**4. Conclusions**

Our results revealed that LMW-ulvan enzymatic production of ulvan extracted from green algae U. pertusa consists of 57.23% rhamnose, 28.76% xylose, 7.42% glucuronic acid, and 1.77% glucose. Its backbone contained (1 →3,4)-linked Rha, (1 →4)-linked Xyl, and (1 →4)-linked GlcA with small amounts of (1 →4)-linked Rha residues; sulfate substitution was at C-3 of rhamnose. In addition, LMW-ulvan was able to relieve intestinal inflammation and oxidative damage caused by DSS. Meanwhile, LMW-ulvan was able to significantly increase mRNA levels of claudin, occluding, and ZO-1, which improve intestinal mucosal permeability. Consequently, the results of this study support the potential application of LMW-ulvan as a functional food ingredient for IBD.

**Author Contributions:** Conceptualization, Y.L. (Yuanyuan Li), R.L., and Y.L. (Yinping Li); Methodology, Y.L. (Yuanyuan Li), R.L., and Y.L. (Yinping Liu); Formal analysis, Y.L. (Yuanyuan Li), H.Y., T.W., P.W., and Y.T.; Investigation, Y.L. (Yuanyuan Li), H.Y., J.Z., and Y.L. (Yinping Li); Data curation, Y.L. (Yuanyuan Li); Writing, Y.L. (Yuanyuan Li), R.L., and Y.L. (Yinping Li); Visualization, Yuanyuan Li; Supervision, R.L. and Yinping Li; Project administration, R.L. and Y.L. (Yinping Li); Funding acquisition, R.L. and P.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Comprehensive Management of Marine Ecological Environment and Risk Emergency Handling No.2019-Environment Protection Project-070, the Key Project of New and Old Energy Transformation in Shandong Province, and Research project of ecological environment protection and restoration of Yangtze River in Zhoushan (No. SZGXZS2020068).

**Conflicts of Interest:** The authors declare that they have no competing financial or non-financial conflict of interests.
