Marine seaweeds increasingly grow into extensive algal blooms， which are detrimental to coastal ecosystems， tourism and aquaculture. However， algal biomass is also emerging as a sustainable raw material for the bioeconomy. The potential exploitation of algae is hindered by our limited knowledge of the microbial pathways-and hence the distinct biochemical functions of the enzymes involved-that convert algal polysaccharides into oligo- and monosaccharides. Understanding these processes would be essential， however， for applications such as the fermentation of algal biomass into bioethanol or other value-added compounds. Here， we describe the metabolic pathway that enables the marine flavobacterium Formosa agariphila to degrade ulvan， the main cell wall polysaccharide of bloom-forming Ulva species. The pathway involves 12 biochemically characterized carbohydrate-active enzymes， including two polysaccharide lyases， three sulfatases and seven glycoside hydrolases that sequentially break down ulvan into fermentable monosaccharides. This way， the enzymes turn a previously unexploited renewable into a valuable and ecologically sustainable bioresource.