Catalyzes the depolymerization of alginate by cleaving the beta-1,4 glycosidic bond between two adjacent sugar residues via a beta-elimination mechanism. Degrades deacetylated polymannuronate alginate more efficiently than non-deacetylated polyM. Is able to degrade its own alginate, but at a lower efficiency than that produced from M.pyriferia and P.aeruginosa (PubMed:10049370). May serve to degrade mislocalized alginate that is trapped in the periplasmic space (By similarity). Eliminative cleavage of alginate to give oligosaccharides with 4-deoxy-alpha-L-erythro-hex-4-enuronosyl groups at their non-reducing ends and beta-D-mannuronate at their reducing end. Monovalent cations such as potassium and sodium enhance activity, as well as a combined action of these cations with magnesium. However, other cations like calcium, cobalt, manganese and zinc, or the presence of EDTA, do not affect the enzymatic activity. Optimum pH is 7.5. Optimum temperature is 30 degrees Celsius. Belongs to the polysaccharide lyase 5 family.