BACKGROUND　Alginate　oligosaccharides　(AOS),　with　various　physiological　effects,　have　been　widely　used　in　the　food,　agricultural,　and　pharmaceutical　industries.　The　biological　enzymatic　method　of　preparing　AOS,　using　alginate　lyase,　has　more　advantages　compared　with　physical　and　chemical　methods.　Cloning　and　heterologously　expressing　alginate　lyase　are　therefore　very　important.　RESULTS　A　novel　alginate　lyase,　BY17PV7,　from　Microbulbifer　sp.　BY17,　isolated　from　Gracilaria,　was　cloned　and　expressed　in　Escherichia　coli　BL21(DE3).　BY17PV7　was　about　27　KDa.　BY17PV7　showed　the　greatest　activity　(150.42　+/-　3.32　U/mg)　at　43　degrees　C　and　pH　8.9.　It　could　be　activated　by　Ca2+,　Mn2+,　Co2+,　Fe3+,　Na+,　and　inhibited　by　Mg2+,　Zn2+,　Ba2+,　Cu2+,　sodium　dodecyl　sulfate　(SDS),　ethylene　diamine　tetraacetic　acid　(EDTA).　BY17PV7　had　a　wide　range　of　substrate　specificity　and　good　degradation　effects　for　poly　beta-D-mannuronate　(polyM)　and　poly　alpha-L-guluronate　(polyG),　demonstrating　that　it　is　a　bifunctional　alginate　lyase.　The　kinetic　parameters　showed　that　BY17PV7　had　a　greater　affinity　for　polyG.　BY17PV7　released　AOS　with　a　degree　of　polymerization　(DP)　of　3-4　in　an　endolytic　manner　from　sodium　alginate.　Alginate　oligosaccharides　showed　strong　antioxidant　ability　of　reducing　Fe3+　and　scavenging　radicals　such　as　hydroxyl,　2,2-azion-bia　(3-ethylbenzo-thiazoline-6-sulfonic　acid　diammonium　salt)　(ABTS)　and　2,2-diphenyl-1-picrylhydrazyl　(DPPH).　CONCLUSION　A　novel　bifunctional　alginate　lyase,　BY17PV7,　was　expressed　and　characterized　in　Escherichia　coli　BL21(DE3).　The　results　were　helpful　for　the　analysis　of　the　molecular　mechanisms　of　degrading　patterns　in　the　polysaccharide　lyase　(PL)　family.　(c)　2022　Society　of　Chemical　Industry.