SPAD values can provide useful information for detecting discoloration in cultivated blades and guiding decisions on fertilization and harvesting.

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The filamentous brown alga was isolated from the coastal area of Uranouchi Inlet in Tosa Bay, Kochi, Japan. To determine its phylogenetic position of this alga, the ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit gene (rbcL) was isolated, and its DNA sequence was analyzed. Phylogenetic analysis of the rbcL sequence revealed that the alga is Acinetospora filamentosa. After determining its phylogenetic position, we analyzed its growth under laboratory conditions at 20 °C and 25 °C, as well as under outdoor culture conditions in a 1,000-L tank. The alga’s growth was similar under both laboratory temperature conditions. In the outdoor culture, we harvested 1 kg fresh weight of algae. Then, we analyzed the content of fucoxanthin that is a valuable compound produced by brown alga using an HPLC. We detected 2.686 mg of fucoxanthin per g dry weight of algae. These results suggest that this alga might serve as a promising source of fucoxanthin due to its successful large-scale outdoor culture and fucoxanthin detection.

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