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 (polyM) alginate from P.aeruginosa more efficiently than non-deacetylated polyM and alginate from M.pyrifera. AlgL from P.syringae degrades also its own alginate, which may indicate a role in cleaving preformed alginate and/or in determining the length of the alginate polymer (PubMed:11029455). 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. The monovalent cation sodium enhances activity but is not absolutely required. Optimum pH is 7.0. Optimum temperature is 42 degrees Celsius. Belongs to the polysaccharide lyase 5 family.