Mannitol is a polyol that occurs in a wide range of living organisms, where it fulfills different physiological roles. In particular, mannitol can account for as much as 20 to 30% of the dry weight of brown algae and is likely to be an important source of carbon for marine heterotrophic bacteria. Zobellia galactanivorans (Flavobacteriia) is a model for the study of pathways involved in the degradation of seaweed carbohydrates. Annotation of its genome revealed the presence of genes potentially involved in mannitol catabolism, and we describe here the biochemical characterization of a recombinant mannitol-2-dehydrogenase (M2DH) and a fructokinase (FK). Among the observations, the M2DH of Z. galactanivorans was active as a monomer, did not require metal ions for catalysis, and featured a narrow substrate specificity. The FK characterized was active on fructose and mannose in the presence of a monocation, preferentially K ؉ . Furthermore, the genes coding for these two proteins were adjacent in the genome and were located directly downstream of three loci likely to encode an ATP binding cassette (ABC) transporter complex, suggesting organization into an operon. Gene expression analysis supported this hypothesis and showed the induction of these five genes after culture of Z. galactanivorans in the presence of mannitol as the sole source of carbon. This operon for mannitol catabolism was identified in only 6 genomes of Flavobacteriaceae among the 76 publicly available at the time of the analysis. It is not conserved in all Bacteroidetes; some species contain a predicted mannitol permease instead of a putative ABC transporter complex upstream of M2DH and FK ortholog genes.
Brown algae (Phaeophyceae) are the dominant macroalgae in temperate and polar regions and thus play a crucial role in the primary production of coastal ecosystems (1). They contain large amounts of different structural and storage carbohydrates. For instance, their extracellular matrices are formed by the accumulation of cellulose, fucanes, and alginates (2-4), while they store carbon by accumulating laminarin and mannitol (5). The potential of this biomass resource for the production of liquid biofuels (6), including ethanol from alginate and mannitol (7,8), and for the implementation of biorefineries (9) has been highlighted recently.Depending on the species, mannitol can represent as much as 20 to 30% of the dry weight of brown seaweed (10). In the genomic and genetic model of brown algae Ectocarpus sp., formerly included in the species Ectocarpus siliculosus (11), it has been observed that the content of this polyol differs according to the diurnal cycle (12) and that it is likely to act as an osmoprotectant or a local compatible osmolyte (13). Mannitol is localized in the cytosol and is also present at the reducing ends of vacuolar laminarin molecules of the M series (in contrast to the G series, which contain only glucose residues) (14). Mannitol in brown algae is produced directly from the photoassimilate fructose-6-phosphate (F6P) by two st...