Some species of brown algae (Phaeophyceae) form large forests that sustain diverse communities and promote ecosystem stability. As an important blue carbon source, its decomposition by marine microorganisms constitutes a key step of the marine carbon cycle. The most abundant macromolecules in brown algae are polysaccharides, such as alginate and fucoidan, which constitute approximately half of their dry weight. In marine bacteria, alginate- and fucoidan-degrading enzymes participate in the initial steps of the polysaccharide assimilation pathway, and these organisms often contain multiple genes coding for these enzymes. In our laboratory, we are exploring metagenomes from different marine habitats in order to assess the polysaccharide-degrading potential of marine microbial communities, to increase our knowledge of the diversity of alginate lyases and fucanases, and to uncover the relative contribution of different taxa of marine microorganisms to the assimilation of the major brown algae polysaccharides. Oligoalginates and fuco-oligosaccharides produced by endo-acting enzymes present remarkable biological activities and applications in the pharmaceutical, nutraceutical, cosmeceutical, agricultural, and food industries. The expression and characterization of alginate lyases and fucanases from uncultured marine bacteria not only contribute to our understanding of the degradation processes, but also is the first step for development of biocatalytic processes for the production of these high-value products from brown algae biomass.