AbstractUnlike proteins, glycan chains are not directly encoded by DNA, but by the specificity of the enzymes that assemble them. Theoretical calculations have proposed an astronomical number of possible isomers (> 10¹² hexasaccharides) but the actual diversity of glycan structures in nature is not known. Bacteria of the Bacteroidetes phylum are considered primary degraders of polysaccharides and they are found in all ecosystems investigated. In Bacteroidetes genomes, carbohydrate-degrading enzymes (CAZymes) are arranged in gene clusters termed polysaccharide utilization loci (PULs). The depolymerization of a given complex glycan by Bacteroidetes PULs requires bespoke enzymes; conversely, the enzyme composition in PULs can provide information on the structure of the targeted glycans. Here we group the 13,537 PULs encoded by 964 Bacteroidetes genomes according to their CAZyme composition. We find that collectively Bacteroidetes have elaborated a few thousand enzyme combinations for glycan breakdown, suggesting a global estimate of diversity of glycan structures much smaller than the theoretical one.