Converts beta-D-mannuronic acid (M) to alpha-L-guluronic acid (G). Has both epimerase and lyase activities. Contributes to abortive encystment by degrading the coat from inside the cyst. Important for cyst germination. 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. [(1->4)-beta-D-mannuronosyl](n) = [alginate](n) Inhibited by zinc. Glycan biosynthesis; alginate biosynthesis. Probably exported via the hemolysin-type secretion pathway. Produced mainly in germinating cells. Composed of one catalytically active A module and three R modules. Each enzyme of this family of C5 epimerases introduces its own characteristic sequence distribution of G-blocks in their substrates, explaining the extensive sequence variability of alginates. These alginates of varying composition have different physical properties and are necessary at different stages of the bacterium life cycle. Belongs to the D-mannuronate C5-epimerase family.