*3.3. Water Extraction Procedure*

The extraction of native fucoidan from *S. crassifolium* was done according to the method reported by Huang et al. [23]. In brief, the algal sample was mixed with 95% ethanol (*w*/*v* = 1:10), shaken for 1 h at room temperature to remove pigments and lipid, and then centrifuged at 970× *g* for 10 min. The residue was then collected, mixed with double distilled water (*w*/*v* = 1:10), and placed in a water bath kept at 85 ◦C for 1 h with shaking to extract the polysaccharides. The mixture was centrifuged at 3870× *g* for 10 min, and the supernatant was collected. Ethanol (95%) was added into the supernatant to give a final ethanol concentration of 20% in order to precipitate alginic acid. The mixture was centrifuged at 9170× *g* for 30 min, the supernatant was collected, and 95% ethanol was added until a final ethanol concentration of 50% was reached in order to obtain fucoidan precipitate. The ethanol-precipitated fucoidan was then recovered by centrifugation at 9170× *g* for 30 min, dried at 40 ◦C, milled, and stored at 4 ◦C for further use. Extraction yield was calculated using the following equation:

$$\begin{array}{l} \text{Extraction yield (\%)} = \text{(weight of the extracted solid, dry basis/weight of the} \\ \text{sample, dry basis)} \times 100 \end{array} \tag{1}$$

## *3.4. Preparation of Degraded Fucoidans*

A sample of native fucoidan weighing 0.2 g was dissolved into 20 mL of distilled water and mixed with 10 mmol/L AA, 10 mmol/L H2O2, or a mixed solution of 10 mmol/L H2O2 and 10 mmol/L AA. The native fucoidan was degraded at RT for 16 h. Then, the degraded fucoidan was precipitated using 75% ethanol, collected, and dried for further use [24].
