The polysaccharide β-mannan, which is common in terrestrial plants but unknown in microalgae, was recently detected during diatom blooms. We identified a β-mannan polysaccharide utilization locus (PUL) in the genome of the marine Flavobacterium Muricauda sp. MAR_2010_75 which resembles PULs in bacteria from diverse ecosystems. Proteomics showed the β-mannan induced translation of 22 proteins encoded within the PUL.