*4.1. Materials*

The brown algae *S. fusiforme* was sourced from Qingdao, China, on August, 2018. Standards of monosaccharides and 1-phenyl-3-methyl-5-pyrazolone (PMP) were purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China). Dextran standards were purchased from American Polymer Standards Corporation (Mentor, OH, USA). Other reagents and solvents were of analytical grade.

#### *4.2. Preparation of Sargassum fusiforme Polysaccharides*

As previously reported, *S. fusiforme* polysaccharides were prepared with slight modifications [11]. Briefly, dry *S. fusiforme* was cut into pieces and pre-treated with 95% ethanol to remove the pigment. Polysaccharides were extracted from the residual materials by hot water for 1 h and 0.1 M HCl at 60 ◦C for 1 h, respectively. The hot water-extracted polysaccharide was further treated with 0.05 M MgCl2 to eliminate the alginate, and ultra-filtered to obtain the SfW. The HCl-extracted crude polysaccharide was dialyzed and precipitated using ethanol to obtain the SfA.

#### *4.3. Structural Analysis of Sargassum fusiforme Polysaccharides*

The chemical analysis of SfW and SfA was based on previous studies [11]. The total sugar content was measured by the phenol-sulfuric acid method using D-glucose as the standard [34]. The content of sulfate was analyzed with the BaCl2-gelation method using Na2SO4 as the standard [35]. The molecular weight analysis was conducted using High-Performance Size Exclusion Chromatography using a Waters 2487 HPLC system with a refractive index detector 2414 (Waters, Milford, MA, USA). The chromatography conditions refer to previous studies [11]. The molar ratio of monosaccharide and fucose content was determined by the PMP derivatization method with minor modification.

SfW and SfA fraction (100 mg) were dissolved in 0.55 mL of D2O (99.9%) followed by centrifugation and lyophilization, and the process was repeated three times. Finally, the resulting polysaccharide was dissolved with 0.55 mL of D2O. 1H NMR spectra were recorded on AVANCE III NMR 600 MHz spectrometer (Bruker Inc., Billerica, MA, USA) at 25 ◦C.
