A Novel Sphingan Containing Enriched Guluronic Acid Residues and Its Hydrogel Properties

In this study, it is shown that the strain Sphingomonas sp. HL-1 can utilize glucose, xylose and mixed sugars to produce high yields (28.7 g/L) of HL gum, which has a high molecular weight of 2.03 × 106 Da. HL gum is a novel sphingan uniquely enriched in guluronic acid (16.74%) and galacturonic acid, with low rhamnose content, distinguishing it from conventional sphingans. It exhibits good solubility in water at 25 °C and 60 °C, pronounced shear-thinning behavior, and concentration-dependent, heat-activated gelation. Remarkably, even at a low concentration of 1% (w/v), HL gum forms a resilient hydrogel upon heating to 90 °C, achieving a gel strength of 2950 ± 130 g/cm2, and maintains gel stability under high salinity (up to 50 g/L). SEM micrographs revealed a clear temperature-dependent morphological transition: from fibrous structures at 20 °C to curled aggregates at 60 °C and finally to a porous fibrous network at 90 °C, consistent with a sol–gel transition. Combined with its ability to efficiently utilize mixed sugars in a short fermentation cycle, HL gum’s unique chemical composition and superior gelation properties distinguish it from conventional sphingans, highlighting its potential for food, biomedical, and industrial hydrogel applications.