Central and storage carbon metabolism of the brown alga Ectocarpus siliculosus: insights into the origin and evolution of storage carbohydrates in Eukaryotes

P>Brown algae exhibit a unique carbon (C) storage metabolism. The photoassimilate d-fructose 6-phosphate is not used to produce sucrose but is converted into d-mannitol. These seaweeds also store C as beta-1,3-glucan (laminarin), thus markedly departing from most living organisms, which use alpha-1,4-glucans (glycogen or starch). Using a combination of bioinformatic and phylogenetic approaches, we identified the candidate genes for the enzymes involved in C storage in the genome of the brown alga Ectocarpus siliculosus and traced their evolutionary origins. Ectocarpus possesses a complete set of enzymes for synthesis of mannitol, laminarin and trehalose. By contrast, the pathways for sucrose, starch and glycogen are completely absent. The synthesis of beta-1,3-glucans appears to be a very ancient eukaryotic pathway. Brown algae inherited the trehalose pathway from the red algal progenitor of phaeoplasts, while the mannitol pathway was acquired by lateral gene transfer from Actinobacteria. The starch metabolism of the red algal endosymbiont was entirely lost in the ancestor of Stramenopiles. In light of these novel findings we question the validity of the 'Chromalveolate hypothesis'.