Catalyzes the depolymerization of alginate by cleaving the beta-1,4 glycosidic bond between two adjacent sugar residues via a beta-elimination mechanism (PubMed:8370530, PubMed:8335634, PubMed:23215237). May serve to degrade mislocalized alginate that is trapped in the periplasmic space. Acts preferentially on non-acetylated alginate or its precursor mannuronan. Is able to catalyze cleavage adjacent to either mannuronate or guluronate residues in alginate. Exhaustive digestion of alginate by AlgL generates dimeric and trimeric products (PubMed:23215237). In addition to its enzymatic function, AlgL appears to be required for alginate export, maybe as part of a multi-protein alginate-secretion complex (PubMed:16177314). 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. kcat is 32 sec(-1) for the depolymerization of mannuronan (dp value of 133). kcat is 1.5 sec(-1) for the depolymerization of acetylated mannuronan (dp value of 133). kcat is 32 sec(-1) for the depolymerization of alginate (dp value of 263). kcat is 1.2 sec(-1) for the depolymerization of acetylated alginate (dp value of 263). The catalytic efficiency of the depolymerization reaction increases linearly with the number of residues in the substrate. Its expression is under the control of AlgB, which also regulates the alginate biosynthetic gene cluster. Deletion of algL is lethal, and microscopic examination of the cells reveals that alginate or a precursor accumulates in the periplasmic space until the cells burst. Belongs to the polysaccharide lyase 5 family.