Bacterial supersystem for alginate import/metabolism and its environmental and bioenergy applications

Distinct from most alginate-assimilating bacteria that secrete polysaccharide lyases extracellularly, a gram-negative bacterium, Sphingomonas sp. A1 (strain A1), can directly incorporate alginate into its cytoplasm, without degradation, through a “superchannel” consisting of a mouth-like pit on the cell surface, periplasmic binding proteins, and a cytoplasmic membrane-bound ATP-binding cassette transporter. Flagellin homologues function as cell surface alginate receptors essential for expressing the superchannel. Cytoplasmic alginate lyases with different substrate specificities and action modes degrade the polysaccharide to its constituent monosaccharides. The resultant monosaccharides, α-keto acids, are converted to a reduced form by NADPH-dependent reductase, and are finally metabolized in the TCA cycle. Transplantation of the strain A1 superchannel to xenobiotic-degrading sphingomonads enhances bioremediation through the propagation of bacteria with an elevated transport activity. Furthermore, strain A1 cells transformed with Zymomonas mobilis genes for pyruvate decarboxylase and alcohol dehydrogenase II produce considerable amounts of biofuel ethanol from alginate when grown statically.