Carbohydrate polymers are industrially and medically important. For instance， a polysaccharide， alginate (from seaweed)， is widely used in food， textile and pharmaceutical industries. Certain bacteria also produce alginate through membrane spanning multi-protein complexes. Using Pseudomonas aeruginosa as a model organism， we investigated the biological function of an alginate degrading enzyme， AlgL， in alginate production and biofilm formation. We showed that AlgL negatively impacts alginate production through its enzymatic activity. We also demonstrated that deletion of AlgL does not interfere with polymer length control， epimerization degree or stability of the biosynthesis complex， arguing that AlgL is a free periplasmic protein dispensable for alginate production. This was further supported by our protein-stability and interaction experiments. Interestingly， over-production of AlgL interfered with polymer length control， suggesting that AlgL could be loosely associated with the biosynthesis complex. In addition， chromosomal expression of algL enhanced alginate O-acetylation; both attachment and dispersal stages of the bacterial biofilm lifecycle were sensitive to the level of O-acetylation. Since this modification also protects the pathogen against host defences and enhances other virulence factors， chromosomal expression of algL could be important for the pathogenicity of this organism. Overall， this work improves our understanding of bacterial alginate production and provides new knowledge for alginate production and disease control.