Polysaccharide lyase that catalyzes the depolymerization of alginate via a beta-elimination mechanism, cleaving the beta-1,4 glycosidic bond between two adjacent sugar residues. Acts specifically on alginate and each of its block structures, with highest activity toward poly-beta-D-mannuronate (poly-ManA). Shows an exolytic mode of action, producing unsaturated monomers. Displays a very low activity against poly-beta-D-glucuronate (poly-GlcA), and is not active on poly-alpha-D-galacturonate, hyaluronan, heparin, heparan sulfate and chondroitin sulfate. Cleavage of 4-deoxy-alpha-L-erythro-hex-4-enopyranuronoside oligosaccharides into 4-deoxy-alpha-L-erythro-hex-4-enopyranuronate monosaccharides. The zinc ion likely plays a structural role. kcat is 34.8 sec(-1) with alginate as substrate. kcat is 62.2 sec(-1) with poly-ManA as substrate. kcat is 34.6 sec(-1) with poly-GulA as substrate. kcat is 22.2 sec(-1) with poly-MG as substrate. kcat is 0.4 sec(-1) with poly-GlcA as substrate. Optimum pH is 8.5. Homodimer. Is an attractive candidate for the broad-spectrum digestion of polyuronides into fermentable monomers for biofuel production. Belongs to the polysaccharide lyase 17 family.