Production and functional characterization of an extracellular galactoglucan G2 secreted by the Sinorhizobium sp. ACT002

Galactoglucans exhibit diverse properties, yet their broad application is limited by production challenges. Herein, we isolated a high-polysaccharide-producing strain, Sinorhizobium sp. ACT002. In shake flasks, ACT002 achieved a productivity of 4.21 ± 0.16 g/L/d, surpassing reported values for Rhizobium/Sinorhizobium species. The purified polysaccharide, G2, had a weight-average molecular weight of 2.23 × 10⁶ Da and displayed a smooth surface as observed by SEM. Structural analyses, including FT-IR, methylation-GC/MS, and 1D/2D NMR, identified G2 as a linear galactoglucan composed of the disaccharide repeating unit →3)-β-D-Glcp-(1→3)-(4,6-Pyr)-α-D-Galp-(1→, closely resembling the EPS II produced by S. meliloti. The 4,6-positions of galactosides were substituted with equimolar pyruvyl groups, while its acetyl group content was lower than that of EPS II. Rheological characterization revealed a critical concentration of 3.23 g/L for pseudoplastic behavior. Among various salts, KCl exerted more pronounced effects on the Carreau-Yasuda flow properties, viscoelasticity, and thermal stability of G2. Moreover, G2 possessed moisture absorption/retention capacities comparable to hyaluronan. It also exhibited notable antioxidant activity, UV absorption, and minimal hemolytic effects. Topical application of G2 effectively mitigated UV-B-induced skin damage in mice, promoting restoration of epidermal integrity and tissue organization. These findings highlight the potential application of G2 in rheology modification, moisture retention, and UV protection.