Converts beta-D-mannuronic acid (M) to alpha-L-guluronic acid (G), producing a polymer with gel-forming capacity, required for the formation of the cyst coat. [(1->4)-beta-D-mannuronosyl](n) = [alginate](n) Inhibited by zinc. Glycan biosynthesis; alginate biosynthesis. Probably exported via the hemolysin-type secretion pathway. Produced during vegetative growth and in encysting cells. Composed of one catalytically active A module and four 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.