Ulvan from Ulva intestinalis of the Caspian Sea: Characteristics, antioxidant, and antibacterial activities

• Sulfated polysaccharides from the green algae Ulva intestinalis (USP) are known as ulvan. • Ulvan is a semi-crystalline biopolymer with a primary crystalline reflection at 27.5 2θ°. • The presence of sulfate esters with (C-O-S) and (S=O) functional groups was identified in USP. • USP demonstrated concentration-dependent antioxidant activity. • USP exhibits good antibacterial activity against four different pathogens. Considering the significance of polysaccharides and the limited knowledge regarding the structure, potential applications, and biological functions of ulvan—a sulfated polysaccharide derived from Ulva intestinalis (USP) of the Caspian Sea—this study aimed to investigate its structural features and bioactivities using hot water extraction. Chemical analysis revealed that USP consisted of 41.8% carbohydrates and 17.18% sulfate groups. ATR-FTIR spectroscopy confirmed the presence of sulfate esters, showing characteristic peaks at 873 cm −1 (C–O–S) and 1260 cm −1 (S=O). Furthermore, XRD analysis revealed a semi-crystalline structure with a prominent reflection at 27.5° (2θ). Biological assays demonstrated a dose-dependent increase in antioxidant capacity. The most effective IC 50 values were 78 μM Trolox/g for total antioxidant activity and 255 μg/mL for superoxide radical scavenging. USP also exhibited antibacterial activity against Escherichia coli, Staphylococcus aureus, Micrococcus luteus , and Salmonella typhimurium , with the highest inhibition zone (12 mm) and the lowest MIC (6.5 mg/mL) and MBC (15.6 mg/mL) observed against M. luteus . These findings highlight ulvan, due to its distinct structural characteristics, as a promising natural and sustainable candidate for various biotechnological applications.