May be involved in the polymerization of mannuronate to alginate. Glycan biosynthesis; alginate biosynthesis. Interacts with AlgX. Cells lacking this gene in the mucoid strain FRD1 are phenotypically non-mucoid, i.e. non-alginate producing, and secrete high levels of dialyzable and low-molecular-weight uronic acids. Belongs to the AlgK family.