**5. Conclusions**

In conclusion, we compared the microbiota composition in the cecal and fecal microbiota of 16-week HFD-fed mice and revealed that HFD dramatically altered the abundance of Bacteroidetes and Actinobacteria, and *Coriobacteriaceae*, *S24-7*, and *Ruminococcus* in both the cecal and fecal microbiota but did not affect the relative abundance of Firmicutes, *Clostridiales*, *Oscillospira*, *Ruminococcaceae* in cecal microbiota and the Simpson's index of fecal microbiota. Co-treatments with SfW and SfA exacerbate body weight gain and partially reverse HFD-induced alterations of *Clostridiales* and *Ruminococcaceae* and alter the abundance of genes encoding monosaccharide-transporting ATPase, α-galactosidase, β-fructofuranosidase, and β-glucosidase. Our findings provide important insights for the study of seaweed polysaccharide-based gu<sup>t</sup> microbiota regulators.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/1660-339 7/19/7/364/s1. Figure S1 1H-NMR spectra of *Sargassum fusiforme* polysaccharides SfW and SfA.

**Author Contributions:** X.-W.X. and H.W. designed the study; B.W.(Bin Wei), Q.-L.X., B.Z., T.-S.Z., and S.-Z.K. performed the animal study; S.-J.W. characterized the structures of polysaccharides; B.W.(Bin Wei) and Q.-L.X. wrote the manuscript; B.W.(Bin Wu) revised the manuscript; X.-W.X. and H.W. supervised the study. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was financially supported by the National Key Research and Development Program (No. 2017YFE0103100) and the programs of the National Natural Science Foundation of China (No. 81903534, No. 81773628, and No. 81741165) and the Scientific Research Fund of Second Institute of Oceanography, MNR (No. JB2002).

**Institutional Review Board Statement:** The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the Ethics Committee of the Zhejiang University of Technology, China (No. 20190219043).

**Data Availability Statement:** The data in this study are available on request from the corresponding author.

**Conflicts of Interest:** There are no conflict to declare.
