The marine yeast strain Metschnikowia saccharicola DD21-2, isolated from sediments in the Yalu River, produces a killer toxin with a lethal effect on Metschnikowia bicuspidate strain WCY, a pathogenic yeast strain that infects crabs. In this study, the killer toxin was purified and characterized. After sequential purification, the purity of the killer toxin was increased 72.2-fold over the purity of the yeast cell culture supernatant. The molecular weight of the purified killer toxin was 47.0 kDa. The optimal pH and temperature for killing activity were 5.5°C and 16°C, respectively. The killing activity was stable over a pH range of 4.0–6.5 and temperature range of 0°C–40°C. The purified killer toxin was only effective against toxin-sensitive integral cells and had no killing effect on the protoplasts of toxin-sensitive cells. When exerting the killing effect, the toxin bind to a cell wall receptor of the treated strain, disrupted cell wall integrity and eventually caused death. The amino acid sequence identified by mass spectroscopy indicated that the purified killer toxin might be a protein kinase, but did not show β-1,3-glucanase activity, consistent with the laminarin hydrolysis results. These findings provide a basis for disease prevention and control in marine aquaculture.

The microwave-assisted carboxymethylation of agar to improve its physicochemical properties was investigated. Microwave power, reaction time, and temperature, ethanol concentration, and amounts of chloroacetic acid and sodium hydroxide were assessed for their effects on synthetic yield and degree of substitution (DS). All factors were positively correlated with DS within a certain range. Using optimized conditions, samples with different DS were prepared, and the physicochemical properties of unmodified and carboxymethyl agars prepared by microwave and conventional methods were compared. Carboxymethylation significantly changed the physicochemical properties of the agar, improving gel transparency and reducing dissolution temperature, gel strength, gel hardness, molecular weight, and molecular size; DS was the key factor. Specifically, higher DS values resulted in greater changes. The microwave-assisted method significantly shortened the reaction time and preserved molecular weight, gel strength, and texture hardness of the agar. Therefore, as an environmentally friendly method, microwave-assisted synthesis shows great promise for producing carboxymethyl agar.

To advance seaweed polysaccharide applications in hydrogel wound dressings, five antibacterial composite hydrogels (groups A~E) were synthesized using oxidized κ-carrageenan (OKC), acrylamide (AM), and progressively increasing concentrations of silver-based metal–organic frameworks (Ag-MOFs). Systematic characterization revealed concentration-dependent effects: (1) positive correlations were obtained for the moisture content (MC, maximized at 82.70% in E) and antibacterial efficacy (dose-dependent enhancement); (2) negative impacts were obtained for the swelling ratio (SR, E: 479% vs. A: 808%); and (3) high-dose drawbacks but low-dose benefits in terms of water resistance (WR), tensile strength (TS), elongation at break (EB), and microstructure were obtained. Group B demonstrated optimal Ag-MOFs loading, enhancing TS and EB, while excessive Ag-MOFs loading in C~E significantly degraded them (p < 0.05). Microstructural analysis showed severe 3D spatial damage in D~E. Furthermore, cytocompatibility assessments revealed that all groups maintained a cell viability exceeding 90%, demonstrating excellent biocompatibility. Among them, A~C showed a viability statistically equivalent to the control (p > 0.05) and were significantly higher than D~E (p < 0.05). In conclusion, group B emerged as the optimal Ag-MOFs formulation and exhibited superior WR, enhanced mechanical strength (TS and EB), and potent antibacterial activity while maintaining microstructural integrity and excellent biosafety. This Ag-MOFs/OKC/PAM hydrogel provides dual infection prevention and tissue support, maximizing seaweed polysaccharide benefits with excellent biocompatibility.

The aim of this study was to investigate the hypoglycemic activity and mechanism of G. lemaneiformis polysaccharide gels (GLP and GLP-HV) based on IR/IRS-2/PI3k/Akt/Glut4 and glycometabolism signaling pathways in HepG2 cells. After H₂O₂-Vc degradation, the molecular weight of G. lemaneiformis polysaccharide gel declined from 1478 kDa to 16 kDa. Molecular weight chromatogram and distribution indicated that GLP-HV had a high molecular weight homogeneity compared to GLP. G. lemaneiformis polysaccharide gels significantly decreased the TC, TG, LDL-C, MDA, and LDH contents and enhanced the activities of HDL-C, T-AOC, CAT, GSH-PX, SOD, insulin, and glycogen in HepG2 cells. Fluorescent staining results showed that G. lemaneiformis polysaccharide gels reduced ROS and calcium ions levels in HepG2 cells. GLP and GLP-HV displayed excellent hypoglycemic activity, with GLP-HV performing better. Furthermore, qPCR and Western blot analysis revealed that G. lemaneiformis polysaccharide gels remarkably strengthened the levels of IR, IRS-2, PI3K, Akt, Glut4, HK, G6PD, PFK, PEPCK, GK, PK genes, and proteins. Spearman’s correlation analysis revealed that the IR/IRS-2/PI3k/Akt/Glut4 signaling pathway played a dominant role in regulating activity. These results show that G. lemaneiformis polysaccharide gels present a prominent hypoglycemic effect mediated by the IR/IRS-2/PI3k/Akt/Glut4 and glycometabolism signaling pathways, with the former playing a dominant role.