**4. Conclusions**

In this work, alginate lyase Alg2951 was cloned from a novel marine bacterium *Alteromonas portus* HB161718T, expressed extracellularly, and characterized. The coldadapted Alg2951 performed its highest activity at 25 ◦C and performed more than 60% activity at 15–40 ◦C and was relatively stable at temperatures lower than 30 ◦C as well. Furthermore, it was a PL7 alginate lyase and could be powerfully promoted by sodium and potassium salts at a concentration of 0.2 M. Meanwhile, it specifically degraded polyG and sodium alginate but had almost no activity on polyM. The TLC and ESI-MS analyses indicated that it could hydrolyze sodium alginate to produce DEH and trisaccharide in an exolytic and endolytic manner. Moreover, the degradation products of alginate by Alg2951 exhibited good antioxidant activities by detecting the scavenging hydroxyl, ABTS radicals ability, and reducing power. This study hopefully provides a potential tool for pharmaceutical and industrial applications.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/1660-339 7/19/3/155/s1: Figure S1: Gellation reactions observed on the plate covered by the CaCl2 solution, Figure S2: Transmission electron micrograph of cells from a 12-hour-old culture on marine agar 2216 of strain HB161718T. Bar, 1 μm, Figure S3: Substrate specificity of the recombinant enzyme Alg2951. **Author Contributions:** Conceptualization, H.H., S.B., and Y.H.; data curation, H.H., S.L., and K.M.; formal analysis, H.H., S.L., and K.M.; funding acquisition, H.H. and Y.H.; investigation, S.B. and D.S.; methodology, D.S. and Y.H.; project administration, Y.H.; software, S.L.; supervision, S.B.; writing—original draft, H.H. and S.L.; and writing—review and editing, S.B. and Y.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by grants from the Key Research and Development Project of Hainan Province (ZDYF2020182 and ZDYF2019133) and the Financial Fund of the Ministry of Agriculture and Rural Affairs of China (NHYYSWZZZYKZX2020).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
