Structure–Function Relationships of a High-Molecular-Weight Exopolysaccharide from Euhalothece sp. with Bioactive and Emulsifying Properties

Cyanobacterial exopolysaccharides (EPSs) are promising sustainable biopolymers, yet halotolerant strains remain comparatively underexplored and the relationships between EPS composition and functionality are still not well-defined. In this study, an EPS produced by Euhalothece sp. AQUAMEB-3 was isolated and characterized to assess its physicochemical properties, compositional features, and multifunctional performance. The EPS was identified as a high-molecular-weight (Mw ∼ 2617 kDa), strongly anionic heteropolysaccharide with a zeta potential of −45.1 mV, composed mainly of xylose, fucose, amino sugars, and uronic acids. FTIR, 1H NMR, 1H–1H correlation spectroscopy (COSY) NMR, and GC–MS analyses indicated a structurally heterogeneous polymer, while XRD and SEM revealed a predominantly amorphous and porous morphology. The EPS also exhibited pronounced shear-thinning behavior and strong water affinity. Functionally, the EPS showed high emulsifying performance, with E24 values reaching 94.2% at 1.0% (w/v), comparable to xanthan gum under the tested conditions, and maintained emulsion stability over extended storage. It also displayed antibacterial activity against Escherichia coli (MIC: 150 μg mL–1) and Staphylococcus aureus (MIC: 300 μg mL–1), together mL–1 with concentration-dependent antioxidant and iron-chelating activities, reaching 82.6% radical scavenging and 96% metal chelation at the highest tested concentrations. Cytotoxicity assays indicated concentration- and cell-type-dependent cytocompatibility. Taken together, these results define a distinct physicochemical and functional profile for the EPS produced by Euhalothece sp. AQUAMEB-3 and expand the available characterization data for halotolerant cyanobacterial exopolysaccharides. The observed associations between its compositional and physicochemical characteristics and functional properties should be regarded as correlative rather than mechanistically established. These findings support the potential of AQUAMEB-3 EPS for further investigation in emulsion stabilization and other biobased formulation applications.